VCO update

[Carl-NC]

Thanks for the ideas below. I was trying to get the circuit to the left to work, but it was really flaky when driven off the PI final gain stage. This same circuit, flipped upside-down, worked great on my fluxgate mag project.

I think a big problem was the new noise I've picked up on my bench. When the VCO is set up as an isolated circuit, the flakiness goes away, but there is not enough loop gain for the thing to oscillate. Instead, it just biases up to a steady state, with the cap voltage held to the same level as the voltage divider.

So, I added some more gain, shown on the right. The LM393 ouptut is npn open collector, so the pnp boosts the (unsaturated) gain by ß. This seems to work OK, gives the needed df/dI. But I'm worried about device skews, as it doesn't take much degradation in gain to make the whole thing flatline.

This noise problem has me baffled. I just built a new 3GHz computer, and set up a network router, and bought a LaserJet5si big honkin' printer, so I figured one of these was the culprit. But I've unplugged everything in the room, except the power supply & oscope, and nothing changes. The output of the preamp is so noisy, that there is no way to do any reasonable testing. And it doesn't seem to be 60Hz-based. Gotta figure this out, in order to keep working on stuff.

- Carl

[Gary]

Re: VCO update

Hi Carl, From your last paragraph, It sounds like you might be trying for too much over-all gain and generating all this noise from resistors and IC's, rather than picking it up from outside sources.

[bbsailor]

Re: VCO update

Carl,

You might want to try building a small Faraday Cage to eliminate external ambient RF waves from combining with desired circuit signals and producing undersirable noise.

I have found a picture of a Faraday cage on the web link below. This shows you how easily one might be constructed.

http://www.cypresssystems.com/Accessories/FaradayCage.html

bbsailor

[Ray Huffman]

Re: VCO update

Many years ago I worked for a company that made sesmic sensors for oil exploration. We had a screened cage that we tested the preamps for the sesmic sensors. These discrete preamps were touted as have 100db gain for the low signals coming from the sensors. It worked for them.
I might do the same thing and build a screened cage to place the PI electronics in so I can test in a low noise environment. First I need to find some window screen at the hardware store that should work. A lot of it is plastic. Hopefully I can find some copper window screen.
Just my thoughts on the subject.,
Ray

[Carl-NC]

Re: VCO update

Problem is, the noise is coming in on the coil, not the circuit. I can verifiy this by rotating the coil, and watching the noise vary. As Eric once noted, noise is minimal when the coil is rotated vertically, though not minimal enough. I suppose if I had a handheld oscope, I could use this to my advantage, by walking around with the detector and finding out where the noise is coming from.

This weekend, I might try moving the setup to other locations and see what happens.

- Carl

[Carl-NC]

Re: VCO update

This is a periodic interferer, not thermal noise. It does not sync on 60Hz, either. I can rotate the coil and the level varies -- see my other post -- so I know it's coming in on the coil.

- Carl

[Beachcomber]

Re: VCO update

Would it be possible/feasible to make a smaller coil. Say something on a 1/2"-1" diameter piece of PVC tubing similar to what is used on Pinpointers for your testing purposes. I would think that would reduce the noise pickup considerably while allowing you to still work on the unit in your shop where your test equipment is located. Then connect the regular coil when you are ready to go outside into an electrically quieter environment for final testing and adjustment.

HH

Beachcomber

[Russ_NY]

Re: VCO update( Noise )

Carl
Just curious,, does the noise vary any at different times of the day, or night? I know that the ignition coil on my furnace causes me problems when it is running. It's kind of a pain to do but you can always trip the breakers to all circuits except for your test setup to see if it goes away. But then you have to go about resetting all the clocks and such. I also have a power substation within sight of my house which I suspect causes interference sometimes. Hope you are able to locate the source.

Russ

[Joe Johnson]

Re: VCO update

Eric, on his forum, recommends to wind a figure 8 coil for testing to reduce the noise pickup.
JJ

[Eric Foster]

Re: VCO update

Hi Joe,

Yes, that really does work well. You can then see the amplifier noise, which is otherwise masked by the coil pickup.

Eric.