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Measuring Resonant Frequency of Secondary (High Frequency)???

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LAN_403

HV Enthusiast

Mon Feb 08 2010, 03:28PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Anyone have any tips on how to accurately measure the resonant frequency of a high frequency secondary (i.e. coils on the order of 4MHz and upward)

I typically measure frequency by putting a small resistor (i.e. 1k) in series with the secondary RF Gnd and hook this up to a frequency generator. Then monitor the voltage with oscilloscope across the resistor. At resonance (series), the impedance is at its minimum, so it should draw the most current and voltage across resistor should be maximum.

Unfortunately, at this high of frequency, i find many of the harmonic modes, so i'm having a difficult time zeroing in on the right one.

Any tips or suggestions?

UPDATE: I guess another possibility is the output of my HP oscillator varies vs. frequency as well.

Thanks

GeordieBoy

Mon Feb 08 2010, 05:03PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Hi Dan,

This is how I did it...

I connected the base of the resonator to the 50 ohm output of an RF generator. Stand the resonator over a ground-plane (if that's how it will be used) or in free-space and hang a scope probe about 18 inches away to monitor the E-field.

Set RF generator to sine waveshape and maximum amplitude then sweep the frequency manually until maximum E-field strength is noted on the scope.

This method gives a reasonably good indication of the free resonating frequency of the coil. However, in my Class E SSTC work I found that two things make it so that this figure is only approximate and of limited practical use:

1. Driving the resonator with a link-coupled primary means that some of the resonator's turns are magnetically coupled to the primary, leaving less inductance free to resonate. This means that the resulting resonant frequency of the secondary increases as you increase the coupling coefficient k.

2. Corona adds a lot of capacitance to the top of the resonator after breakout. This lowers the resonant frequency after breakout and is particularly noticeable if no toroid is used since Ctop is otherwise quite small for a physically small resonator.

In my 4MHz and 8MHz SSTCs I made the decision to fix the drive frequency to 4MHz or 8MHz respectively and then tune the resonator to this drive frequency. I found this more effective than the usual method of tuning the drive to match the resonant frequency of the coil. The reason for this is that it lets you optimise the design of the RF power amplifier and drive amplifiers for a specific operating frequency like you might do in a radio transmitter design.

I must say that I always found the tuning surround the resonator itself the most awkward part of HF-SSTC debugging. Especially as high-power Class-E amplifiers aren't particularly forgiving of load mismatching!

I hope this helps,

-Richie,

HV Enthusiast

Mon Feb 08 2010, 07:11PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Excellent tip Richie. That should certaintly makes things much easier.

Thanks!
Dan

ragnar

Mon Feb 08 2010, 07:31PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Hi Dan,

For my smaller coils, I built a signal generator that uses paralleled gates of a 74AC14 as the output driver; i.e. a signal of known voltage and fast enough for my 13.7MHz coil, but that I didn't mind to cook!

I set up the secondary in what is intended to be its final form, including a solid primary mount and the counterpoise. (Usually the metal box enclosing the to-be driver). The signal generator connects through a series capacitor to the primary.

I'm a big fan of Richie's dangling probe technique. For the tiny coils, 1" was a good distance for a clear signal. Then I can sweep and note the peak or peaks.

I felt this technique eliminated some of the "primary proximity" mystery, especially since dramatic alterations may pose a near-MHz change; as for corona loading, you will discover a good "fudge factor" to apply to your pre-breakout sweep measurements!

radiotech

Mon Feb 08 2010, 07:33PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

For coils at that frequeny I use a Boonton 160-A Q meter. This finds the inductance and Q at F . The resonant capacity is noted. Then at another, 0.5F the test is repeated. The self capacity can be calculated with these two sets of data.

If the coil is powered, how about using a grid dip meter, or its solid state equivalent. Use it in the wavemeter mode, ie just a simple wavemeter.

HV Enthusiast

Mon Feb 08 2010, 11:40PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

The hanging probe technique worked great. No more mysterious peaking that I got with the HP oscillator.

doctor electrons

Tue Feb 09 2010, 12:40AM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Dan! When i wound the 4mhz secondary for your kit i used a 8 inch piece of the coil wire as an antenna. Scrape off the insulation
clip it in the probe standing parallel with the coil about 20 inches away. You can REALLY see the magnitude on the scope increase.
I tuned that bugger to exactly 3.855 mhz! By the way, i completely love that thing!!!!! Keep up the good (wayyy understated) work!!
Thanks for everything youve taught me in the past few months, youre the best!!

HV Enthusiast

Tue Feb 09 2010, 03:09PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

doctor electrons wrote ...

Dan! When i wound the 4mhz secondary for your kit i used a 8 inch piece of the coil wire as an antenna. Scrape off the insulation
clip it in the probe standing parallel with the coil about 20 inches away. You can REALLY see the magnitude on the scope increase.
I tuned that bugger to exactly 3.855 mhz! By the way, i completely love that thing!!!!! Keep up the good (wayyy understated) work!!
Thanks for everything youve taught me in the past few months, youre the best!!

Thanks.
Be sure to check out the new manuals on my website for the 4MHz coil.

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