Before we get started with What Makes a Receiver Great

Before we get started with What Makes a Receiver Great
Please mute your mike by clicking the Mike button. Your mike is muted when the Mike Symbol is RED. Un-mute only when you want to ask a question. Mike symbol is Black Mute again when you are done talking. This reduces everyone’s background noise and makes the class more enjoyable. Please hold your questions until the end of my presentation. There will be plenty of time for questions after the class The reason is to generate a high quality recording of the class. The class will start promptly at 7 pm Pacific (02:00 UTC) and will be recorded. Thanks! Read Slide Northern California DX Club ncdxc.org

What makes a receiver great? Understanding receiver specs
Good evening, I’m John Eisenberg K6YP The subject of my class today is: What makes a receiver great? My goal is to give you a solid understanding of the meaning of some key receiver specifications. This will help you evaluate what is on a product data sheet, or know what SHOULD BE there , but isn’t. Some of the concepts are a little challenging but I’ll try my best to make them clear. For those of you who are not RF system engineers this material should: Help debunk some of the outrageous misconceptions often heard on the air Make you able to read and understand a QST product review and Aid you in picking out you next radio In a few days this recorded class will be posted at John Eisenberg K6YP Northern California DX Club ncdxc.org

Agenda Introduction Receiver fundamentals Sensitivity Linearity
Dynamic Range and AGC Function Selectivity Stability Conclusion I will start with a brief introduction, This will be followed by a discussion of what a receiver should do. The presentation is not focused on any particular receiver architecture, but a single conversion super-hetrodyne block diagram will be used to illustrate some key points. Next we’ll look at the factors that determine receiver sensitivity and discover how important low receiver local oscillator phase noise is when detecting weak signals. Linearity is also critical in a pileup as intermodulation of strong adjacent or non-adjacent stations can bury a weak signal. I will review the concept of SFDR which determines the range over which your receiver can receive a signal free of intermodulation. We will also look at why proper AGC design is so important to receiver performance and then talk about BDR. The next topic is receiver selectivity, the ability to separate the desired sigal from others. Then I will briefly touch on receiver stability, which just isn’t much of an issue today. Finally I’ll draw some slightly biased conclusions. Northern California DX Club ncdxc.org

Introduction If you can’t hear him, you can’t work him!
Northern California DX Club ncdxc.org Introduction If you can’t hear him, you can’t work him! Hearing him depends on: Is he on? Is there decent propagation? Do you have enough antenna? How much QRM/QRN is present? The performance of your receiver. Today’s we will focus on receiver performance. Read the slide

What are you up against? Weak signals (CW or SSB or a digital mode)
Atmospheric and man made noise (QRN) Interfering signals (QRM) such as: Strong signals adjacent to your frequency Strong signals far removed in frequency Fast or slow fading (QSB) Read the slide Northern California DX Club ncdxc.org

What are your weapons Key receiver performance factors include:
Sensitivity (Weak signal reception) Selectivity (Bandwidth matched to signal, Ability to reject adjacent QRM) Optimum detector for desired signal modulation type Linearity (Spurious free dynamic range) Blocking dynamic range (From strong adjacent signals) Stability (Keep the signal in the pass band) What are your weapons? Sensitivity: The ability to receive a weak desired signal. Selectivity: The ability to reject adjacent interferers (QRM) close to the desired signal. The Detector: The RX should have an optimum detector for each signal modulation type you wish to receive. SSB, CW etc. Linearity: The RX should have sufficient linearity to prevent IM products generated in the receiver from burying the weak signal you are trying to hear. BDR: Good BDR is needed to prevent strong close in signals from reducing total RX gain and making the desired weak signal inaudible. Stability: The receiver must be sufficiently stable to keep a signal in its narrowest IF pass band. Even greater stability is required by some digital modes. Many of these performance factors interact in different real world situations! Northern California DX Club ncdxc.org

What this talk will address
Key Receiver Specifications What are they? Why each is important? How are they defined? What they mean to you. . Read the slide Northern California DX Club ncdxc.org

Receiver fundamentals
What must a receiver do? Amplify a weak signal delivered to the receiver by the antenna. Filter out undesired interfering signals and noise . Detect the desired signal, extract its intelligence and present the content in a useful format. Read the slide Northern California DX Club ncdxc.org

Receiver fundamentals
What must a receiver not do? Add additional excess noise to the received signal (Degrade SNR) Generate additional spurious signals or mixer images which can corrupt the detection process Drift off the desired signal frequency Read the slide Northern California DX Club ncdxc.org

