WC6W wrote:
> The values are close enough if the operating Q is sufficiently high
>(>10) and considering tube capacitance plus circuit strays the errors
>typically go unnoticed in the real world.
>
>
Even then, they may not be close enough to distinguish between tank
"resonances".
The test of accuracy of the design formula is to analyse the resulting
network with the load present, and back-calculate to the actual value of
Q. These are basic series/parallel network calculations.
Rich wrote:
>RL = 2000 ohms. R out equals 50-ohms. Freq = 7.00MHz. Q = 10
>Therefore, C1 = 2000 ohms/10 = 200 ohms. From calculation, C1 = 114pF,
>C2 = 562pF, L = 5.06uH.
Rich evidently used an approximate formula - it fails the analysis test,
which indicates a Q of 11.4 at the design frequency, with the load
present.
The accurate formula - in ARRL Handbooks since 1995 - gives C1=102.1pF,
L=5.57uH, C2=463.9pF. When analysed with the load present, this checks
out with Q=9.98. (I don't know where the small difference from Q=10
comes from - it could be either arithmetic errors in Excel or a very
minor approximation in the formula.)
It makes a big difference to the resonant frequency whether the load is
connected or not. The tank is designed to operate with a load of 50 ohms
in parallel with C2. If the load is not present, it changes the
effective value of C2 in series with C1, so the resonance will indeed
shift.
With either the accurate formula or the inaccurate one, the resonant
frequency with the load present is between 7.00 and 7.05MHz.
Without the load, the resonant frequency of Rich's network is indeed
7.266MHz, but that isn't a valid indication of resonance in operation.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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