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 ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
