1. Chelmsford Amateur Radio Society Advanced Course (4) Receivers Part-2 – Receiver Architecture

2. Receiver Architecture
Receiver architecture – block-level arrangements
Superhet receivers
Double-conversion superhet receivers
Mixers
IF frequencies
Image frequencies
Use of high and low IF frequencies
Local Oscillators in transceivers

3. Superhet Receivers
The Superhet (super-heterodyne) receiver converts the input RF frequency to another IF frequency
All today’s broadcast receivers are superhets. LO
Mixer
AGC RF
amp IF AF
Detector
145MHz
10.7MHz
300Hz-3kHz
audio
134.3MHz

4. Superhet Receivers
Tunable RF input frequency is converted to a single IF frequency by a tunable Local Oscillator (LO)
Multiple tuned circuits are used to get selectivity in the IF
This is easier to do for a fixed-frequency IF
We can also use filters using crystals that can’t be tuned
At high RF frequencies it is impossible to make sufficiently narrow filters for narrow band signals
…but we can do it at a lower IF
Amplifiers that work over wide frequency ranges (and with AGC) are difficult to make
But relatively simple for one fixed IF frequency

5. Choice of IF Frequency
Practical filter bandwidths depend on Q of circuits
Practical bandwidths for crystal filters
Depends on temperature drift, and initial accuracy
Hand-tuned crystal filters narrower but larger and cost more
Ceramic filters also often used
Standard frequencies are preferred 455kHz, 1.4MHz, 10.7MHz, 21.4MHz, 45MHz, 70MHz
Standard crystal and ceramic filters are low cost
Oddball frequencies & bandwidths much more expensive
For an IF of 470kHz, and BW 6kHz,
Q=470/6 = 78. Realistic with LC circuits

6. ~ Mixer as a Converter Mixer 145MHz–123.6MHz=21.4MHz IF frequency
Mixer may be used a frequency converter
Changes the selected RF frequency to the IF frequency using a tunable LO signal.
Mixers have spurious responses – image frequency, half the RF…
LO can be above or below the RF
IF can be above or below the RF
Mixer
145MHz–123.6MHz=21.4MHz
IF frequency
RF 145MHz ~
LO 123.6MHz
Image frequency is
123.6MHz-21.4MHz=102.2MHz

7. Image Frequencies
Image is normally 2x the IF away from the RF frequency
On the same side as the local oscillator
Image has a band of frequencies that corresponds to tuning range
Frequency MHz
Amplitude IF
LO frequency
RF frequency
Image frequency
Front-end RF filter may
look like this

8. Choice of IF Frequency Image is 2x IF away from the wanted frequency
Larger IF frequency makes suppression of image easier
Too low an IF and the RF input filters are too difficult
LO radiation is also a problem if it leaks up the antenna
Tuning range of receiver cannot cross the IF
Hence HF receivers often have a very high 1st IF, >60MHz
Realistic RF filtering usually forces the choice of 1st IF.
This may not be good for selectivity!
Hence a second lower IF is often used – DUAL CONVERSION
High 1st IF gives good image rejection
Low 2nd IF gives good selectivity
NBFM (2.5kHz dev) demodulation also requires a low IF, 455kHz
For WBFM (75kHz dev) it can be greater, 10.7MHz

9. Dual Conversion Superhet
Block diagram
Mix 1
IF1
AGC
Mix 2
IF2
Filter
SSB CW
CIO AF
amp AM FM
LO2
LO1

10. RF Input Filter May be one BPF covering band of operation
eg HF band, 2m band
Low cost
May be several switched filters for specific amateur bands
For HF general coverage, may be a set covering sub-octave bands
Generally 6 or more required
Fully tunable filters (preselector)
Usually expensive
To run on a site with multiple transceivers better filters are required

11. Transceiver Block Diagram
Shared Local oscillators in transceivers
In transceivers, some parts are frequently shared
Frequency synthesisers, local oscillators, IF crystal filters PA
IF2 AF
amp
Demod
LO2
LO1 RF
Mic
Mix 1
IF1
Mix 2
CIO
Mod