Instability in detectors.

[Sean Goddard]

I noticed something which I had never taken into account before that occurse with ALL Tesoro machines.

If you have a large target, then the frequency of the transmit signal changes, as it would as Tesoro only use a simple Colpitts oscillator in most cases.

Now, and maybe Dave J. could throw some light on this for me. By the same token, surely all the front end circuitry is designed to measure the phase shift of a target at a certain frequency, but this shift will cause inaccuracies to occur as the frequency changes, so the target phase response changes.

I think what I'm trying to say is that unless you use a Xtal oscillator, then inherently your disc circuit is going to change it's reference point and thus it's "response" to an object, depending on what the object is.

OR, is this a feature that Tesoro use and understand, and if so, can you explain why this systems is used?

I've looked at the math but can't derive a formula to predict the response....Yet!

[Carl]

Re: Instability in detectors.

Actually, this would not be unique to Tesoro as lots of single-frequency detectors use Colpitts oscillators. However, the frequency shift should be pretty small, maybe a percent or less. Consider the old BFO's, which ran at 100kHz but the mixer frequency shift was only a few 100 Hz.

- Carl

[BJ]

Re: Instability in detectors.

The reson for Tesoro is as i presume "cost"!

1:Its a cheap way to do a oscillator.
2:Since the coil is a part of the (tuned tank)
oscillator big objects will shift its frequency.

3: Since the coil is a part of the oscillator
its a cost effective way in regard to impedances
and output transistor driving. And yes several
other parameters to.

4:You dont need Xtal, do a simple function generator tri/squ cost effectivness is almost the same and stability are better and by far easier to adjust operating frequency.

5: the clear advance of that sheme is its ability to create a resonable sine wave.

BJ

[Carl]

Another thought...

I expect that with Colpitts transmitters, you will get more frequency variation just from different coils, than from a target. Since the coil Z is a primary factor in the osc. freq., I expect that manufacturing tolerances in the coils will shift the frequency around, probably by several percent.

- Carl

[Reg]

Re: Instability in detectors.

Hi Carl,

I suspect you are right and the phase shift would be minimal as far as any discrimination goes.

Now, as for using a Colpitts oscillator, or any other oscillator for that matter, this type of design should actually be more efficient and provide a greater energy transfer into the ground.

Since the coil is part of the oscillator frequency, then there should be minimal loses do to a mismatch. That can occur if the coil isn't perfectly tuned to the crystal frequency, if a crystal is used.

Now, how does this all translate into depth loss. I suspect very little except maybe in the case the coil and associated caps are tuned to a frequency that isn't close to the crystal frequency.

My guess is if a person were wanting to increase the depth a little, they would simply have to increase the current in the coil and then it takes a significant current increase to realize a depth increase.

If I remember correctly, George Payne once told me that you have to double the power into the coil to increase the depth an inch or so. The next inch requires that the power be doubled again.

It doesn't take too many times of doing that before you need to drag a car battery in a little red wagon behind you just to power the detector.

Reg

[Carl]

Re: Instability in detectors.

I agree, the Colpitts is a pretty efficient way to do it. Drawback is, the voltage swing is limited to the supply rails. I guess you could use a transformer, though, and I think some have.

Doubling the power gives a 12% depth increase, theoretically (isotropic radiator). That's about an inch for today's detectors.

- Carl

[Reg]

Re: Increasing power

Hi Carl,

George Payne used a switching regulator to increase the voltage to the oscillator circuit on the Discovery modules. Instead of the 12V battery voltage, the voltage was increased to about 35V if I remember correctly. I think this is what he did on the older ST (Tek) models also. Both gained an inch or so in an air test.

Now, as for a transformer type design, Bill Hays uses this basic principle on his little TR's he makes and sells today. For a TR, this is quite a sensitive little detector. Too bad it doesn't have a voltage regulator stabilize things.

Reg

[pawel]

Re: Instability in detectors.

Well... Using a JFET Colpitts (17kHz) fed with 5V I obtain a 20V p-p swing at the coil...

Regards,

pawel

[pawel]

Re: Instability in detectors.

Without ANY trafo, of course.

pawel

[Robert Hoolko]

Re: Instability in detectors.

If you drive a tuned coil with a current pulse the coil voltage can exceed the supply voltage. Up to about twice the supply voltage you still get a very good sine wave. As the voltage goes above that the current pulses have to get narrower and the voltage waveform may start to distort.

If you drive the White's BM coils with a frequency within 1% of the center frequency the response is very flat. The XLT goes almost 10% off center at the extreme frequency settings, but the response is pretty far down at that point.

Robert

[Dave Johnson]

Re: Instability in detectors.

The target is so small in relation to the transmitter that its effect on transmitter phase shift is almost unmeasurable.

A target large enough to cause as much as 1 degree of phase shift in the transmitter, would have already saturated the receiver circuitry.

--Dave J.

[Sean Goddard]

Re: Increasing power

Reg,

U R absolutley right, I just happen to have a Treasure Baron (dead) on the bench at the moment, and I managed to light my eyes up by holding the inverter module before I realised what it was...OUCH!!