IDX-PRO+VDI

[jlsilicon]

The signals in a Rx ,, superhet are say 100MHz IN, 110MHz L.O. so result in a 10MHz IF. So here you need 100 in for 10 out as you imply.



It doesnt hold like this on VLF IB dets,

The Tx and Rx are on the same frequency.
BUT The Rx tank, has to be detuned often between 50 and 75% of the Tx freq - this eliminates a dominant step change in phase of the Rx front end that would occur if Rx was co tuned in frequency with Tx. Some machines use PLL techniques etc to allow co tuned Tx Rx but simple kit like here does not.




if these machines are static, there are no a.c. signals fed to the op amps.

when you sweep a target the signal from the target/coil has 2 important properties, a phase the samplers can view! (due to when they sample) and an amplitude whose value changes rapidly - which is due to the movement over the target..


It is the rate of change of these signals that allow the opamps to amplify them. (As they form high gain differentiators ) So amplify only signals whose amplitude is changing. At a specific rate (linked to shaft length and arm sweep speed ) Results in signal energy in the op amps at approx 8Hz.

This is why machines are sweep rate dependant, go too slow and the rate of change of amplitude is too sluggish and the opamps dont have gain ( due to the open cct pole, or capacitor )

Too fast again the gain is limited due to the cap on the feedback.


So the op amp would not need filter poles shifting/retuning as the signals here are from shaft length and resultant rates of change of signal amplitude.

But, what about the ne5534 - and its C1,C2,C32 - aren't these creating a band filter ?
- And, the C22 and C23 ... ?

- Are you saying that I can modify the TX Transistors Frequency - with NO Sensitivity or Range loss ?

[golfnut]

Oh I see, I mis understood your question.. When you said Opamps I thiought about the x,y channel amps.


Sure the preamp will benefit from a minor tweak, I made the cap C32 a little smaller maybe 180n 150n 120n something like that.

Primarily the Tx freq, the Rx coil freq. Tweak on the preamp. You cam doe the preamp easily in Pspice free DL


I used my IDx at 12.5kHz on homemade coil and found these.



I used it at 14.5kHz on a NEL Hunter coil and found plenty.

[jlsilicon]

Oh I see, I mis understood your question.. When you said Opamps I thiought about the x,y channel amps.


Sure the preamp will benefit from a minor tweak, I made the cap C32 a little smaller maybe 180n 150n 120n something like that.

Primarily the Tx freq, the Rx coil freq. Tweak on the preamp. You cam doe the preamp easily in Pspice free DL


I used my IDx at 12.5kHz on homemade coil and found these.

[ATTACH]32175[/ATTACH]

I used it at 14.5kHz on a NEL Hunter coil and found plenty.

Golnut,
Thanks for the update !
If its not too much, can you supply those values ?
I am not so quick with Analog - as I am a Digital skilled.

- I was thinking of having a Frequency Switch.
Say: 6.6KHz / 13KHz / 26KHz - or something like this.
- So, I need to know what Caps/etc to adjust for the Broader Range of Bands.

Would be useful for different Soils and/or Targets ... yes ?

Thanks !

[golfnut]

I think the place to start is the Tx - just work empirically, as its not logic ! I used a smaller Tx coil cap try 0.47uF or so. I used a smaller L value for the coil also to push mine to to 12.5KHz

The preamp I think I used 120nF for the C32. The feedback cap I left alone, it sets the upper roll off.



If you want to calculate values use 1/ (2 PI* SQRT(L C ) for freq or rearrange for C= 1/4 PIsqd Fsqd L

[jlsilicon]

Ok - thanks.
I will try it in the next few days.

I am slow with the Analog - I understand the theory - but don't have the intuitive tweaking skills yet ...

I am trying many different configurations and coil changes ... but it is slow as ever with work during the day ... little energy left in the evenings to debug.
I have clear results - but at a slow pace.

[golfnut]

leave everything and get some reference performance, as is.

Get the freq of Tx u have now.


Swap the Tx coil cap for smaller one and see where this gets the Tx freq..

Assume u have scope access

[jlsilicon]

leave everything and get some reference performance, as is.

Get the freq of Tx u have now.


Swap the Tx coil cap for smaller one and see where this gets the Tx freq..

Assume u have scope access

The equation seems to work.

But, isn't the C37 also part of the TX oscillator circuit ...
- Does the C37=100nF=0.1uF in the TX circuit need be changed ?

[golfnut]

I cant see a C37 0,1uF cap in the Tx?

There are about 10 schematics where is the part..

[jlsilicon]

Ok sorry, C21,

BC327 Collector to BC237 Base with C21=0.0022uF - from Eduardo on the First page here.

- How do I calculate Freq of 6.6KHz , from the RC of C=0.0022uF and R=2.7K and 1.8K resistor ?

Or, do I just change C21 from 0.0022uF to 0.001uF - to switch from 6.6KHz to 13.2KHz ?

...Or is this not part of the RC - and so does not change ... ?

[golfnut]

I see. The connector terminal for the Tx has no coil on the drawing.

ok, the Tx tune 99% done in the search coil (NOT on the Schematic)and the capacitor that is in parallel with the search coil (NOT on the schematic)


The 2n2 coupling cap is just that. Its not really tuning just passes the signal to the 2nd half of the oscillator.

[golfnut]

did a spice model quickly 0.5mH coil and 330nF should do about 12kHz

[jlsilicon]

Thanks !
I wanted to try to build 1 or 2 more Metal Detectors this weekend.
- So I will give this a try.

Would like to bring 1 or more of my models to the Detector Club soon (a few members seem to be interested).

[jlsilicon]

did a spice model quickly 0.5mH coil and 330nF should do about 12kHz

Ok - one more question though :
- What about the RX Coil and Cap ?

Only guessing gives me about: 30.0mH and 12nF ... maybe ??

[golfnut]

perfect if you were doing about 8k Rx sounds good.

[jlsilicon]

perfect if you were doing about 8k Rx sounds good.

Sorry I am lost,
I am following your 12khz TX - so I assume same 12khz for RX ?