Simple Superhetrodyne receiver
Antenna Pre- Select Filter Here is the block diagram of an analog, single conversionm, Superhetrodyne Receiver The Pre Select Filter rejects strong OOB signals and mixer image responses. The RF Amp amplifies the incoming signal. We want min RF gain before the mixer, just enough to nail a reasonable RX NF. The RX should have the capability to switch the RF amplifier out of the signal path. The RF amp should not add its own IM products to the received signal. I’ll discuss NF and IM shortly. The Image Reject Filter follows the RF amplifier. Its job is to prevent signals at the mixer’s image frequency from reaching the mixer and being converted to the IF frequency. The mixer converts say a 7 MHz I/P signal to a 9 MHz IF using a 16 MHz LO (16-7 = 9 MHz) The mixer also produces a 9 MHz IF with a 25 MHz input. 25 MHz is the image. (25-16=9MHz) We need to be sure that when RX is tuned to 7MHz, that image rejection at 25MHz is adequate. The Mixer either adds or subtracts the LO and signal frequencies to produce an IF freq. The mixer usually sets the IM performance of a RX. We want to keep gain ahead of the mixer low to avoid hitting it with large signals which produce mixer IM products. Some RX’s use an IRM to reduce the required rejection of the image reject and pre-selection filters ahead of the mixer. The quality and cleanliness of the LO signal are very important. Low phase noise is critical. The Roofing Filter knocks down big, near by signals before they compress the IF amp The IF Amp brings the weak signal up to a level that can be detected The Pre Detect Filter must be matched to the signal characteristics . The roofing filter should not be too much wider than the pre detect filter. Of course the Detector should also be matched to the signal characteristics as well. An Audio Amp and Loudspeaker or Phones follow the detector and produce sound that you can hear. The AGC System must respond to strong signals in such a way that overall RX gain is reduced to prevent RX saturation. IF gain is reduced before RF gain. AGC timing (ie attack and decay times) must also be matched to the signal type being received. AGC Line AGC System Mixer RF Amp IF Amp Image Reject Filter IF Roofing Filter IF Pre- Detect Signal Filter Detector BB Amp Local Oscillator Northern California DX Club ncdxc.org

dB’s and dBm’s Power ratio in dB = 10log(P2/P1)
Gain in dB = 10log(Pout/Pin) 3 dB is a factor of 2, dB is a factor of 1/4 10 dB is a factor of 10, -20 dB is a factor of 1/100 39 dB is a factor of 2x2x2x10x10x10 = 8000 39 dB is dB 0 dBm is 1 milliwatt Thus +13 dBm is 20 mW, -9 dBm is 1/8 mW Let’s discuss dB’s and dBm’s. Understanding both of these is critical to understanding RX performance. Given two power levels P2 and P1 Their ratio is P2/P1 Their ratio in dB is 10*log10 (P2/P1) = 10*log(P2) – 10*log(P1) All logs are base 10 Try to remember a few power ratios in dB A power ratio of 2 is 3 dB or 3dB is a power ratio of Log(2) = A power ratio of 4 is 6 dB because 3 dB + 3 dB = 6 dB and 2*2 = 4 A power ratio of 10 is 10 dB Log(10) = 1.00 A power ratio of 1/10 is -10 dB A power ratio of 100 = 10*10 which in dB is or 20 dB Log(100) = 2.00 -20 dB is a power ratio of 1/100 Cascaded Power ratios Multiply and dBs add So 39 dB is db or a power ratio of 2 * 2 * 2 * 10 * 10 *10 = Log(8000) = 3.903 Gain is the power ratio Pout/Pin of a component or system Zero dBm is defined as 1 milliwatt or 1/1000 of a watt, +30 dBm is a watt, +60dBm = 1kW So +13 dBm is 13 dB more than a milliwat or 2 * 10 or 20 mW And -9 dBm is 9dB less than a mW or db below 0 dBm or ½* ½ * ½ = 1/8mW dBc is usually in reference to spurs etc. So if a mixer image is -30 dBc it is 30 dB below the desired signal. Northern California DX Club ncdxc.org

Noise power Noise is distributed over frequency.
Northern California DX Club ncdxc.org Noise power Noise is distributed over frequency. Noise Power is measured “per unit bandwidth” Example: A noise signal has a uniform power spectral density of -120 dBm/Hz. Noise power increases by 10log(Bandwidth in Hz) 1 Hz Uniform Noise PSD Bandwidth Total Noise Power 1 Hz dBm 10 Hz dBm 100 Hz dBm 1 MHz dBm PSD dBm/Hz In evaluating RX performance we often deal with the concept of Noise Power. Noise power is distributed across a range of frequencies. It is measured “per unit bandwidth”. For example, a noise signal has a power spectral density of dBm/Hz. In the graph, the pink trace is the noise signal which is constant over frequency. If you measure the noise power in a 1 Hz BW it would be -120dBm. Seen in a 10 Hz BW, noise power increases by a factor of 10 or 10 dB to -110 dBm Seen in a 100 Hz BW, noise power increases by a factor of 100 or 20 dB to -100 dBm Seen in a 1MHz BW, noise power increase by a factor of or 60 dB to -60 dBm So noise power increases by 10log(BW in Hz) over the noise power measured in a 1 Hz BW. Receiver specs are often specified in dB, dBm, dBc, dBc/Hz and dBm/Hz It is important to have an intuitive feel for each of these. Freq

Receiver sensitivity Signal/Noise ratio at RX Input
Noise Figure Ratio = Noise figure = Input S/N (dB) - Output S/N (dB) Signal/Noise ratio at RX Input Signal/Noise ratio at RX Output Input S/N = 40 dB Output S/N = 30 dB Now we are ready to take a look at receiver sensitivity which is classically set by the RX NF Noise figure is a measure of how much the SNR of a signal is degraded when it is processed by a device such as an amplifier. The SNR at the input of the device is as good as it can be. The SNR at the devices output will be degraded by the device’s NF. NF = 10*log(Input SNR/Output SNR) or NF = Input SNR in dB – Output SNR also in dB Input and output SNR must be measured in the same BW The device in the slide has a NF = SNRin(dB) – SNRout(dB) = 40dB – 30 dB = 10 dB The text books tell us that NF sets receiver sensitivity. This isn’t always true in the real world. Other factors can degrade RX sensitivity more than NF. However if the RX has a poor NF you are off to a bad start. Most good HF receivers should have about a 10 dB to 15 dB NF. Look at the product review, the manufacturers rarely specify NF. They do often specify sensitivity in uV, which is a good surrogate. Device with NF = 10 dB Northern California DX Club ncdxc.org

