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

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

2. 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 Receiver Architecture

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

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 145MHz–123.6MHz=21.4MHz IF frequency RF 145MHz ~ LO 123.6MHz Mixer as a Converter • 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 Image frequency is 123.6MHz-21.4MHz=102.2MHz

7. IF IF Image frequency LO frequency Front-end RF filter may look like this RF frequency Amplitude Frequency MHz 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

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

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