Instability arises from two causes: feedback via the positive supply rail and feedback via the negative supply rail. The positive supply rail is readily fixed with RC decoupling and voltage regulation. The negative supply feedback is prevented with grounding and layout. Let's look at each audio stage and see what was done.
Audio Amp:
Since I'm running several watts of audio output we have to watch feedback carefully. The circuit board puts several capacitors close to the audio amplifier chip. But that is only the start and active regulation is used elsewhere to reduce feedback via the positive supply rail. Now the subtlety of layout takes over. The power is connected close to the audio amplifier chip. Just like a "star ground", the remainder of the receiver draws power from the far end of the track on which the capacitors sit. Don't think of the track as being 13.8 volts. Instead, consider it as high current. We want to power the remainder of the receiver from a point that has the lowest current. The point I use still has to be decoupled further and I run the rest of the receiver from a low drop out regulator of 12 volts.The same concept applies to the negative rail. Current in the negative rail is what gives rise to a voltage differential between the point the negative supply connects to the PCB, or local ground, and any other point connected to the local ground via a trace. To minimise the current flowing between local ground and anything other than the audio amplifier ground, I break the ground into two major sections.
The first is just around the audio chip and power connector on the bottom sides of the PCB. The attach image tries to explain this. The green tracks are ground on the bottom layer. The only parts sharing this ground plane are the audio amp, the supply capacitors and the power connection. The top ground plane only connect to this green layer via the power supply pin. This prevents any circulating current from flowing through the ground plane on the top side of the board.
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| Ground Traces for Audio Amp |
With a grounding scheme that hopefully minimises feedback on the negative rail we can look at the other audio stages.
Audio Pre-amp:
The first audio stage uses a discrete bipolar amplifier. In practice I never found an op amp in my junk box to be as quiet as this circuit which I first saw in the August 1992 edition of QST in an article by Rick Campbell. My implementation is a bit casual in comparison. I simply used what I had to hand that was close to Rick's circuit. I have found the active decoupling circuit on the positive rail sufficient when taken from the regulated 12 volt line.Before I used a regulated 12 volts I had instability problems, but that could have been also partly due to layout.Audio Low Pass Filter:
The output of the audio pre-amp drives a low noise op-amp for further gain before an 8 pole low pass filter. I have used both switch capacitor filters and active filters based on op-amps. I don't have a strong preference either way. Both approaches work really well but I have stuck with the op amp approach this time.In the past I used a rail splitter to bias the op amps. The drawback is any noise or feedback on the positive and negative rails finds itself at the input connected to the rail splitter. It will be attenuated, but is still present. This time I took the regulated 12 volts and inverted it to negative 12 volts. I was hoping for a magic bullet but I'm not sure if this extra complexity made much difference. But since the receiver is working I'll follow the wise saying "If it ain't broke, don't fix it!"
Other Grounding Issues:
One issue I had when building this receiver took some time to understand. When completed it was not as sensitive as I expected. The clue to fixing this arose when I disconnected the local oscillator and the loudspeaker noise increased. This occurred even when I removed the inductor between the audio pre-amp and the product detector.With the oscilloscope probe grounded at the negative power supply pin I poked around and found the 74HC04 squarer I was using was generating lot's of hash with no local oscillator input. Forcing the input pin low with a screwdriver blade eliminated the noise and hash. As the circuit to be posted later showed there was heavy decoupling on the power supply to this part. Holding a tantalum capacitor across the existing decoupling capacitors next to this IC made no difference. Looking at the circuit board revealed the cause:
As current flowed from the ground pin of the IC it was in part flowing past the grounding points of the pre-amp (Shown with * in the diagram above). Since current flowing through a finite resistance gives rise to a voltage this was injecting noise into the pre-amp via the grounded components. The fix was a via to the top layer at the ground pin and for good measure a via at each group of decoupling capacitors on the positive supply to this part. (Shown with # in the diagram above). The result was an increase of sensitivity of around 15dB!
Summary
Curing audio instability and feedback is a dark art. I am only just starting to get my head around this issue so any useful advice is really appreciated. One thing I have learned is that connecting the ground lead of your cro to the negative supply rail of your power supply where it connects to the pcb then poking around is very illuminating.Regards
Richard VK6TT
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