Best possible receiver sensitivity
The noise power from a resistor at 25°C (or a matched antenna in signal free environment) is kTB (Boltzmann’s Constant (k) x Temp (°K) x Bandwidth (Hz). kTB = -174dBm/Hz This is the noise floor of a noise free receiver at 27 °C . kTB = x 10^-21 watts/Hz at 27°C or about 0.01 V in a 500 Hz bandwidth. What is the ultimate limit of receiver sensitivity. The noise power output of a matched antenna located in a signal free environment is the same as that of a resistor at the same temperature. This is the ultimate noise floor of any noise free system. Of course no system can ever be totally noise free. The NPO of any impedance matched antenna in a signal free environment is kTB where k is Boltzman’s constant T is temperature in degrees kelvin =degC +273 B is bandwidth in Hz kTB = -174 dBm/Hz or -174 dBm measured in a 1 Hz bandwidth at 27 deg C. kTB = 3.98 * 10^-21 watts/Hz at 27 degC or about 0.01 uV in 500 Hz BW. A receiver with a 500Hz IF BW, 15 dB NF, and a signal 8 dB above the noise (just about MDS for most folks) would have a MDS sensitivity of -124 dBm or about uV if the RX sensitivity was NF limited. kTB=-174dBm/Hz=3.98E-21W/Hz=4.462E-10V/rtHz*sqrt(500)=0.01uV kTB at 500Hz RX sensitivity = =-124dBm (500Hz BW)=10^(-124/10)=3.98E-13mW=3.98E-16W E=sqrt(3.98E-16*50)=1.41e-7V=0.141uV Northern California DX Club ncdxc.org

Minimum detectable signal
Input Noise Floor = KTB + NF + 10log(BW in Hz) Input MDS = KTB + NF + 10log(BW in Hz) + 8 dB Maybe for OH2BH, MDS = Noise Floor + 6 dB (The 8 dB factor is subjective !) Often other receiver problems such as reciprocal mixing further degrade MDS Read the slide OH2BH is Martti Laine a world famous Dxer from Finland with legendary ears. In real world contesting and DX situations reciprocal mixing and IM are often far more important than NF. I will have some examples in the slides to follow. Northern California DX Club ncdxc.org

MDS for CW and SSB signals
Northern California DX Club ncdxc.org MDS for CW and SSB signals SSB Filter 3 KHz BW (35dB) CW Filter 500 Hz BW (27dB) Minimum Detectable SSB signal -121 dBm Minimum detectable CW signal -129 dBm SSB MDS -121 dBm CW MDS -129 dBm SSB Noise Floor -129 dBm To further clarify the effect of BW on MDS, let’s compare how CW and SSB signals fare in the same receiver. The levels shown on the graph are at the RX INPUT. The RX’s BW is set to 500 Hz for CW and 3 kHz for SSB. 10log(500Hz) => 27 dB 10log(3000Hz) => 35 dB RX NF = 10 dB CW noise floor = kTB + NF + 10log(BW) =-174dbm/Hz + 10 dB + 27dB = -137 dBm SSB noise floor = kTB + NF + 10log(BW) = -174dBm/Hz + 10dB + 35 dB = -129 dBm Thus CW MDS is -137 dBm + 8 dB = -129 dBm and SSB MDS is dBm + 8 dB = -121 dBm CW has an 8 dB advantage!!! The take away from this exercise is that RX filters play an important role in determining RX sensitivity and selectivity. You must have roofing and pre-detection filters matched to the characteristics of the signals you wish to hear! CW Noise Floor -137 dBm Noise Power PSD is -174dBm/Hz +10 db NF or -164 dBm/Hz Noise Floor = -174 dBm/Hz + 10dB NF +10log(BW)

The “standard”S meter Receiver S Meter 1 ‘S unit’ = 6 dB
Northern California DX Club ncdxc.org The “standard”S meter Receiver S Meter 1 ‘S unit’ = 6 dB S meter reading Signal level in V Signal Level in dBm S dB S dB S dB S dB S S S S S S S Let’s examine the receiver’s S Meter. I say “standard S meter” as I have used a standard 6 dB S unit. This is the definition of an S unit. S9 is defined a 50uV or -73 dBm. S8 is 25uV because voltage follows a 20log law whereas power follows a 10log law. The signal level of an S8 signal is -79 dBm. Every time we go down in input level by 1 S unit the voltage drops by a factor of two and the power drops 6 dB. So at S1 the voltage is 0.2 uV or -121 dBm a very weal signal indeed. At 60 dB over S9, a huge signal, the voltage is 50000uV or -13dBm. Not quite enough to light an LED but getting close. Receivers can vary widely from the standard both RX to RX and over the RX’s dynamic range. Values between 4 and 9 dB per S unit are not uncommon. This means an S6 signal on a 4 dB/Sunit RX and an S6 signal on a 9 dB/Sunit RX could be very different. Analog electronics has improved a lot these days alowing more accurate S meter circuits. This is far better than reading the AGC voltage as was done back in the day. In a receiver with its IF signal processing done using a DSP, the S meter can be virtually perfect as it may be realized digitally.

