Question-Unsteady tone/output

[Anonymous]

Hi all,
I have a question and wanted to see if any of you have run into this before. Maybe someone has an idea of what could be causing my problem. I wired together a ciruit. I'm using pretty standard PI circuits. Out of the integrator the signal feeds another (TL072) op amp with a gain and threshold control. It is directly coupled through a 10k resistor. The output feeds a VCO circuit. The tone fluxuates and I'm not sure what is making it so unsteady. Looking at the output of the intergrator, it would appear that the signal level is pretty steady but looking at the output of the following stage, the voltage does fluxuate confirming what I hear from the VCO. Adjusting the gain lower doesn't seem to eliminate the problem. Other than this, everything seems to be ok. I thought that I might be picking up noise from the coil but I disconnected it and the problem persists. Has anyone had a problem like this? Any suggestions are appreciated.
Thanks,
FJ

 

[Anonymous]

Hi FJ,
Yes, the problem you are experiencing is not that uncommon. The solution or solutions can be difficult to determine without some experimenting and knowing just how the circuit is designed.
First, assuming you are using a mono type coil, then disconnecting the coil doesn't completely disconnect the xmit from the receive. Some noise will still get through via the damping resistor and possibly noise generated or capacitvely coupled by the FET.
I found this noise to be quite noticeable if the gain is significant.
Now, you do not mention just how much gain you have in the amp after the integrator but if it is significant, then you will not be able to readily see the noise at the integrator, yet see it at the output of the next stage. So, what you are experiencing is quite possible there also.
I would make sure that the power supplies are rock solid. Any noise there will be reflected in the output.
Also, if the gain is too much in the second stage, then, depending upon just how the VCO is built, the overall sensitivity might just be too much allowing noise to cause an erratic output. In other words, the VCO might allow too small of a signal to cause a significant audio tone change.
I would make sure I could adjust the pulse rate also. Sometimes just altering this rate a little will greatly reduce the noise one experiences. The key is to shift the rate so noise signals are not sychronized with the pulse rate.
Things like TV's, computers, lighting, and other electrical devices are great noise generators and the signals from them can extend for quite some distance. High impedance parts become little antennas as well as noise generators themselves. So, the noise doesn't necessarily have to get into the circuitry through the coil.
Finally, PI's are noisy by nature, so any reduction in filtering in the integrator stage and in the next stage could easily cause what you are experiencing. You might try increasing the filtering in these stages. I found that cutting the capacitance in half at my integrator caused a significant increase in noise. So, I went back to the original value. Actually, a little large cap helped but slowed things down too much. In other words, a little experimenting will probably be necessary.
I hope this helps.
Reg

 

[Anonymous]

Thanks Reg, I see all these detectors out there and I wonder if it's just me but it sounds like I'm not alone <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt=""> Some very helpful suggestions. Since I posted, I reprogrammed the processor to stop pulsing and the output was still unstable. I looked at the output of the final amplifier and could see a change of about 2, 3, or even 5mv occationally on the scope which would coincide with what I'm hearing. I then realized that I may have such a minute change out of the integrator that I can't see directly out of that stage but see and hear out of the next stage. I then reprogrammed the processor and stopped pulsing the gates and the output was solid. I re-enabled the transmit pulse and it was still rock solid output. At this point I don't know if it is one of the problems you mention or if my code is not exactly precise in the timing and causing an ever so slight change out of the integrator. I guess somemore experimentation is in order. BTW, if I remember correctly, I have about 10mv of switching noise out of the charge pump. Do you think this is too much? I thought that was pretty good. Thanks for your help.
FJ

 

[Anonymous]

Hi FJ,
If you are using the charge pump output as your + supply directly, I would say, yes, the noise level could be the cause or part of the problem.
When I was working on a PI that used a charge pump, I found I needed to regulate the output to assure a good solid + supply voltage.
It sounds like you might be working on a circuit similar to the Magenta PI. They use a circuit similar to the one you described. If it is, I would suspect that the LM 318 is a little noisy also.
Reg

 

[Anonymous]

This is a composite of circuits published on the net and some of my own ideas. I am using an LM7662 to generate the +5V rail. I'm using the NE5534 for the front end. I'm feeding -12V into the LM7662 to generate +12V and then regulating it down with a 7805. There are 2 filter caps on the input and on the output. Also using 2 larger than recommended caps on the LM7662 as well as all electrolytics are low ESR caps. Any other ideas to reduce the noise? I didn't realize 10mv switching noise was that much.
Thanks for the help,
FJ

 

[Anonymous]

