[R-390] R-390 Q of Caps and tuned circuits (was: Tracking down low sensitivity 0 to 8 mhz)

[Larry H]

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

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On Fri, Apr 5, 2024 at 2:30 PM Tom Frobase <tfrobase at gmail.com> 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
>
>