LO phase noise & reciprocal mixing
Imagine that you are copying a weak signal and all of a sudden a very strong clean carrier pops up 20 to100 KHz from your frequency. Nothing happens. It is rejected by your receiver’s battery of filters. Right???? No! Your receivers sensitivity may be degraded by reciprocal mixing caused by your local oscillator (LO) phase noise. Read the slide This is an important and somewhat difficult concept to comprehend. LO means local oscillator The LO mixes with the RF signal to produce an IF signal. The phase noise of each LO in a multiple conversion RX are important, but usually the phase noise of the highest frequency LO, typically the 1st LO dominates. This is particularly true in a general coverage RX with a high 1st IF frequency where a high frequency, wide tuning range local oscillator is required. Such oscillators usually have higher phase noise. Reciprocal mixing can often degrade RX sensitivity much more than RX NF. This is especially true in an environment full of strong signals like a contest, Field Day or a DX pileup. Northern California DX Club ncdxc.org

LO Phase noise VLO = (A + Nam(t)) sin[LOt +  + pn(t)] FLO
Im(VLO) LO Phase noise VLO Amplitude A + Nam(t) Phase  +  pn(t) Re(VLO) VLO = (A + Nam(t)) sin[LOt +  + pn(t)] The phase noise term pn(t) usually dominates the AM noise Nam(t) LO Spectrum with phase noise 10 kHz Offset At the top of the slide is a phasor diagram of the LO signal shown with an equation for LO output voltage V_LO for the mathematically inclined. Here we see the LO voltage vector in pink. Its length represents its amplitude. AM noise causes small variations in amplitude of the length of the vector The phase angle of the vector is shown in green. Small variations in the phase are caused by LO phase noise. Variations caused by AM noise and Phase noise are depicted by the oval path at the tip of the LO voltage vector. In the equation A is the amplitude of the LO voltage V_LO. It is modulated by a small AM noise term Nam(t). The sum of these multiply a sinusoid at the LO frequency with a phase angle of theta modulated by a small Phase Noise term THETApn(t). For the more pragmatic, here is a look at LO phase noise on a spectrum analyzer. Phase noise is specified as dB below the carrier (dBc) in a 1 Hz BW at a specified carrier offset. In the illustration we have phase noise specified in a 1 Hz BW offset 10 kHz from the LO frequency. Phase noise is usually expressed in dBc/Hz at some offset. All oscillators operate with some form of soft limiting or closed loop feedback to hold the oscillator’s output voltage constant. Thus LO noise is usually dominated by phase noise. dBc/Hz Phase noise is often expressed in: dBc/Hz at some carrier offset 1 Hz FLO Northern California DX Club ncdxc.org

Reciprocal Mixing Process
Northern California DX Club ncdxc.org Reciprocal Mixing Process Interferer contaminated with RX LO Phase noise Strong clean Interferer IF Filter Bandwidth Weak Signal LO phase noise on interferer Receiver RX RF input signals RX IF Output The strong receiver input signal is W6YX at Stanford, transmitting a clean cw carrier. He is <2 miles away, uses a 5 element beam and runs a booming 1.5 kW The weak signal is FT5XO on Kerguelen Island, miles away, long path, using a vertical and 100 watts You can see both of these stations at my receiver’s input. The pink curve is my RX LO phase noise spectrum. W6YX’s strong carrier and the weak FT5XO signal mix with my LO yielding IF signals contaminated with LO phase noise. The strength of W6YX’s signals makes its IF phase noise sidebands quite large. The wide LO phase noise spectrum places noise in my IF filter bandwidth. Often that noise is stronger than the desired signal which is then buried by the noise and thus becomes undetectable. Even though I have a low noise figure RX and could otherwise hear FT5XO , my LO phase noise prevents me from being able to hear the weak signal from Kerguellen Island due to the strong interferer far from FT5XO in frequency. Phase noise from the RX LO signal mixing with a strong clean interferer even 100kHz away can degrade your ability to receive a weak signal. THIS IS RECIPROCAL MIXING. Buried Weak Signal Local Oscillator with Phase Noise LO phase noise on weak signal

