TGSL 29.2 KHz

[homefire]

Ok, I'm trying to bump up the operating frequency of the TGSL to double the 14.6 KHz as is.


I have reduced the Transmit and Receive section Caps by as close to half as allowed using standard Cap values.

I have moved the voltage pump drive from Pin 12 to Pin 11 of the 4024 trying to keep the frequency at the 7.2KHz as it was. Maybe I should leave it on 12 to accommodate the increased Current required by the higher frequencies?


I have Rerouted one audio feed from Q4 to Q6 of the 4024 in order to bring the audio feed as it was running at 14.6KHz.


Should I adjust the High pass filter following the Receive Op Amp?


Any More things you think would need to be accommodated?

[homefire]

Better schematic I hope!

Ok, that is just the Original.

Can someone delete this post please?

[mikebg]

Better schematic I hope!

Ok, that is just the Original.

Can someone delete this post please?

Homefire, please delete your idea :-)
The resonance frequency of a tank circuit is inverse proportional to square root of capacitance. That means to double resonance frequency, you should reduce capacitance 4 times.
When you increase TX frequency, the phase difference between targets decreases. That means the increased TX frequency worsens target discrimination.
When you increase TX frequency, the AIR&GND signal increases. That means reduced sensitivity of RX because you should decrease the gain of preamp to avoid its saturation.
Can you point an advantage of increased TX frequency?

[homefire]

"The resonance frequency of a tank circuit is inverse proportional to square root of capacitance. That means to double resonance frequency, you should reduce capacitance 4 times."

Yes your right! Messed that one up! Thank You!

But still with those values it brings it up to 19.67 KHz and that is good enough for me. The 29KHz was a bad Idea as it's in the U.S. Citizens Radio Band and would have been subject to RFI up the
Wazzoooo!

"When you increase TX frequency, the phase difference between targets decreases. That means the increased TX frequency worsens target discrimination."

This is true but it also stimulates more Eddy Currents in Smaller objects and less Magnetic conductors such as Gold!

"When you increase TX frequency, the AIR&GND signal increases. That means reduced sensitivity of RX because you should decrease the gain of preamp to avoid its saturation."

Again Right on! But the ability of finding smaller gold Nuggets and bits has made up for the loss of depth.

I may have to Use better filtering after the first Receive Amp and that is what I am asking.

The Fisher Gold Bug operates at 70.01KHz and is one hell of a small gold machine.
Using a two stage Receive amp may be required but introduces yet more noise.

Not after coins here.

Not after Relics.

This is for Prospecting.

As long as I can get the Ground Balance to zero, See a 1/16Th gram of gold at 7" I would be happy.

"Can you point an advantage of increased TX frequency? "

The Quest for Gold !


Original Caps in Transmit







Caps As I changed.

[DOOLEY]

hello homefire ,

29 khz is not close to cb radio frequency,
27 mhz is cb radio,or there abouts.
i used to be a cb nut years ago,forgoten all the band clasifications now,
starts at about 26 mhz for low band,up to about 28 mhz for high band.

[homefire]

Yes your right!

[Dr Vel]

hello homefire ,

29 khz is not close to cb radio frequency,
27 mhz is cb radio,or there abouts.
i used to be a cb nut years ago,forgoten all the band clasifications now,
starts at about 26 mhz for low band,up to about 28 mhz for high band.

Really unimportant but for accuracy it is 26.965 MHZ (CH1) to 27.405 MHZ (CH40).

[homefire]

So what else about jumping up the frequency?

[Skippy]

I think a home-build detector operating at 20 - 30 KHz is a good idea, especially as there is a lack of commercial machines at these freqencies. It would make an interesting relic-hunter, too. Some of the smaller hammered silver coins we get in Europe have 'resonant' frequencies in the 15 - 25KHz area.
You might want to consider the use of smaller diameter search-coils, too. Skin effect will become more significant, too, so bifilar/trifilar winding of the coils should be considered.

[Comp]

????? I keep seeing references to 'skin effect' on here,... I am an ex military Telecomms engineer, we were taught skin effect didn't come into play pretty much till you got to UHF, (400 Mhz, and above), {from there on up, we silver plated coils/Lecher lines..} I cant see it having much effect at audio, or a touch higher,...
or am I wrong???

Cheers, Fred

[homefire]

I know microwave does that. I think he is referring to Eddy currents.

[Skippy]

I'm sure a bit of search engine use will tell you the details, but I recall a skin depth of 0.5mm is common at MD frequencies, so you're probably OK with a single conductor, especially as the Q factor doesn't need to be very high. But I'm sure some Geotech-ers will have more practical experience.

[Skippy]

Homefire:
You will also need to change the receive-coil load capacitance, keeping the same 'doubling of frequency' relationship. The ground balance RC time constants will need halving.

[homefire]

If you look at the schematic, I changed the front ends of both Receive and Transit on Coils.

As soon as the Taxman pays off his loan I gave em, I plan on ordering a kit from Silverdog and building AS IS!

Once working I will make the changes and see what happens.

I will try as schematic is shown and see what is what.

Mods will follow I'm sure.

If that works out, I may go up to a higher freq like double or more.

I want a cheap to me GOLD machine.

Time will tell and Cost more then I have but willing to try.

[Skippy]

Ah, now I see on the enlarged cct, I thought the red text was referring to the cap on the amplifier feedback resistor. The RC after the Rx pre-amp will need retuning in proportion to the frequency, too, I think. I would also think that changing the searchcoil parameters would be wise, though not essential. Ie. halving L, halving the parallel C, and choosing coil R so that Q remains the same. I'm not really familiar with the TGS, I'm sure the experts will have better detailed info for you.