We all know that the R-390 and many other rx's rely on high 'Q' tuned circuits to operate correctly. Silver Mica (SM) caps have been used a lot in these circuits because of their high 'Q'. The resultant high 'Q' of a tuned circuit is normally the result of the design of the inductor used in them (for the most part), but the Q of the cap is also important. If the Q of either is not high, the overall Q of the tuned circuit will not be high.
For many years, the high 'Q' of SM caps was unequalled, but a few years ago better dielectrics in ceramic caps were developed and the Q was considerably improved. These caps are in the 'class 1' category and are usually NP0's or C0G's. Some datasheets for 'class 1' caps specify the Q of the dielectric, and some specify the inverse factor, DF (dissipation factor). The formula to convert one to the other is: Q = 1/DF, but this rating is usually done at 1 MH. Some include graphs that help in determining the Q at higher frequencies, like 17 MH. The dissipation factor of Vishay's series 561R 'class 1' C0G is 0.1 % max at 1 MH, so according to the formula Q = 1/DF, the Q for this cap would be 1000. The PAN OVERSEAS (GUANGZHOU) ELECTRONIC CO.,LTD. TEMPERATURE COMPENSATING CERAMIC DISC CAPACITOR (POE-D01-00-E-16) for values of 30 PF & ABOVE, have a specified Q of 1000+ at 1 MH. CDE's SM caps of type CMR meets requirement of MIL-PRF-39001, in the capacitance range of 100-1000 pF, have a dissipation factor of 0.00075 max at 1 MHz. That's a Q of 1333.3333. Lower pF value caps have a higher Q. I'm not sure what the Q is of the SM caps used in the R-390, but suspect it is above 1000 at 1 MH, so I'm not sure that ceramic caps are ready to replace SM's in sensitive tuned circuits, yet. Regards, Larry ------------------------------------------------------------------------------------------------------------------------------------------------------------------ On Fri, Apr 5, 2024 at 2:30 PM Tom Frobase <[email protected]> wrote: > I have had the 100pf mica go bad in the 17mhz drive circuit. Interestingly > it did not check bad, but greatly limited the amplitude of the mixer drive. > … Tom, N3LLL > > ______________________________________________________________ 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