Reciprocal mixing -30 dBm Interferer RX NF = 15 dB,
Northern California DX Club ncdxc.org -30 dBm Interferer RX NF = 15 dB, IF filter bandwidth = 250Hz A -30 dBm strong interferer is 20 KHz from desired signal LO phase noise = -120 dBc/Hz at 20 KHz carrier offset IF Filter Bandwidth Desired Signal RX IF Output 20 KHz Lets use some numbers to see how reciprocal mixing compromises RX sensitivity. Here we have a receiver with a 15 dB NF and an IF BW of 250 Hz. A -30 dBm strong, clean interferer appears at the RX input 20 kHz from the desired signal. The receiver’s LO phase noise is -120 dBc/Hz at a 20 kHz offset from the LO frequency. So the receiver’s noise floor is kTB+NF+10log(BW in Hz) = -174dBm + 15dB + 24dB = -135dBm At 20 kHz away from the -30 dBm interferer Phase Noise in the 250 Hz IF BW referred to the RX input is: LO Phase 20kHz offset + Pinterferer + 10log (BW) = -120dBc/Hz + -30dBm + 24 dB = -126 dBm Adding the RX noise floor noise power of -135 dBm to the noise caused by reciprocal mixing of -126 dBm we obtain a total noise power of dBm. Thus the RX noise floor with reciprocal mixing is degraded from -135dBm to dBm or 9.5 dB. The Equivalent RX NF has increased by 9.5 dB to 24.5 dB. Thus RX sensitivity is reduced 9.5 dB enough to bury a weak signal. Unfortunately the interferer is VERY LOUD i.e. 43 dB over S9 so it significantly degrades receiver sensitivity! RX input noise floor = KTB+NF+10log(BW) = -174dBm/Hz+15dB+24dB = -135 dBm At 20 KHz away from the -30 dBm interferer phase noise in a 250 Hz BW at the RX input is LO Phase noise+Pinterferer+10 log(BW) = -120dBc/Hz -30 dBm +24dB= -126 dBm Adding noise powers in a 250 Hz bandwidth yields dBm. Thus the Equivalent RX NF with phase noise is degraded by 9.5 dB to NF = 24.5 dB.

Reciprocal Mixing Dynanic Range
From the previous example Step 1: The receiver NF is 15dB, its IF bandwidth is 250Hz. Determine Receiver input noise floor = kTB + NF + 10log(BW) = -174dBm/Hz +15dB + 24dB = -135dBm Step 2: Place a single very low phase noise interferer 20 kHz away from the signal. Receiver LO phase noise is -120dBc/Hz at a 20kHz offset in the RX 250 Hz IF BW Determine the interferer level that results in 3dB more noise power from the detector This occurs when the noise introduced by Reciprocal Mixing equals the NF noise. Noise Floor–Phase 20 kHz-10log(BW)= -135dBm dBc/Hz-24dB = -39dBm Step 3: RMDR = Interferer Level - Noise Floor = -39dBm dBm = RMDR = 96 dB a good but not incredible RMDR at 20kHz offset. Reciprocal Mixing Dynamic Range is a hot new spec being prominently mentioned by various transceiver vendors. Let’s understand what this specification means. Using the same receiver and LO phase noise as was considered in the previous slide i.e. RX NF = 15dB, IF BW = 250 Hz, LO Phase Noise = -120 dBc/Hz at 20 kHz offset The RX Input Noise Floor remains equal to kTB+NF+10log(BW in Hz) = -135dBm We again place a low phase noise CW interferer 20 kHz away from the signal. Now we must determine the interferer level that causes the detector’s noise output to increase exactly 3 dB. At this point noise caused the RX NF and noise caused by reciprocal mixing are equal. The measurement is made with an AC milli-voltmeter at the RX audio output. This is the condition at which RMDR is defined. The level of the interferer is RX Input Noise Floor – Phase noise at 20 kHz offset – 10log(BW in Hz) = -135 dBm dBc/Hz -24 dB = -39 dBm. RMDR is defined as Interferer Level –RX Input Noise Floor = -39dBm dBm = 96dB. This figure is a good but not a stellar RMDR at 20 kHz offset from the LO frequency.

Receiver sensitivity summary
Noise figure, predetection bandwidth and total gain ideally set receiver sensitivity. Predetection bandwidth and the detection process must be matched to the signal characteristics. Spurious signals and mixer images generated in the receiver must be suppressed LO phase noise in the presence of strong interfering signals can severely degrade receiver sensitivity and usually sets MDS in real world situations. Read the slide Northern California DX Club ncdxc.org

Receiver linearity Why worry about linearity?
Strong signals close to a weak DX signal can saturate your receiver’s front end or its IF amplifiers dramatically reducing total gain. Pairs (or multiple) strong interferers can place unwanted intermodulation products on top of that all time new one you are trying to put in the log. These issues are compounded by the previously addressed reciprocal mixing problem. Now let’s change gears and focus on the upper end of the RX DR Read the slide Northern California DX Club ncdxc.org

Gain compression Gain (dB) Linear region Input Power Small Signal Gain
@ 1 dB Gain Compression Small Signal Gain SSG - 1 dB Nonlinear region Let’s examine gain compression, the simplest type of nonlinear behavior. All components and systems can only deliver a finite amount of output power. In the linear region the small signal gain is constant with increasing input power so that output power increases 1 dB for each dB of additional input power. If the system can no longer continue to deliver 1 dB more output power in response to each additional dB of input power its gain is starting to compress. When the gain is reduced by 1 dB, the power input for 1 dB gain compression is reached. If the input power continues to increase the saturation region is reached where power output can not increase with additional power input. Gain may fall below unity as the system saturates. Gain compression occurs in any nonlinear component such as an amplifier, mixer or complete RX. Gain compression is important because as the receiver begins to compress IM products are generated. Saturation region Receiver Input Signal Level (dBm) Northern California DX Club ncdxc.org

