1. Home
  2. Electronics Design
  3. Q meter question

Q meter question

Hi guys, I tried to measure SRF of an air core inductor with my Boonton 260A. I set both capacitors to zero and assumed it would find peak Q at the SRF. It didn't, the Boonton found 1.745Mhz when it is actually 4.2Mhz. I suppose the Boonton shouldn't be used this way, but I don't know why. Also the the Boonton has a vernier capacitor the goes from +3pf to -3pf, how does it do a negative capacitance? MikeK

Below are some measurements I made on the way realizing the Boonton 260A doesn't measure SRF by the method I used. Included just because it experimentally proved what I thought I new about apparent and real inductance, and it came out so good!

----------------------------------------------------------------------------------------------- I included to much info, for clarity pay attention to inductance values only, the first time through. I have a 56 turn, 2.25" radius,4.85" length coil with solid #18 (.043") wire, spaced 1 wire width apart. Using the formulas and calculator found at

formatting link
The "single layer calculator" with these numbers as input; 56 turn, 2.25" radius, 4.85" length, The calculator gave an answer of; L=231uh Measurements were made using a Boonton 260A Q meter to get the following numbers.

550khz Q=345 with 360.6pf calculate 232uh 1000khz Q=410 with 105.0pf calculate 241uh 1700khz Q=398 with 32.0pf calculate 274uh

-disappointing inductance agreement, but cleared up later-

The inductance is only close at 550khz, I know self capacitance will modify the apparent inductance, so I used the formula* Co=(C2-4C1)/3 to find the self capacitance of the inductor. Answer: 6.23pf. Ok, so I add 6.23pf to the capacitance at each frequency and recalculate the inductance.

550khz Q=345 with 366.83pf calculate 228uh 1000khz Q=410 with 111.23pf calculate 228uh 1700khz Q=398 with 38.23pf calculate 229uh

Well, that's certainly good agreement and close to the 231uh the calculator gave. At this point I'm happy, But; The Boonton measured SRF of the coil is 1,745khz with a Q of 398. The acyual SRF is about 4.2 Mhz Why can't the Boonton be used this way? So something is way off. Any thoughts? Thanks, MikeK

*C2=221.5 @ 700khz C1= 50.7 @ 1400khz

begin 666 icon_smile.gif M1TE&.#EA#P`/`+,.`/_J`$5%10```/_.`/_)`/^T`/Z=`/_^D__]$_______ MQS,S,___Z__E`````````"'Y! $```X`+ `````/``\```1;T$D9:IW85:;. M&1

Reply to
amdx
Loading thread data ...

I found the Boonton has a minimum capacitance of 30pf so when I tried measured for SRF, I had 30pf across the inductor. Resonant frequency of 230uh and 30pf +6.23pf (inductor self cap) is 1,744khz very close to the 1,745 khz I measured. MikeK

-----------------------------------------------------------------------------------------------

Reply to
amdx

That's an incremental capacitor, connected across the main capacitor, and arranged so that,when it is at zero, the main capacitor calibration is correct. You add both readings.

Some (better?) Q meters, like the Marconi Instruments TF1245, actually do the incremental using the main capacitor with a geared-down drive (taut band in the case of the TF1245).

Don't you have the 260A manual?

One standard way of measuring self resonant frequency [1], hence self capacitance requires that you have an inductor that will resonate at the DUT's self resonant frequency within the tuning capacitor range, which we'll call the substitution inductor:

Set the main capacitor to maximum capacitance.

Connect the DUT to the inductor terminals.

Adjust the oscillator frequency to obtain resonance. Call the values of frequency and capacitance f1 and C1

Find, or make a substitution inductor that will resonate at where you think the DUT self-resonant frequency is, within the instrument's capacitance range.

Remove the DUT, and substitute the substitution inductor.

Increase the frequency about 7.5 times, and adjust the capacitor for resonance. You might have to play with both frequency and capacitance.

Connect the DUT across the substitution inductor.

Readjust the capacitor for resonance, and note whether it had to be decreased or increased.

If it decreased, you're above the self resonant frequency, remove the DUT, decrease the frequency, resonate the substitution coil again, and reconnect the DUT.

If it increased, you're below the self resonant frequency, increase the frequency,and proceed as above.

What you're after is the frequency at which connecting the DUT doesn't affect the test circuit resonance (though it will affect the Q). call this frequency f0,

That's the self resonant frequency. The DUT is purely resistive at that frequency.

The self capacitance of the DUT is given by: C0 = C1 / ((f1/f0/)^2 -1)

Ref[1] "Marconi Instruments Circuit Magnification Meter TF1245 Manual OM1245" (Marconi Instruments Limited, England, 1960)

Boonton used to make sets of standard inductors to go with their Q-meters. You should keep your eyes open for some. Marconi ones won't fit, the pin spacing is different.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)
Reply to
Fred Abse

Thanks for all that, Yes I have other ways I could find SRF. I have recently pulled out my Boonton after owning it about 10 years and started learning how to use it. I learned; you can't measure SRF directly, the internal capacitor has a minimum of 30 pf and I think I learned the vernier capacitor is a 6pf capacitor labeled 0pf when it is 3pf, the main cap is labeled 30pf when it is 27pf, so adding the 3pf and 27pf makes the labeling correct. I recently lost an ebay bid on standard inductor grr... Yes, I do have the manual. Thanks, MikeK

Reply to
amdx

Yup, that's right.

The really ancient Marconi (forgot the number), single range, up to 50 MHz only, actually rocked the main capacitor stator with a cam to get the incremental.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)
Reply to
Fred Abse

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.