[R-390] Filters

[Robert Newberry]

What about the softrock lite? Cheap and easy to build.
On Feb 16, 2013 4:17 AM, "Charles P. Steinmetz" <
charles_steinmetz at lavabit.com> wrote:

> Bob wrote:
>
>  If you are going to do an op amp filter, you need to do the math first.
>> Op amps are fine for something that's 40 KHz wide and stable to a few KHz.
>> One percent at 455 KHz is 4.5 KHz. The individual resonators in the 4 KHz
>> filter are set up to within < 400 Hz of their desired frequency.
>>
>
> Additionally, Q that high at 455 kHz would require op-amps with an
> insanely high gain-bandwidth product.  A quick analysis of a 4th order
> Chebyshev design similar to the existing 4 kHz mechanical filter indicates
> that op-amps with a GBW > 4.5 GHz would be required.  And it would have to
> work in the steaming confines of the 390A IF chassis, with 6 resistors and
> 4 capacitors that hold their values to within about 0.01% (a tempco of
> around 0.0001% -- or one part per million -- per degree C).
>
> Then there is DSP.  Mike wrote:
>
>  A DSP filter would be an ideal replacement for a mechanical.  DSP chips
>> that work at 455KHz are readily available for very reasonable $.  The
>> problem is the software.  To do a nice, flat topped, steep skirt filter
>> would not be a trivial piece of work.  The upside is that one relatively
>> simple circuit could be made to do everything from super-narrow CW to full
>> bandwidth AM SWL.     *   *   *     A savvy ham with the proper background
>> could homebrew a very nice filter for a relatively cheap price. Soldering
>> the hardware together would be a fairly easy weekend project, assuming you
>> had a surface mount adapter board for the DSP chip. The software would take
>> many weeks to write and even longer to debug.
>>
>
> For anyone who has done it a time or two, the filter code would be an
> evening's work.  The problems would be (1) DSP horsepower and (2) feature
> bloat.  Once you digitize, why would you want to implement just the IF
> filters and convert back to 455 kHz to feed it through the 390A
> IF/detector/audio chain?  You'd be nuts not to provide lots of bandwidths
> from 10 Hz to 10 kHz or more, all-mode detectors, passband shift, several
> notch filters, noise blanker, synchronous AM, AGC, etc., etc.
>
> With either of these solutions, once you have it designed and debugged,
> hams and SWLs might be willing to pay $100 for a fully assembled unit, with
> warranty support (and perhaps installation) included.  Maybe $25 for a
> board and complete parts kit with a detailed, 55-page manual.
>
> It's probably easier (and much cheaper) to set the 390A to 16 kHz (jumper
> the filter position if the 16 kHz mechanical filter is dead) with slow AGC
> and feed the 390A's 455 kHz IF output through a suitable attenuator to your
> choice of commercial DSP radios tuned to 455 kHz.
>
> Best regards,
>
> Charles
>
>
>
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>
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