Northern California DX Club ncdxc.org
Intermodulation When 2 or more signals are presented to a nonlinear device, the results are harmonics of each signal and sum and difference products of the signals and their harmonics. These sum and difference products are called intermodulation products. What is Intermodulation and how are IM products generated? Again the nonlinear device could be an amplifier, mixer or a complete receiver. Only Signals at frequencies F1 and F2 are present at the device’s input At the device’s output we see the input signals at frequencies F1 and F2 Additionally we also see: Harmonics of F1 and F2 i.e. 2F1, 3F1, … and 2F2, 3F2 … Even harmonics have even indices 2, 4, 6 etc. Odd harmonics have odd indices 1, 3, 5 etc. Even order IM products such as F2+or-F1, 2F2+or-2F1, 4F2+or-2F1 … The sum of the indices of even order IM products are even Odd order IM products such as 2F2+or-F1, 3F2+or-2F1, 4F2+or-3F1 … The sum of the indices of odd order IM products are odd. The order of the IM product is given by the sum of its indices. The IM product 3F1-2F2 is 5th order. Power F1 F2 F1 F2 Nonlinear Device Even Odd Odd Even 2F F2 dc Freq

Intermodulation Odd order difference products (nF1-mF2) with n+m odd, cluster close to the original signals and can interfere with another weak close in signal. Even order sum products (nF1+mF2) with n+m even, can also cause interference. Usually the receiver’s preselect filter takes care of even order products. (Unless your neighbors are guys running legal limit amplifiers and big antennas) Read the slide Northern California DX Club ncdxc.org

Even order intermodulation
Interfering Signal Pair IM product = mF1 ± nF2 Product order is m+n (1F1+1F2), m=1 n=1, Order is 2nd F1 F2 7.10 MHz MHz W6YX W6XX Interference from Even Order IM Products. This problem is caused by extremely strong out of band signals. If thes signals or their harmonics reach the mixer they can produce interference. Here are W6YX and W6XX both operating on 40 meters at 7.10 MHz and 7.14 MHz respectively. They are both close by and running legal limit power. You are listening on 20 meters. Your RX pre-selection and image reject filters reject the strong 40 meter signals but not enough. They reach the mixer and generate the second order IM product F1 + F2. The second order IM products of their signals are F2-F1= 40 kHz and F2+F1=14.24 MHz. I am chasing the very weak A92BR in Bahrain at MHz, 3 KHz away. He is clobbered by W6YX and W6XX’s second order IM generated in my receiver’s mixer at MHz. Phase noise on W6YX and W6XX signals also can also reciprocally mix further obscuring A92BR. F1+F2= MHz IM2 Receiver IF Passband F2-F1= 0.04 MHz IM2 2F2-2F1= 0.08 MHz IM4 2F2+2F1= 28.48 MHz IM4 A92BR MHz Northern California DX Club ncdxc.org

Odd order intermodulation
Interfering Signal Pair F=F2-F1 = =0.1 MHz IM product = mF1 ± nF2 Product Order is m+n (3F1-2F2), m=3 n=2, Order is 5th F1 F2 14.1 MHz MHz W6YX W6XX 2F2-F1= 14.3 MHz IM3 2F1-F2= 14.0 MHz IM3 Receiver IF Passband Lets consider interference from Odd Order IM products. F1 and F2 pass unimpeded through the pre-selection and image reject filters and directly hit the mixer. The odd order difference products (n*F1-m*F2 and n*F2-m*F1) cluster close to the generating carriers F1 and F2. The third and fifth order IM products are shown. Here F1 is W6YX at MHz and F2 is W6XX at MHz at my RX input. They generate odd order difference products at: 2F2-F1 at 14.3MHz and 2F1-F2 at 14.0MHz at my receiver’s mixer input. The IM3 products bracket the W6YX and W6XX signals. Note the spacing of all signals and IM products are 100 kHz apart the same spacing as between F1 and F2 The 14.3MHz product clobbers DX0K in the Spratly Islands at MHz, 2 kHz away and right in my IF filter. DX0K MHz 3F1-2F2= 13.9 MHz IM5 3F2-2F1 =14.4 MHz IM5 F F F F F Northern California DX Club ncdxc.org

Intercept point IF Output Power (dBm) Intercept Point Fundamental
Signals The intercept point of a receiver is a term you hear tossed around a lot on the air. Let’s see what it means. It is a small signal phenomena occurring at relatively low input signal levels. Note that as the input signal increases 1 dB, the output power also increases 1 dB. The slope of the fundamental signal Pout vs Pin curve is one. 1dB out for each dB in. The slope of each IM product is simply its order. So third order IM products have a slope of 3dB for each dB of additional input power and 5th order IM products have a slope of 5dB for each additional dB of input power. In theory the plot of the fundamental signals and the output of all the various orders of IM meet at a single point called the Output Intercept Point. In reality this is not often exactly the case so we specify OIP2, OIP3, OIP5 etc. Thus OIP3 is where the 3rd order IM curve hits the extended fundamental (slope of one) curve. Fortunately the levels of the IM products are well below the signals that cause them. Still IM products can severely degrade receiver performance. If the Intercept point is referred to the receiver INPUT we talk about IIP3, IIP5 etc. OIP3 and IIP3 differ simply by gain. IIP3 + G = OIP3 Linear Region Slope=1 IM3 Slope=3 IM5 Slope=5 RF Input Power (dBm) Northern California DX Club ncdxc.org