Hi FJ,
It sounds like you have already tried several things to try to determine the source of the noise and well as reduce it.
I went back and reread what you posted earlier to try to come up with some different ideas.
I am assuming that when you reprogrammed the micro to pulse again and the output was solid, you did not activate the gating to the diff amp at the same time. Is that correct?
If the noise stops when you cease gating the sampling FETs, then it points to the front end picking up noise.
If you activated both the pulse and the gating and the problem went away, I am wondering if you changed the pulse rate. If so, then you might have changed the relationship between either the external noise (lights, tv, etc), or you changed any possible interaction between the 7662 and the sampling.
I am not sure if the noise is coming from the 7662 or not, but it would be interesting to see if you were to change the switching speed of the 7882, if the noise would change. If you keep the pulse rate the same then the relationship between the external noise is a constant so any change in noise output that occurs with a change in the 7662 switching rate should point to the interaction of the pulse rate and the converter. At least, I think it should.
Just changing the pulse rate instead does change the relationship between the external noise, so I would expect some changes there.
If the noise changes when you just adjust the 7662 repetition rate, I would seriously consider trying to sync the 7662 to the pulse and or sampling. If nothing else, I would disable or try to sync the 7662 during the pulse time and sampling.
Anyway, it is just another thought I had. Hope it helps.
Reg

 

[Anonymous]

Hi FJ,
I am getting tired. The last paragraph in my previous post doesn't make any sense. What I was trying to say was, if altering the 7662 switching rate seems to have an effect on the noise level, I would either sync the pulse and sampling and the 7662 so the 7662 was not switching during the critical times like just before or during the decay or during the sampling. I might try disabling the 7662 during both the pulse and sample if the time isn't too long.
If I had a real long pulse on time, I might switch during it, but make sure the 7662 switching was complete several usecs before the end of the pulse on and not switching near or through the sampling time.
I forgot to ask, do you have a means to vary the pulse rate? If so, does changing the rate seem to help? I know it should with some external noises.
I hope this time it makes a little more sense.
Reg

 

[Anonymous]

Hi Reg and FJ,
Another area to look at is the VCO. I have encountered interference from this source if the grounding and decoupling of it are not correct. It is important that the VCO grounding is not shared by the previous receiver circuitry. It is best to take it back separately to the main reservoir capacitor. If you disable the VCO, is the noise still there on the dc amplifier output, or does it reduce? There will always be some fluctuation at this point, but it should be random and is largely the amplified noise of the front end.
The 7662 charge pump ground should also be returned direct to the main reservoir and not through the receiver ground.
I also had a problem once, with a voltage regulator that was oscillating at high frequency. Make sure that all linear regulators have sufficient value capacitors on input and output, mounted close to the regulator.
Eric.

 

[Anonymous]

I am assuming that when you reprogrammed the micro to pulse again and the output was solid, you did not activate the gating to the diff amp at the same time. Is that correct?
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Yes that is correct.
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If the noise stops when you cease gating the sampling FETs, then it points to the front end picking up noise.
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When I stop the sample gates but turned the xmit pulse back on, the output was quiet. The tone was very steady.
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If you activated both the pulse and the gating and the problem went away, I am wondering if you changed the pulse rate. If so, then you might have changed the relationship between either the external noise (lights, tv, etc), or you changed any possible interaction between the 7662 and the sampling.
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I then activated the gate sampling and was back to the same problem. I don't have a means to change the pulse frequency via switches or a knob yet, I just reprogrammed the processor to change the pulse rate during these experiments and went from the original 500hz to 700hz and then 200hz and really no change from that. I did notice that when I changed to 700hz that the overall output seemed to drift up. The output still varied.
Thanks for you're help. I'll keep playing with it and see what I come up with.
FJ

 

[Anonymous]

Another area to look at is the VCO. I have encountered interference from this source if the grounding and decoupling of it are not correct. It is important that the VCO grounding is not shared by the previous receiver circuitry. It is best to take it back separately to the main reservoir capacitor. If you disable the VCO, is the noise still there on the dc amplifier output, or does it reduce? There will always be some fluctuation at this point, but it should be random and is largely the amplified noise of the front end.
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While I have the pulsing circuitry ground going directly to the reservoir cap and the front end, integrator and amplifier ground going to the reservoir cap (the 2 only connected together at the reservoir cap), I'll bet I don't have the VCO ground separated and connected back at the cap. Its probably down the chain from the final amp running back through the integrator back to the front end (hope this makes sense). This could be a problem. I will try disabling the VCO while I have the signal on the scope and see if that affects it.
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The 7662 charge pump ground should also be returned direct to the main reservoir and not through the receiver ground.
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Off the top of my head I don't recall if my 7662 is connected directly back or not but if it isn't, I'll change that and see if it helps.
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I also had a problem once, with a voltage regulator that was oscillating at high frequency. Make sure that all linear regulators have sufficient value capacitors on input and output, mounted close to the regulator.
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I had this problem too once so now I put a 22uf in parallel with a .1uf on the inputs and outputs up against each regulator. I haven't had this problem again since doing that
Thanks to you and Reg for all your suggestions. I have a lot to look at now. All thoughts and suggestions are appreciated and welcome.
Thanks,
FJ