Help needed with TGSL RX oscillator,coil setup

[slug54]

Hello,
I am having problems getting my RX oscillator to tune or adjust. The frequency pretty much stays around 14.285 KHZ when changing value of cap C6.
The waveform does change when changing the cap value but mainly just the amplitude and not much frequency change.
With a 15nf cap in for C6 the wave form is a square wave at 11.2V PP. When I attempt to adjust the overlapping of the coils I can get a sine wave but the lowest voltage is 8.8V PP. This is being measured with a scope at TP4 pin 7 of U101. The TX oscillator is running at 14.430KHZ 13.2V PP sine wave with 2 caps in parallel for C2 to give 21nf.

Interesting thing I found out on accident is without C6 installed I get a 14.285KHZ sine wave but I can overlap the coils and get the voltage down as low as 1.7V PP.

Just a little detail on the project that may help figure out what I am doing wrong. I took the schematic from the main TGSL project and re-created it in Eagle and added the 2 tones modification a header connector for VDI connection and headphone jack and volume control. I had the board manufactured I did not create it my self.
Attached is a section of the schematic. My coils have the shielding on them and are connected with a USB cable using the "Improved performance in wet grass" configuration from the project document. One coil is fixed down and the the other is loose for adjustment. Here are the measured specs on my coils:
TX Coil 5.96mh 23.5 Ohms , RX Coil 6.74mh 25.7 Ohms.

Thanks for reading and for any help you can give me. -Jeff

[kt315]

The waveform does change when changing the cap value but mainly just the amplitude and not much frequency change.
---
sorry, but it is RECEIVE coil, not transmitter one. you can not change the frequency by C6.

[dbanner]

The rx coil will always receive THE EXACT FREQUENCY of the tx coil transmit oscillator. However, the value of the parallel capacitor is simply to tune the RESONANT FREQUENCY OF THE RX COIL. This you must do by hand calculation using mathematical formula, or just use a resonance calculator. Your Targets are 14.5KHZ for the tx transmit frequency and 16.1KHZ for the rx RESONANT FREQUENCY. The spread is important to get proper discrimination and adjustment of ground balance trim reseistor.

[kt315]

DB

a resonant frequency of the coil may be measured by especially projected LC tester.

[slug54]

Thank you both for your help.
So if I understand this correctly, I need to tweak my TX frequency a little more to get the 14.5KHZ and then calculate what value of cap with my with my 6.74mh RX coil will give me 16.1KHZ resonant frequency which I should be able to measure on pin 7 of U101. Interesting circuit KT. Thanks for helping. -Jeff

[homefire]

It's not working as per the schematic ?

[slug54]

Hey Homefire,
From what I understand as I overlap the coils when the voltage drops and before it rises again I should see the resonating frequency of the RX coil and C6.
The lowest voltage I'm getting measuring at pin 7 of U101 is 8.8 volts and I haven't been able to read anything near the 16.1 KhZ by changing cap value of C6.
I just installed a header socket for C6 so that will make changing the value of the cap easier. I will do some more testing tonight and see if I can tune it and get better results. -Jeff

[dbanner]

Hey Homefire,
From what I understand as I overlap the coils when the voltage drops and before it rises again I should see the resonating frequency of the RX coil and C6.
The lowest voltage I'm getting measuring at pin 7 of U101 is 8.8 volts and I haven't been able to read anything near the 16.1 KhZ by changing cap value of C6.
I just installed a header socket for C6 so that will make changing the value of the cap easier. I will do some more testing tonight and see if I can tune it and get better results. -Jeff

C6 value is selected by calculation for the resonant frequency of the RX coil. The RESONANT FREQUENCY IS ENTIRELY DIFFERENT TO THE ACTUAL FREQUENCY OF THE RECEIVED SIGNAL. The output of U101 pin 7 is going to be the same as the TX frequency(14.5KHZ)
Regardless of the value of C6, you will always get 14.5KHZ at output of receiver.
C6 is selected to give rx resonance of 16.1KHZ. Regardless of the resonance calculation of the rx, it will still always receive 14.5Khz.
This is why we say that this receiver coil is tuned slightly "off resonance", because the tx oscillator is transmitting a signal of 14.5KHz whereas the rx coil is set at resonance of 16.1khz.BUT IT WILL STILL ONLY RECEIVE A SIGNAL OF 14.5KHZ.
So although she is tuned for 16.1khz, she will still receive 14.5khz albeit at less gain. This is how the designer made it for this particular detector.

[kt315]

you have just to encrease the resonant frequency of RX coil to TX resonant one. i do not sure know how much but approx on 1-2kHz.
you have to measure RX resonant frequency offline WITHOUT connecting of power supply on board.
it may be done with especial projected LC coil tester. i had gave a link.

[dbanner]

Measure inductance of RX coil.
Then use resonance calculator or whatever hand calculations to select value of C6 for a resonant frequency of 16.1Khz.
Set this value of C6. That's it.

[slug54]

Thanks guys,
Dbanner this makes sense now. I was under the impression that I should be able to measure the 16.1KHZ.
I understand what needs to be done now to get this stage going. Thanks for taking the time to explain this clearly.
I'm going to work through this thing stage by stage and hopefully some day soon it comes to life -Jeff

[kt315]

sorry i mistaken. the resonant frequency of RX coil is below than TX one.