Estimating IM level Power (dBm) Input Order Order Order Order
Intercept Point (dBm) +40 dBm Order Order Order Order  (dB) dB Signal Level (dBm) -13 dBm (P-1) (dB) dB This chart shows a simple way to estimate IMD levels. It is INPUT referred. +40 dBM is a superb input IIP3. The very best elite class transceivers just about achieve a +40 dBm input intercpt point. It is a goal to shoot for as mixer technology improves. P in this chart is the order of the relevant IM product. In this illustration P = 3. Here the input signals are both: -13 dBm. These are both exactly 60 dB over S9 The RX IIP3 is +40 dBm. Delta = IIP3 – Pin = +40 dBm dBm = 53 dB So the 3rd order IM are (P-1)Delta below the signals = (3-1)*53dB = 106 dB Thus the 3rd order product level is -13 dBm – 106dB = -119dBm Note that -119 dBm is only 2 dB greater than S1 with two 60dB over S9 input signals. Other examples are shown on the slide. You can see that 3rds and 5ts are usually all that matter. One can not ignore the even order IM. 7.1MHz +7.2MHz=14.3MHz. It is the way the ham bands were designed. We get to own most of our own dirty laundry! P Order IM Level (dBm) th -119 dBm Frequency (kHz) Northern California DX Club ncdxc.org

Spurious free dynamic range
Power (dBm) Input Intercept Point (dBm) +40 dBm Rcvr NF =15 dB, IF Filter BW = 3 kHz  =54.66 dB, 3rd Order (P=3) Signal Level (dBm) dBm SFDR = ( dBm) - (-124 dBm) = dB (Noise floor = IM3 level) SFDR is another useful way to compare receiver performance at both ends of the DR. SFDR is defined when IM products generated by the large signals, just pop up above the RX noise floor. SFDR is the difference between the large signal level and the noise floor. Again we use a elite class transceiver with an IIP3 of +40 dBm We are considering the third order IM as IM3 are the strongest close in IM products. So in this case P = 3. The RX has a 15 dB NF and a 3 kHz IF BW filter. First we calculate the RX noise floor = kTB + NF +10log(BW) = -174dBm +15dB + 35dB = dBm referred to the RX input. Next we calculate the input signal level that yields 3rd order IM products at the RX noise floor of -124 dBm dBm does not hardly move the S meter. Its < S1! Delta = (RX IIP3 – RX Noise Floor)/P = (+40dBm dBm)/3 = 54.7 dB The Input signal level = IIP3 – Delta = +40 dBm – 54.7 dB = dBm or S9+58 dB Now SFDR = Input signal level - 3rd order IM level (which is equal to the Noise Floor) = -14.7 dBm dBm = 109.3dB This is an outstanding SFDR. SFDR (P-1)= dB, 3rd Order (P=3) P Order IM Level (dBm) th -124 dBm Noise Floor = log 3000= -124 dBm Northern California DX Club ncdxc.org Frequency (kHz)

AGC function AGC reduces the gain of the receiver RF and IF amplifiers in the proper ratio to maintain sensitivity and SFDR in the face of rapidly changing signal levels (QSB). The analog or DSP detector suite (one for each mode) drives the AGC function. The AGC algorithm should also be optimized for each mode. Read the slide Northern California DX Club ncdxc.org

AGC function AGC rate must adapt to the mode in use and if possible to the QSB conditions. Fast attack to minimize pops and thumps Adaptive decay matching signal characteristics AGC holds the detector input level approximately constant as receiver input signal level varies. Modern DSP based AGC systems can offer vastly improved capability. Read the slide Northern California DX Club ncdxc.org

Blocking dynamic range
How large can a single CW interferer 20 KHz away from a weak signal be, before the desired signal’s detected level drops 1 dB? Blocking dynamic range is the difference in level between the weak signal and strong interferer when the receiver gain is compressed 1 dB. What happens as the interferer moves closer to the desired signal? How about a situation with many close in intereferers as in a pileup. One more important measure of RX linearitye is BDR. Read the slide Northern California DX Club ncdxc.org

Blocking dynamic range
Northern California DX Club ncdxc.org Blocking dynamic range Signal dBm Interferer -29 dBm Po dBm Gain to pre-det’n filter o/p (100dB) Blocking Dynamic Range = 71 dB (3 kHz) Offset Total Gain Interferer from Fo SSG Comp Level (kHz) (dB) (dB) (dBm) 1 2 3 4 5 7 9 11 13 15 100 99 82 50 49 47 -47 -59 -71 -83 -95 ? -1 -0.7 -0.3 AGC +20 +19.3 +17.7 -76 -88 -100 -112 Noise Interferer -29dBm Gain to roofing filter o/p (50dB) Lets look carefully at BDR with some numbers. Here the gain applied to the weak signal is 100 dB because the signal is centered in the pre detection filter’s BW. That gain is reduced if the interferer compresses the RX. With the interferer 3 kHz away from the signal, the receiver SSG seen by the interferer is only 50 dB. It sees only the gain up to the roofing filter and not the total IF gain ahead of the pre-detection filter of 100 dB. Data taken on the radio shows that the output 1 dB GCP is +20 dBm at this point in the receiver. At 1 dB gain compression the small signal gain seen by the interferer is reduced from 50dB to 49dB. Thus a -29 dBm interferer causes the receiver to reach its output 1 dB gain compression point of +20 dBm. At the 1 dB GCP the gain seen by the weak signal is also reduced 1 dB. This is the level at which BDR is measured. BDR is the difference between the level of the interferer causing 1 dB GC and the weak signal. Measured at the RX input BDR = Pin(-1dB compression) – Psignal = -29 dBm dBm = 71 dB a quite poor result This slide illustrates another reason why the gain ahead of the mixer should be as low as possible. 50 dB is at least 35 dB too much RF gain ahead of the roofing filter!! The interferer will always drive the IF to saturation for interferers within in the pre-detection filter’s BW where the total small signal gain is 100 dB. BDR Signal -100dBm Fo-6 kHz Fo-3 kHz Fo+3 kHz Fo+6 kHz Fo

