[R-390] Filters

[Bob Camp]

Hi

Pretty much everything you get from a Softrock comes from the PC. It really doesn't have much in it.

Bob

On Feb 16, 2013, at 5:57 AM, Robert Newberry <N1XBM at amsat.org> wrote:

> 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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