Hello
Lately I've been trying to make superheterodine receiver and i'm having some problems with the IF chain stability.
the receiver's first frequency block is:
- input cascode amplifier, gain controlled by split emitor resistor - working
- 3db 50ohm attenuator pad
- SBL-1X mixer - working
- 10.7MHz band-pass tunable diplexer, 50 ohm terminated - working
- BC547 untuned amplifier, gain controlled by split emitter resistor - working
- 3dB 200ohm attenuator
- 10.69850 2KHz crystal filter
- 3dB 200ohm attenuator
This entire block is rock stable and because of the good filter, very narrow band.
I've tested it until overload and will not oscillate / do anything strange
The IF chain was build using 3 tuned stages that provided some decent gain and an untuned stage connected to the mixer via 3db pad
Each stage followed this schematic:
L1 / L2 / L3 where arranged in such a way that it presents ~ 5000 ohms in the transistor's collector.
Those are not the actual values used on the board.
Also, the Q is quite high - 10-20kHz bandwidth.
The first problem: when connecting the output of stage 1 (L3 ) to the input of stage 2 ( C3 ) the entire thing begins to oscillate at 100-150MHz.
The oscillation is strong enough to be seen just by keeping the scope's probe next to C2.
The same behavior was apparent with only one stage connected to the signal generator: when I pushed the RF-OFF key the entire
thing begins oscillating at almost 200MHz.
The generator shorts it's output to ground when the RF output is disabled.
The only idea i had was that the thing would become something close to:
( the same type of oscillator is used by the frequency generator )
I could stop it oscillating by adding a resistor in series with C3 -> i could get 3 ~ stable IF amplifiers to work stand-alone.
The next step was too add a buffer amplifier( the same configuration as the ones above) that is feeding the second mixer via a 3db 50ohm pad.
The problems started when i begin to apply the second IF oscillator signal: the IF chain restarts oscillating almost on the 2'nd IF frequency and acts as a mixer by itself
Because of that the 2nd mixer's output is just junk - it's mixing 3-4 very close frequencies.
If i remove the 2nd IF oscillator then the IF chain either becomes stable or keeps oscillating at 10.7MHz.
My question: if I have that narrow band filter, is there any need to have a narrow band IF ?
Can I get away with 3-4 untuned amplifier stages that will also allow for AGC ( by changing the bias point )
Or should i keep 1 tuned stage just before the 2nd mixer to clean up added noise ?
The entire board is build dead-bug style on double sided copper clad, with the mixers and filter on the back, pins passing trough the PCB
Also, any hints on how to make a DIY diode ring mixer that will work on 10.7MHz ?
It seems that some of the mixers I have are junk and I can't properly test them.
I have some T50-2, FT37-43 and FT37-61 cores and some small signal schottky diodes.
I've seen W2AEW's clip
and notes
http://www.qsl.net/w/w2aew//youtube/Diode_ring_mixer_operation.pdf but I don't know how to choose the number of turns for the transformers. 20 turns on T50-2 cores should work ?