Receiver Linearity Summary
When the receiver’s gain is compressed by a single or multiple large signals, harmonics or IM products are generated. IM products in the receiver’s IF passband can bury the weak desired signal you are trying to copy. A receiver Input Intercept Point (IIP3) of at least +30dBm will greatly reduce the level of the generated IM products and result in an acceptable Spurious Free Dynamic Range. A receiver with a decent IIP3 will likely have a high input power at 1 dB gain compression, resulting in a larger Blocking Dynamic Range. Read Slide Remember it is close in IM performance that matters. Choose IF filters for each mode you plan to use. Look for third order IM specified 2 kHz from the received signal not 20 kHz. Two kHz is more relevant to those who like to contest using CW or find themselves in CW DX pileups. 20 kHz spacing is useless for all modes. The manufacturers sometimes use specsmanship to hide receiver flaws. Don’t fall into that trap.

Receiver selectivity Selectivity is determined by the final IF filter
The filter must be matched to the signal characteristics. Crystal filters are good but they are expensive and can suffer from ringing and delay distortion. DSP based filters are generated in code and can be designed for a wide variety of bandwidths, and shape factors. Thus additional filters are almost free. Best of all DSP filters can greatly reduce ringing. Read the slide Analog filter technology can not realize many DSP filter characteristics! Northern California DX Club ncdxc.org

Receiver selectivity An excellent receiver has at least 2 crystal roofing filters wide enough to avoid ringing, but narrow enough to reject close in interferers and IM products. For example: 3 kHz for SSB, 1 kHz for CW These would be followed by a choice of DSP filters optimum for various conditions. For example: 2.8, 2.4 and 1.8 kHz for SSB, 500 and 250 Hz for CW Read the slide Northern California DX Club ncdxc.org

Receiver selectivity The set of DSP filters should allow for various operating conditions such as comfortable local rag chewing and intense contest or DX situations. DSP based filter suites should contain an adaptive notch filter to reduce CW beat notes in the IF pass band (Tuner uppers) A variable IF band pass filter with selectable center frequency and bandwidth can also be very useful. Read the slide You really want both these features! Northern California DX Club ncdxc.org

Receiver frequency stability
All modern radios employ synthesized LOs. Make sure tuning resolution meets your needs Verify that the synthesizer reference source is stable enough for the digital modes A <0.5 ppm TCXO is often a good option to invest in. A 10 MHz reference output is also a useful feature Most important .... How’s the phase noise? Read the slide Northern California DX Club ncdxc.org

DSP based superhetrodyne receiver
Northern California DX Club ncdxc.org DSP based superhetrodyne receiver The block diagram shows a single conversion superhetrodyne receiver with an IF DSP handling the pre-detection filtering, notch filtering, pass band tuning, Detection and AGC tasks. The DSP is moving toward the antenna in modern radios. Today there are radios available With the a?d converter abd DSP immediately following the pre-selection filter and RF amp. Antenna Pre- Select Filter AGC Line AGC System Today’s transceivers all use DSP technology to handle part or all of the signal processing chain. Today there are transceivers where the DSP based circuits have moved all the way to the output of RF amplifier.The pre-selection filter and RF amp are the only bits of analog circuitry left in the RF chain. These transceivers are called a software defined radios or SDRs. In the RX an analog to digital converter follows the RF amplifier and all processing is done digitally after that. The Elecraft KX3, Flex Radio 6000 series and ICOM 7300 and others are true SDR radios. These radios offer good to great performance but expect even greater improvements to come in SDR technology in the rather near future. It will be the wave of the future because it is better and cheaper! Mixer RF Amp Image Reject Filter IF Roofing Filter IF Amp IF Matched Signal Filter Detector BB Amp Analog DSP Local Oscillator

Conclusion My Priorities
Close in (2 kHz) phase noise Phase noise usually sets receiver sensitivity, not noise figure. If you can’t hear him in the pileup, you can’t work him! Close in (2 kHz) input intercept. You still can’t hear him if he is wiped out by IM3 from strong stations. Close in (2 kHz) blocking dynamic range. Analysis has convinced me that long before BDR becomes an issue, reciprocal mixing has buried the new one I am trying to hear. Read the slide Northern California DX Club ncdxc.org

Northern California DX Club ncdxc.org
Conclusion Rigs with great dual receivers, terrific specs and good bang for the buck are very important but Don’t neglect front panel ergonomics, an intuitive user interface and well thought out menus and control functions. You will most likely using this radio for many years. Get the rig that is right for you! Read the slide

Are there any questions?
Thanks for coming. See you in the pileups! And now I’ll take your questions. Click on your microphone button so that it turns from red to black to ask a question. Try not to generate a “pileup” as WebEx does not have a great SFDR. Please click your mike off i.e. back to red when you are done. Thanks for coming. I hope that this class was useful. If additional questions occur to you later you can me at k6yp at arrl.net.

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