It looks like the filter is designed for 50 ohms in and out. The transformer
inductance is also a part of the filter. It's value should be about the same as
L1 so to use it at 50k to 200K, you'll need to rewind for a lot more turns on
the primary and both secondaries if you keep the same core material.

Scaling it all and then optimizing for Coil Q of 40 and regular cap and coil
values, I get new values like:

L1 680nH - 100uH
C1 470pF - .022uF
C21 180pF - .027uF
L2 1.8uH - 100uH
C22 470pF - .022uF
T1 680nH (13:6:6 Turns) - 100uH (1911:882:882 Turns)

I strongly recommend changing T1 to a much higher mU core material.

This should result in -3dB points at 50 and 200 kHz, flat pass-band and -30dB at
20 and 500 kHz.


Good luck finding a 50 Ohm antenna at these frequncies. An active antenna may be
the way to go, otherwise you'll need many acres for all the wire.



On Sun, 11 Sep 2005 12:09:16 -0400, TRABEM wrote:

I am trying to scale an existing front end receiver (butterworth
bandpass) filter to a different frequency range. Unfortunately, it has
a transformer in the original design, so I'm stuck. I also don't know
how to handle the load presented by the active front end component
other than it's probably not significantly reactive.

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.

The filter components and transformer winding details are in the
document at:

http://www.amqrp.org/kits/softrock40...0Assy%20v1.pdf

The input chip is an FST3126, spec sheet is at:

http://www.fairchildsemi.com/ds/FS/FST3126.pdf

The spec for the T30-2 transformer core is at:

http://partsandkits.com/T30-2.htm

I have aade filter design software, but it isn't allowing me to plug
in the transformer into the design page of the software....so, I need
to know it's equivalent circuit I think.

The transformer winding details are on page 4 of the document and the
schematic of the front end is page 9. Ultimately, I need new values
for L1, L2, C20, C21 and C22.

If someone can give me a reasonable guess as to the equivalent circuit
of the transformer and the IC (U5), I can do the rest of the job
myself using the aade filter software.

Thanks,

T