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

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

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

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

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

~ 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

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

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

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

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

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