Tx and Rx coil inductance ratio

[dbanner]

I'm no expert, but I think it's has to do with the coil impedance. Otherwise you "load" the coil and change its characteristics, adversely affecting the search loop.

[Carl-NC]

I've tried different value resistors for TGSL front end. As the resistor values gets smaller than the original 4k7 the Rx signal gets more noisy and less in amplitude with the same gain ratio of 47x aprox. I tried 1k-47k, 2k2-100k but the best was with 4k7-220k.

Does the coil gets loaded if the resistor value gets smaller, could somebody explain this?

If the TGSL runs at 10kHz then a 6.5mH RX coil has a ~400 ohm impedance (2*pi*f*L). You want the input resistor to be at least 10x this to minimize loading. 1k will load it and cause a signal reduction.

Lowering the input R also increases its current noise (4KT/R) that gets reflected into the coil impedance, which might explain why it looked noisier.

[dbanner]

I'm reminded of just why measuring instruments typically have high input inpedance, precisely to avoid loading the circuit being measured, and in so doing, avoid incorrect readings.

[Nandor]

That was the information I was looking for!!!
Thank you very much Carl.

[Nandor]

I just made some experiments with different Tx coils and finally found out why different frequencies have different coil values. It was interesting to see that the oscillator cant run if the tank capacitor is too high in value but then the Tx inductance also has to be changed and then power consumption is also very important, look:

This is the Raptor's oscillator with my homemade TGSL coil


Tx-12.85kHz 14.8Vpp 1.56mA


R1-1K R1-680ohm
Vpp-9.44v Vpp-14.5v
A-1.3mA A-1.8mA


L-Tx 1mH~~R=3.7ohm R1-100ohm


TX-8.5kHz 13.1Vpp 6.45mA
Tx-10kHz 14.8Vpp 6.06mA
Tx-13.9kHz 16.4Vpp 5.11mA
Tx-15.7kHz 17.2Vpp 4.80mA


Tx R=25.8ohm~5.96mH


Tx-13.9kHz 14.8Vpp 0.94mA

As I lowered the Tx frequency the power consumption went higher. Also the sinewave got smaller so it needed more current and voltage

[Nandor]

I forgot it and maybe did not understand it well, but now it's all clear to me thank's dbanner.