>
>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.
>
? There is more than one way to figure Q.
>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
? [chortle]
> or a very minor approximation in the formula.)
ok
>
>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.
? To do it right, we really need to connect 2000-ohms across C1 and
50-ohms across C2. Want to make a wild guess as to whether this will
increase or decrease resonance?
>
>With either the accurate formula or the inaccurate one, the resonant
>frequency with the load present is between 7.00 and 7.05MHz.
>
? hardly, Ian. The calculated resonance is now 7.371MHz.
>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.
>
? Agreed. 7.371MHz is probably more accurate.
-- Cheers, Ian
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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