1. Intermediate Course (4) Receivers Karl Davies East Kent Radio Society EKRS 1

2. Receivers Summary  Block diagrams of receivers  The Superhet receiver  Detection of AM, FM, and SSB  How a Diode AM Detector works  Use of BFO and CIO  AGC 2

3. Crystal Receiver Block diagram  Single tuned circuit; poor selectivity  No gain; poor sensitivity  Requires large aerial and earth; only receives strong signals Tuned circuit Diode detector Earphone 3

4. Crystal Receiver Circuit L1, C1 - Tuned circuit – selects signal D1 – Detector diode – demodulates C3, R1 – Low-pass filter for audio Antenna D1 AA119 R1 100k C2 500pF C3 100pF L1 Coil X1 C1 100pF RF Earth Crystal Earphone 4

5. TRF Receiver (Tuned Radio Frequency) Block diagram  RF amplifier gain increases sensitivity  One or more tuned circuits  All the gain is at one frequency – feedback is a problem  AF amplifier provides more power for loudspeakers RF amplifier Demodulator AF amplifier BFO 5

6. Superhet Receiver Block diagram  Mixer changes variable RF frequency to fixed IF frequency  IF amplifier provides selectivity with several tuned circuits  Fixed IF can use non-tunable crystal or ceramic filters Local oscillator Mixer AF amplifier IF amplifier Demodulator BFO 6

7. Mixer as a Converter Mixer may be used as 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

8. AM Envelope Detection Demodulation of a modulated audio signal Detector output follows envelope of RF  Otherwise known as an “envelope detector” Demodulated Audio Modulated RF Envelope 8

9. Diode Detector Circuit Diode D1 rectifies AC into DC C1/R1 is a lowpass filter - filters out the RF D1 conducts on positive half-cycle only Output Voltage Time Input D1 R1 100k C3 100pF RF Input Demodulated AF Output 9

10. Diode Detector Operation Volts and Current in the diode detector (SPICE Simulation) Diode Voltage / V -2 0 1 2 200 0 0 Time/µSecs 0.511.522.533.54 Diode Current / µA 400 600 800 D1 anode input D1 cathode output D1 current Diode conducts on peaks only AnodeCathode 10

11. CW Demodulation - BFO BFO = Beat Frequency Oscillator  Slightly offset BFO added to carrier to generate the “beat note” + Carrier WaveBFO Envelope Detector Beat Note …the origins of the “BFO” ! 11

12. SSB Demodulation SSB filter selects only the wanted sideband Product detector mixes to baseband CIO is at the frequency where carrier would have been Product detector is a balanced mixer AF Amp Sideband Filter IF Amp Product Detector IF CIO = Carrier Insertion Oscillator CIO ~ 12

13. USB Demodulation SSB demodulation is essentially mixing to baseband IF Filter Mixer Amplitude Frequency MHz Lower Sideband Upper Sideband Amplitude Frequency Upper Sideband Amplitude Frequency MHz Upper Sideband Unwanted sideband may contain noise and other signals Mixing with the carrier frequency is product detection 13

14. LSB Demodulation SSB demodulation is essentially mixing to baseband Typically, IF filter is not moved; the local oscillators are offset. When mixed down, the LSB spectrum becomes inverted. IF Filter Mixer Amplitude Frequency MHz Lower Sideband Upper Sideband Amplitude Frequency Lower Sideband Amplitude Frequency MHz Lower Sideband 14

15. Demodulation Summary CIO = Carrier Insertion Oscillator  A fixed local oscillator used to demodulate SSB  It reinserts the carrier that was removed in the transmitter  Wanted sideband is directly translated directly to Audio BFO = Beat Frequency Oscillator  A slightly variable local oscillator used to demodulate CW  CW demodulated by envelope detector BFO/Detectors can resolve SSB. A product detector can resolve CW Two Issues  Input Signal Levels can vary greatly  Detection methods described so far are for Amplitude based modulations, not FM 15

16. AGC AGC = Automatic Gain Control AGC adjusts the gain to keep carrier level constant Signal level varies widely, but audio volume stays constant Mixer IF Amp Demod RF Amp AF Amp LO AGC Circuit 16

17. FM Receiver Uses FM Discriminator to detect small frequency deviations FM receivers use IF limiter amplifiers; no AGC needed Squelch detects high-frequency noise and gates audio Local Oscillator Mixer AF amplifier IF amplifier Discriminator Squelch circuit 17