Designing & Building an RF Oscillator Sniffer Coil w/ X1 (1:1) Oscilloscope Probe

One of my viewers reached out and requested more detail on the coil I used to detect and display the oscillator carrier on my o-scope. In this video, I demonstrate a M&P that can be used to detect the oscillator carrier/frequency using a hand built coil (Inductor ~80 uH) and O-scope X1 (1:1) probe (capacitance of ~126 pF). The design assumes an I.F. frequency of 455 kHz with the oscillator operating above the BC receiver range, i.e., 530 kHz + 455 kHz = 985 kHz to 1600 kHz + 455 kHz = 2055 kHz.
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#restoreoldradios

[@jimnewman5596]

Another great tip to check the oscillator without loading it down Don.

[@RestoreOldRadios]

Thank you Jim, yep definitely provides that feature. I’m looking forward to trying it out on more receivers in the future to better evaluate its potential. Hope you’re doing well. Best, Don

[@jimnewman5596]

@@RestoreOldRadios Hi Don, I just built my sniffer today using Mike's RC calculator and it works much better than the coil pickup that I was using before. The sweet spot for my coil was around 1.1MHz. Thanks for the knowledge you pass on to everyone.

[@RestoreOldRadios]

@@jimnewman5596 Sounds good Jim, glad it helped.

[@cornelolteanu942]

Thank you sir, all the best !

[@W1RMD]

Thanks for doing this! I'm studying on this subject and it's nice to see do a video on it. Take care and happy Easter to you and your family.

[@RestoreOldRadios]

Ryan, you’re welcome. I hope you have a blessed Easter as well. Best, Don

[@EngineeringVignettes]

Ah. You are using the scope probe capacitance as the reactive part of the tank circuit... clever. Up at 2 MHz, I wonder to what effect the inter-winding capacitance comes into play as well... The measurement effects of the instrumentation, on the DUT, are commonly forgotten. This short video helps to "poke" others to say that they may be getting a wrong answer in their measurements just by the act of trying to measure it! Kind of like a _Heisenburg effect_ on LC circuits.

[@RestoreOldRadios]

Eddy - great to hear from you. I’m sure the winding capacitance comes into play, but since I’m covering such a broad frequency range +/- from the measured resonance point just to confirm oscillation is occuring, I didn’t spend anytime trying to measure or calculate it. Hope this finds you well. All the best. Don

[@roncaruso931]

Thank you. This was very helpful.

[@RestoreOldRadios]

Glad it was helpful! Thanks for reaching out Ron. All the best. Don

[@Wil_Bloodworth]

At the very end of the video where you show your scope, there are four wires. It looks like the probe ground is on the left lead and the scope probe is on the right lead. What are the other two alligator clips attached to? Maybe a signal generator generating a 455 Hz signal? Thanks Don!

[@RestoreOldRadios]

Hi William - the red lead is not attached to anything, just laying in the coil former and being fed from my RF signal generator while varying the frequency from the resonant frequency of ~1.64 MHz being displayed on the scope. The black lead ties back to the SG too; ground connection. Hope this helps. Best, Don

[@Wil_Bloodworth]

@@RestoreOldRadios that answers my question perfectly. Thank you.