Chelmsford Amateur Radio Society Advanced Course (4) Receivers

Part-2 – Receiver Architecture

Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 1

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

Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 2

Superhet Receivers

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The Superhet (super-heterodyne) receiver converts the input RF frequency to another IF frequency

– All today’s broadcast receivers are superhets.

AGC 145MHz 10.7MHz

RF amp 134.3MHz

Mixer LO IF amp IF amp Detector AF amp 10.7MHz

10.7MHz

300Hz-3kHz audio

Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 3

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 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 4

Choice of IF Frequency

• • • •

Practical filter bandwidths depend on Q of circuits

For an IF of 470kHz, and BW 6kHz, Q=470/6 = 78. Realistic with LC 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 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 5

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

~

RF 145MHz

Mixer

~

LO 123.6MHz

145MHz –123.6MHz=21.4MHz

IF frequency

Image frequency is 123.6MHz-21.4MHz=102.2MHz

Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 6

Image Frequencies

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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 IF IF Front-end RF filter may look like this Frequency MHz Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 7

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 1 st IF, >60MHz

Realistic RF filtering usually forces the choice of 1 st

– This may not be good for selectivity!

Hence a second lower IF is often used – IF.

DUAL CONVERSION

– High 1 st – Low 2 nd IF gives good image rejection 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

Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 8

Dual Conversion Superhet

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Block diagram AGC Filter Mix 1 IF1 Mix 2 IF2 LO1

Chelmsford Amateur Radio Society Advanced Licence Course

LO2

Anthony Martin M1FDE

AM SSB CW CIO AF amp FM

Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 9

RF Input Filter

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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

Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 10

Transceiver Block Diagram

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Shared Local oscillators in transceivers

– In transceivers, some parts are frequently shared – Frequency synthesisers, local oscillators, IF crystal filters

RF amp Mix 1 IF1 Mix 2 IF2 Demod AF amp LO1 LO2 CIO PA Mix 1 IF1 Mix 2 IF2

Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE

Mod Mic amp

Slide Set 10: v1.0, 4-Nov-2004 (4) Receiver Architecture 11