[Lowfer] Homebrewed SSB Filter question

[Chris Trask]

>
> I want to experiment a little, and try building a SSB filter that
> will pass the USB of my Osc. at 184.322 KHz. I have never attempted
> such before, but want to try just to see what happens. Once I
> finish putting together the DSB Modulator, I'll need to shave off,
> or greatly reduce the LSB component.
>
> *** QUESTION *** ; If I use crystals to build the filter, will my
> Center Frequency ( Fc ) need to be 185.822 KHz ?? I have a handfull
> of 184.322 KHz crystals, but that would pass my carrier freq. and
> not the USB audio. Am I correct on this ?? Not sure, so asking.
>

    If you're making a ladder filter, then you can use crystals that have a
series resonant frequency of 184.822KHz.  In shunt, they will produce a
multiple of series resonant transmission zeroes at the carrier frequency and
the parallel resonant frequency will be above the carrier frequency.  If you
were making an LSB filter, then you would use parallel resonant frequencies
in a seties configuration so as to have multiple parallel resonant
transmission zeroes at the carrier frequency and the series resonant
frequency will be below the carrier frequency.

>
> *** SECONDLY *** ; Since this will be used with a signal of about
> 33mW; what are some thoughts on possibly using Op Amps to design
> a Bandpass filter for the USB audio ?? If this approach will work,
> the Op Amps will also serve as amplifier stages ( I Think ).
>

    That would be an interesting approach, and may be realizable at that
frequency if you use the current-feedback opamps.  You'll need an elliptical
highpass filter with quite a few poles to pull that off, and you'll need to
do it as a cascade of a single 3-pole section and a number of 2-pole
state-variable sections so as to overcome the time delay problems even with
those parts.  Each 2-pole section will require three opamps and the 3-pole
section will require four if you do it correctly.

Chris

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