PI-1 question

[FJ]

Hi all,
I put a PI-1 together and the output of the integrator has a negative going pulse on the signal. Of course it is also present on the output of the following amplifier stage. Is this normal? Also, the output bounces around a fair amount. Are others seeing the same things? I'm wondering if there is too much noise being picked up and if I take it out and away from the workbench if it will stabilize. Other than that it seems to work. The signal bouncing up and down is a little annoying with the tone unsteady in the headphones. It also makes it tougher to hear slight changes as a target is at the fringe of the detection range.
Thanks,
FJ

[Sean Goddard]

Re: PI-1 question

Sounds like you could do with "upping" the time constant on your integrator, and beefing up your supplies (larger reservoir capacitors) etc.

[FJ]

Re: PI-1 question

This is Carl's stock circuit so I guess I'm curious if I'm the only one seeing this. I'm surprised Carl himself hasn't jumped in. I built option 2 and the only thing I did different from the schematic was using .47uf instead of the .1uf caps in the integrator, everything else is per the schematic including the power supply components. The resorvoir cap is a 4700uf which is larger than what Eric recommends, it should be plenty. Am I the only one seeing this problem or is this normal?
Thanks,
FJ

[Eric Foster]

Re: PI-1 question

Hi FJ,

Try winding a figure of 8 coil for testing in the workshop. Although it won't give you the range of a standard coil, it will cancel out ambient noise and enable you to see if your electronics is noise free and working OK.

As for the negative pulse on the integrator output, check where you are grounding the scope. It is easy to superimpose a bit of the transmitter waveform if you ground in the wrong place. Make sure you are grounded to the receiver ground only.

Eric.

[Carl]

Re: PI-1 question

Integrator output should go positive, not negative. Check the preamp input configuration, make sure it is hooked up in inverting mode. Also make sure the preamp offset is adjusted to a reasonable point. Have you 'scoped the preamp output?

Yes, on the workbench you will get a lot of noise. Esp. if you have any CRTs around.

- Carl

[FJ]

Re: PI-1 question

Thanks for the grounding suggestion, I will try that!
FJ

[FJ]

Re: PI-1 question

Sorry I didn't make it more clear. The output overall does go positive but there appears to be a low going square wave or pulse superimposed on the output signal of the integrator. I've now built 2 PI-1s to connect with both types of audio and they both do this. I'm trying to understand if this is normal since both do this or is it something I'm doing wrong.
Thanks again,
FJ

[Carl]

Re: PI-1 question

Certainly not normal... if you 'scope the output of the integrator or the next gain stage, you should see a DC level that floats up and back down, as you pass a metal target over the coil. Where are you seeing a low-going pulse? Does the DC level bounce down quickly, then float up?

- Carl

[FJ]

Re: PI-1 question

Hi,
The output does float up and then back down as it should. The signal has a pulse riding on it that goes low. I will take Eric's suggestion and move my ground from the battery lead and ground the scope at the integrator's ground. Also, I will look at the transmit pulse and see if that is what I'm seeing on the integrator output.
Thanks,
FJ

[Eric Foster]

Re: PI-1 question

Hi Fj,

From what you say, I suspect this is the problem. You are seeing the voltage drop on the battery lead when the transmitter pulse is on. This will superimpose a negative pulse of several millivolts on the integrator output. Moving the scope ground should cure it.

Eric.