Prevent frequency counter noise from affecting VFO signal

Hi, in my "Super VFO" **broken link removed** I have added this frequency counter **broken link removed** connected it directly at the output.

As it is normal, noise comes out of it and affects the oscillator tone.

What has to be done so that the frequency counter has no effect in the oscillator tone?

1. More buffers with attenuation in between them?
2. Other...?

What do you mean it 'affects the tone'? Wideband noise? Amplitude variation? Frequency variation? Ringing? A better input buffer sure wouldn't hurt.

barry said:

What do you mean it 'affects the tone'? Wideband noise? Amplitude variation? Frequency variation? Ringing? A better input buffer sure wouldn't hurt.

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The tone of the demodulated carrier in a nearby receiver is not clean. I do not know if it is frequency or amplitude variations.
I can also hear the GATE clicks as the MCU samples the signal.

I suspect the problem isn't that the counter is injecting to the VFO but that you are hearing a combination of signals from both on the receiver. Certainly the counter will radiate some interference, especially if you have long connecting wires but nothing should be leaving the input connection. Also check you have a good low-impedance ground connection between the VFO and counter so you are not observing a voltage drop across the impedance of the output ground connection.

Brian.

betwixt said:

I suspect the problem isn't that the counter is injecting to the VFO but that you are hearing a combination of signals from both on the receiver. Certainly the counter will radiate some interference, especially if you have long connecting wires but nothing should be leaving the input connection. Also check you have a good low-impedance ground connection between the VFO and counter so you are not observing a voltage drop across the impedance of the output ground connection.

Brian.

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This is an unshielded bare board. I have read in other similar counter projects that the counters must be shielded.
Now that I am thinking it, because the ALC loop of the VFO is fed at the output, maybe the counter somehow affects the level (AM modulates).
Any ideas?

First thing that I would recommend (if is not done already) is to supply the frequency counter circuit from a totally separate power supply.

It shouldn't produce any signal from the counter input pin but yes, in theory any unwanted signal at the output could enter the amplitude feedback path. However, the time constant of the feedback is quite long so I would think it very unlikely that audible modulation changes would enter through that route. My suggestion would be to first screen the entire counter and ensure good spacing between any wiring (including the display wires) from any VFO components. Also add a small resistor (100 Ohms max) in series with C1 and use a screened cable to couple the VFO output to it. You might find some improvement if you add an RF choke in series with the positive supply line to the counter but make sure it is fitted inside the shield.

Brian.

i would use a cheap silicon amplifier chip and an attenuator pad to isolate the input of the counter. Some ICs are designed for a very high reverse isolation

biff44 said:

i would use a cheap silicon amplifier chip and an attenuator pad to isolate the input of the counter. Some ICs are designed for a very high reverse isolation

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I have used a two stage transistor amplifier instead of one. It becomes more sensitive. I think I cannot distinguish any difference now.