How to measure SRF of a DD coil with Parallel Damping Resistor

[autitch]

Hi brains trust,

First of all I would like to thank Joseph Rogowski “bbsailor” for having penned and shared the “Making a Fast Pulse Induction Mono Coil” paper. I have found it very informative and a great help in my quest to make my own coil to suit a GPX Minelab. I’m looking to measure the SRF of coils so to be able to determine the parasitic capacitance of coils. I have used the SRF measuring circuit described in the Fast Coil paper on some of the mono coils that I have and I was unable to get a sensible reading with the 1pF capacitor before the scope probe. I repeated the measurements without the capacitor and I manage to get the mono coils to exhibit the SR phenomenon on the scope and hence I was able to determine their respective SRFs. If someone may be able to make some sense of why with the capacitor in place the measuring circuit didn't work I would greatly appreciate the explanation.

I tried to use the same circuit on my DD coils and while attempting to measure the SRF of the RX coil and I wasn’t able to get any readings, the scope output was basically a flat signal at any frequency. As I understand the RX coil on a GPX Minelab has a parallel damping resistor soldered in the coil housing. Would this fact necessitate a different SRF testing method or would I need to remove the damping resistor prior to measuring the SRF. From my basic understanding of electronics a mono coil can be essentially described as a series LC circuit and the DD RX coil is a parallel LCR circuit. Guidance on measuring the SRF of a DD RX coil would be greatly appreciated.

AuTitch

[homefire]

Think of Both Mono and DD as Tank Circuits. Only the Capacitance is Caused by the Coil Winding + Cable + Fet or Transistor. The Damping Resistor presents a load on the Circuit to Reduce Oscillations or Ringing. https://en.wikipedia.org/wiki/LC_circuit

[green]

Have you tried monitoring amplifier out of an operating PI with an oscilloscope while you bring the test coil near the operating PI coil to measure SRF(self resonant frequency)? DD coil, short the Tx coil leads when measuring Rx. Short the Rx leads when measuring Tx. If the amplifier is fast enough you will see a decayed resonance when the operating PI coil turns off.

Test coil leads needs to be open circuit with no damping resistor.

[Carl-NC]

The 1pF cap isn't really necessary if you have a 10x probe, and including it will attenuate the signal going into the scope.
The 1M resistor between the sig gen and coil is too high IMO, I generally use 10k or 100k as the SRF is fairly independent of this value. Making it bigger (1M) just reduces signal amplitude, no point in doing that.
Finally, a damping R doesn't do much to the SRF, but it does act as a signal attenuator so, again, reducing the 1M resistor will help. I'm not sure exactly how the ML DD coils are wired but you should be able to measure the SRF. Might want to try reversing the pins used for signal & ground.

[green]

Have you tried monitoring amplifier out of an operating PI with an oscilloscope while you bring the test coil near the operating PI coil to measure SRF(self resonant frequency)? DD coil, short the Tx coil leads when measuring Rx. Short the Rx leads when measuring Tx. If the amplifier is fast enough you will see a decayed resonance when the operating PI coil turns off.

Test coil leads needs to be open circuit with no damping resistor.

Three questions. (1)Has anyone tried the method? (2)If yes, how did SRF measurement compare with your other test method? (3)Is there any reason the method would give a wrong SRF reading? I don't have a signal generator so I haven't tried that method. The method gives a higher reading than the other methods I've tried, pulse circuits exciting resonance with R damping removed.

[autitch]

Thanks gents for the input. I’m heading back into the workshop in the next few days to try out some of the suggestions.
Carl, I will try a smaller resistor to see if I can get to drive the coil. I suspected that the 1pF capacitor might have been interfering with the 10X probe. Certainly the results I achieved without the capacitor were in line with results obtained by others.
AuTitch

[green]

Thanks gents for the input. I’m heading back into the workshop in the next few days to try out some of the suggestions.
Carl, I will try a smaller resistor to see if I can get to drive the coil. I suspected that the 1pF capacitor might have been interfering with the 10X probe. Certainly the results I achieved without the capacitor were in line with results obtained by others.
AuTitch

It's been a few days and I am curious if you have had a chance to try some measurements. If not, no problem.

[autitch]

Hi Green,

I've resorted to pulling apart the DD Coiltek 14" coil I've been looking to test. I haven't been able to make any other arrangement work. It's been a pretty painstaking process to disassemble the coil without destroying it. I should be at a point to be able to remove the damping resistor tonight and than putting a the shielding back and run a few tests. Slow process.


AuTitch

[6666]

AuTitch

is there any chance you could take a few pictures as you go along ? thanks.

[autitch]

After some painstaking tedious work I’ve managed to document a bunch of measured parameters for a Coiltek DD 14” round coil I decided to sacrifice for the benefit general knowledge. All inductance readings are measured at 1kHz and all SRF measurements were made using the test circuit as shown in SRF circuit sketch attached.
Test 1: Complete coil, as is.
RX ( pin 1-2) TX ( pin 4-5 )
Inductance 500 uh 280 uh
Resistance 6.9 Ohms 0.57 Ohms
SRF Not Measurement 464 kHz

Test 2: Coils was opened and shielding removed to expose the damping resistor. Damping resistor was removed and all shielding was replaced and coil shell refitted.

RX ( pin 1-2) TX ( pin 4-5 )
Inductance 511 uh Not Measurement
Resistance 6.9 Ohms Not Measurement
SRF 272 kHz Not Measurement

Test 3: Coax coil was removed and short enameled copper wires were soldered. Shielding was refitted prior to testing.

RX ( pin 1-2) TX ( pin 4-5 )
Inductance 511 uh 280 uh
Resistance 6.6 Ohms 0.47 Ohms
SRF 331kHz 1040 kHz

Test 4: As per Test 3 without shielding.

RX ( pin 1-2) TX ( pin 4-5 )
Inductance 511 uh 280 uh
Resistance 6.6 Ohms 0.47 Ohms
SRF 333kHz 1060 kHz

Damping resistor 559 Ohms (soldered in parallel in the coil across the Rx wires)

Coax Cable 1525mm long

RX ( pin 1-2) TX ( pin 4-5 )
Capacitance 216 pF 316 pF

Feel free to post any additional measurements that may be of interest.
AuTitch

PS I'm unable to attach the jpgs. When I work it out I'll post them.

[6666]

posting pics Click reply
click go advanced
click manage attachments
add files
browse
select file from where ever the picture is stored click open

click upload and wait till finished
click done

[bklein]

Am I interpreting this correct? I took a coil and attached it to a pi board's TX output. I placed the test coil on top of it.
I see what the attached photo shows (sorry for the off-rotation). The test coil is a whites DF. What does this indicate as far as goodness/badness?
Using the 1M resistor, 1pf capacitance (twisted wires), and signal generator method this coil peaks at 500Khz (in the photo the frequency is 319KHz).

[6666]

Green will put you on right path

[Polymer]

Am I interpreting this correct? I took a coil and attached it to a pi board's TX output. I placed the test coil on top of it.
I see what the attached photo shows (sorry for the off-rotation). The test coil is a whites DF. What does this indicate as far as goodness/badness?
Using the 1M resistor, 1pf capacitance (twisted wires), and signal generator method this coil peaks at 500Khz (in the photo the frequency is 319KHz).

The ringing curve shape looks right, can't say if good/bad for a DF coil ....
green describes making a loop with the scope probe to its GND and holding this near/in the excited coil. Coil ends open I believe.
This may give you a higher SRF. Found the thread about this, hope it helps.

https://www.geotech1.com/forums/show...ires-questions

[bklein]

Here is scope probe method (pi excites test coil DF)). 500-560KHz.