Did I calculate the Self Resonant Freqency of my coil correctly?

[dbanner]

Saw this little project while browsing through old mags.
I think this circuit can be used for playing around with coils. So to find Q, SRF, inductance, etc.
Anyways, it looks interesting for the person who likes to experiment with self made coils. And has nostalgia for vintage gear.

[kt315]

Few Additioal Details

Posted by: Eric Foster (---.proxy.aol.com)
Date: March 19, 2005 04:52AM

Hi bbsailor,
The 5uS is measured from the start of the transmitter switch off, so it includes the high voltage flyback time. The scope was externally triggered by the drive pulse to the Mosfet.
The shielded coil plus cable resonant frequency is 555kHz. This is measured by inducing a pulse from another coil, so it does not include any extra capacitance contribution from the electronics.
Coil inductance is 344uH and coil resistance 3.8 ohms, inclusive of cable.
The transmitter pulse is 20uS wide and the two sample pulses 5uS wide, when running at the 5uS delay. TX p.p.s. is 20,000. At the other end of the delay control, the TX is 60uS wide, with delay and samples at 15uS, and p.p.s. is 6500.
The considerable reduction in useable sample pulse delay is the sum of attention to three areas. 1) the coil 2) transmitter output and drive circuits 3) front end amplifier. At this point in time, this detector is purely experimental, just to see if the limits can be pushed out a bit further.
Eric.

[dbanner]

Few Additioal Details

Posted by: Eric Foster (---.proxy.aol.com)
Date: March 19, 2005 04:52AM

Hi bbsailor,
The 5uS is measured from the start of the transmitter switch off, so it includes the high voltage flyback time. The scope was externally triggered by the drive pulse to the Mosfet.
The shielded coil plus cable resonant frequency is 555kHz. This is measured by inducing a pulse from another coil, so it does not include any extra capacitance contribution from the electronics.
Coil inductance is 344uH and coil resistance 3.8 ohms, inclusive of cable.
The transmitter pulse is 20uS wide and the two sample pulses 5uS wide, when running at the 5uS delay. TX p.p.s. is 20,000. At the other end of the delay control, the TX is 60uS wide, with delay and samples at 15uS, and p.p.s. is 6500.
The considerable reduction in useable sample pulse delay is the sum of attention to three areas. 1) the coil 2) transmitter output and drive circuits 3) front end amplifier. At this point in time, this detector is purely experimental, just to see if the limits can be pushed out a bit further.
Eric.

"The shielded coil plus cable resonant frequency is 555kHz.......Inducing a pulse from another coil"
Can you show such a setup circuit?

[kt315]

setup circuit? desolder overdamping resistor from any pi board. connect o-scope to preamp output. you see sinewave signal. measure T between neighbor emissions.
calculate f=1/T.

[yatahaze323]

Hey everyone,

I tried the diagram that Carl posted and I was having trouble getting the oscope to trigger so I moved on to diagrams that Elliot and kt315 posted.

For the diagram Elliot posted I don't have 10 meg resistors and I wasn't sure if I could use a different resistor type then whats in that diagram.

I did have everything that kt315 posted so I setup everything according to that diagram and I was able to get that setup properly.

BUT, on my function generator I have an "Attenuation (dB)" dial and I'm not sure what I should set that at to get the proper measurement. I attached a picture of the dial.

Does anyone know what I should set the Attenuation to?



Thank you!

[kt315]

interesting. but dB is just relative scale, kinda 'in times more-less'. can you give voltage value P-P on output in position 0?

[Carl-NC]

I tried the diagram that Carl posted and I was having trouble getting the oscope to trigger...

You don't need it to trigger. Just need to find where it peaks.

[yatahaze323]

Kt315, I'm not by my oscope right now but I did take pictures,

This is the settings on the function generator I set the freqency to 1kHz and for the Attenuation I set the outer dial to 0 and the inner dial turned all the way to the right:

This was the oscope output from those settings:

When I adjusted the Attenuation the p-p increased fairly quickly, but the max peak of the wave never increased when I continued to adjust the attenuation outer dial. I measured the wave and it came out to 712 Hz which can't be the srf.


Hi Carl, I'll try your design again, maybe I screwed up the connections when I tried the design posted.

On a good note, I attached the coil to my mpp and it works really well. I did a air test indoor in a emi noisy area and I could detect a us quarter at around 10.5" and a us nickel around 10".

Thank you again for you help!

[kt315]

this table. if you have 2V p-p then 10dB is 2*3.16~ 6V p-p
20dB is 2*10= 20V.

[green]

I would be interested in SRF comparison using siginal generator method vs method https://www.geotech1.com/forums/show...575#post261575 thanks.