Chelmsford Amateur Radio Society
Advanced Course
Receivers
Part-3 – Receiver Circuits
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
1
Receiver Circuits
Common circuits in receivers:• IF amplifiers
• IF transformers
• LC coupled pairs
• AGC amplifiers
• Obtaining the AGC voltage
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
2
Colpitts Oscillator
• The LO may be provided by a tuneable LC or fixed crystal oscillator
• Often followed by a buffer amplifier
– To increase mixer drive level
– To prevent feedback from strong input signals affecting the oscillator
R1
6.8k
+9V
+9V
R3
1.5k
R3
1.5k
R1
100k
Out
TR1
TR1
C1
R2
1.5k
100n
C
XL1
Crystal
Chelmsford Amateur Radio Society
Advanced Licence Course
C2
C1
R2
68k
L
R4
1.0k
Out
5-35pF
Anthony Martin M1FDE
C2
R4
1.0K
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
3
IF Amplifier
+9V
IFT2
• Tuned circuits provide
selectivity
• Transistor provides gain
• AGC line varies the bias
current to control the gain
Next
stage
Previous stage
IFT1
• Used in multiple stages –
higher gain, more selectivity
TR1
100n
100n
AGC line
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
4
Coupled-Pairs
• Coupled pairs can be used to provide frequency selectivity in IF stages
– Used to be common in Valve IFs (fewer stages)
but transistor IF transformers are single-tuned
Critical coupling
Primary
Secondary
Overcoupled
C
Undercoupled
C
L
L
Mutual inductance coupling
Chelmsford Amateur Radio Society
Advanced Licence Course
Frequency Response
Anthony Martin M1FDE
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
5
Coupled-Pairs
• Inputs and outputs can be inductive-taps or capacitive taps
• Coupling may be by physical L or C as well as mutual inductance
• Usually seen in various forms in VHF transceiver frontends
Coupling
Output
Input
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
6
Shape Factor
• Shape Factor is a measure of rejection for large unwanted signals
• Shape Factor can be related to the number of tuned circuits in the IF
0dB
• Shape Factor is the Ratio
of the 6dB BW to 60dB BW
–6dB
–6dB BW
–20dB
• Typical shape factors are
about 1.5 to 2.5
–40dB
–60dB
–80dB
–15kHz –10kHz
• Crystal filters may be used
for shape factors narrower
than 1.5
–60dB BW
–5kHz
Chelmsford Amateur Radio Society
Advanced Licence Course
0kHz
+5kHz +10kHz +15kHz
Anthony Martin M1FDE
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
7
Generating AGC Voltage
• Can be achieved by a simple diode detector
+9V
IFT4
Detected AGC Voltage
Last IF Amp
TR6
C2
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
R2
D1
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
8
AGC Issues
• The Detector “sense” must match the IF amplifiers
– Decrease GAIN with increasing SIGNAL.
• AM detectors are fine for AM or other modulation with a carrier
• SSB has no carrier. No signal is present between words so more
complex arrangements are used:– Fast attack, slow decay
– Fixed “hold” time
– Manual AGC controls may be provided
• AGC may drive a meter for S-level
• When using a separate Tx and Rx, your own Tx will overload the Rx:– Will take a long time for the Receiver AGC to recover
– Can use a fast AGC, or implement a “receiver de-sense” line
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 11: v1.4, 23-Dec-2012
(3) Receivers Circuits
9