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February 5th 05, 02:12 AM

In article ,
(gudmundur) writes:

The source is 50 ohms, I am feeding through a 1000 ohm resistor to
my tuned circuit at 30mhz, and the load is over 5000 ohms (FET input).

I want to use a parallel resonant circuit to ground to form a pass
filter. If I want a Q of around 5, would I need 5000 ohm XL/XC ??

This would seem nearly correct. Have I calculated correctly?

Not quite. Loaded Q is dependent on ALL losses in parallel
with a parallel-resonant circuit. If you want a Q of around 5
then the R:Xc or R:Xl ratio is 5.

A problem seems to be the loading of the 50 Ohm source plus
a series (?) resistance of 1000 Ohms. That alone puts 1050
Ohms in parallel with the resonant circuit. If the total parallel
R is 1050 Ohms, the total from FET input and coil's own Q is
5000 Ohms, then the total parallel R will be 868 Ohms. The
inductive reactance would have to be 1/5th of that or equivalent
to 0.921 uHy. The Xl of 0.921 uHy is 173.6 Ohms at 30 MHz.

The problem is compounded by using a series resistor to feed
the parallel resistance. If the resonant circuit has a total parallel
resistance of 5000 Ohms, the magnitude of the impedance is
that value, 5000 Ohms. But, with a series resistance from the
50 Ohm source there's a power loss from the equivalent voltage
divider action equal to 5:6 or about 1.6 db at 30 MHz resonance.

It would be better to use another way to couple the 50 Ohm
source, such as a tapped inductor. Tapping down at 1/10th of
the turns has an approximate 1:100 impedance change which
would transform the source to be about 5000 Ohms across the
parallel L-C. That would change the needed inductance to get
the loaded Q but there would be no extra losses involved. The
new inductance value at resonance would be about 2.65 uHy for
an Xl of 500 Ohms (1/5th of 5000 in parallel with 5000 Ohms).

Note: At resonance (and only at that frequency), the impedance
across the L-C is entirely resistive. At any other frequency it is
a complex number (both R and jX).

retired (from regular hours) electronic engineer person