[R-390] R-390 Q of Caps and tuned circuits

[David Wise]

I estimated the Q of some caps using my HP 4342A Q Meter.
You put on a coil that you can resonate at your test frequency.
Record C and Q, and zero the meter in deltaQ mode.
Add a cap.  Readjust C to resonate and record deltaQ.

I tested only 100pF caps, and I ignored part to part variation.
The formula for Q of the cap is
Q = Q1Q2Cp/(Q1-Q2)C1

I used a 100uH inductor that resonates at 1MHz and C1=285.2pF with Q=98.
Its self-resonant frequency is 12MHz so we can ignore its distributed C.
Since I pretend my test C is always 100pF, Cp/C1 = 100/285.2 = 0.3506 .
Except for the 3kV ceramic which is only 80pF.

Here's my data.

Polyester, Cornell Dubilier type PM, Q2=66 so Q = 98*66/32 * 0.3506 = 71

Ceramic
3kV deltaQ = 5.0 so Q = 98*93/5 * 0.3506 * 80/100 = 511
dogbone dQ = 3.0, Q = 98*95/3 * 0.3506 = 1088
mlcc dQ = 2.0, Q = 98*96/2 * 0.3506 = 1649
NPO Erie Ceramicon dQ = 1.8, Q = 98*96.2/1.8 * 0.3506 = 1836

Mica
antique Aerovox dQ = 5.4, Q = 98*92.6/5.4 * 0.3506 = 589
10% molded dQ = 1.2, Q = 98*96.8/1.2 * 0.3506 = 2772
Sickles Silvercap (molded) dQ = 1.1, Q = 98*96.9/1.1 * 0.3506 = 3027
5% 500V dipped SM dQ = 1.0, Q = 98*97*0.3506 = 3333

The above tests violate one of HP's rules.  I was supposed to pick a frequency and inductor that resonated with C1 about 30pF to 100pF above the test cap Cp; in other words, resonate near the bottom of the 4342A C dial.  Repeating the tests at 1.4MHz, I get:

REPEAT AT 1.4MHz, Q = 95.5, C1 = 144.9
All caps ~100pF except 3kV ceramic = 80pF

3kV dQ = 10, Q = 95.5*85.5/10*80/144.9 = 451
dogbone dQ = 5.0, Q = 95.5*90.5/5*100/144.9 = 1193
mlcc dQ = 4.0, Q = 95.5*91.5/4*0.6901 = 1508
ceramicon dQ = 3.2, Q = 95.5*92.3/3.2*0.6901 = 1901

Aerovox dQ = 9.5, Q = 95.5*86/9.5*0.6901 = 865
10% molded dQ = 2.0, Q = 95.5*93.5/2*0.6901 = 3081
Sickles Silvercap dQ = 1.9, Q = 95.5*93.6/1.9*0.6901 = 3247
5% 500V SM dipped dQ = 1.9, Q = 3247

As you can see, my ceramic caps did way better than Larry's.
Want to trade caps to compare our measurement systems?

My best ceramic is better than my worst mica but that might be an outlier.
All the other micas outperformed the hottest ceramic.

Dave Wise