Hi,

Hi,

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.

Notice that the primary to secondary (total) turns ratio is
almost 1:1, that both shunt capacitors in the band-pass filter are
of the same value and that the path resistance on either side of
the secondary, through the FST3126M to the low-pass filters is
around 20-ohms. So what you are looking at (to a crude
approximation) is a design impedance of 50-ohm all the way
through.

As for the toroid, for optimum Q you might want to look for a
mix more suitable for the frequency range you quoted (maybe 15?)
and increase the diameter to take the extra wire as you will need
to scale the impedances for use at 50kHz. The FT series, I think,
need less turns for the same inductance and so should be easier to
wind.

Finally, a simple low-pass filter would probably be all you
need here as the transformer primary impedance will be a limit the
low end. Filter response is dependant on source as well as load
impedance and most antennas that you would use at those
frequencies will be quite reactive and difficult to design a fixed
input network for. You may be aware that a lot of HF receivers use
a separate hi-Z antenna terminal for LF/MF.

Having said that, if you do want to use a simple low-pass
configuration, remember that a symmetrical 'Pi' filter with source
and load impedances of 1-ohm and a cut-off of 1-rad/sec would use
two 1-Farad capacitors and a 2-Henry inductor. To scale to new
impedances and frequency, divide C (and multiply L) by the design
impedance and then divide both C and L by the new cut-off
frequency in rads/sec.


Cheers - Joe