[R-390] Filters

[Charles P. Steinmetz]

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