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

1
Chelmsford Amateur Radio Society Advanced
Course(4) ReceiversPart-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 todays 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 cant
  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, Q470/6 78.
Realistic with LC circuits
6
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-21.4MHz102.2MHz
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

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,
  gt60MHz
• 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

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