[Premium-Rx] RE: Mods for Racal 6790GM

[John Reed]

----- Original Message ----- 
From: <ToddRoberts2001 at aol.com>
To: <premium-rx at ml.skirrow.org>
Sent: Thursday, February 17, 2005 5:23 PM
Subject: Re: [Premium-Rx] RE: Mods for Racal 6790GM

Here's more on that roofing filter mod.  I do a lot of DXing for lowfer 
beacons.  In the LF band my main problem is noise.  I designed a noise 
blanker that could go in either a Racal (RA6793A with factory LF option), or 
a Harris RF-590.  The blanker worked well in the Harris, but not well at all 
in the Racal.  The reason for this is that the tighter bandwidth roofing 
filters in the Racal were stretching out the noise pulses too much and 
making them difficult for the blanker to identify as noise.  I'm sure of 
this because I tried smaller bandwidth roofing filters in the Harris with 
the same results.  Small bandwidth roofing filters are good sometimes, but 
not with noise blankers.

The addition of post-IF filters was a big help for the Racal.  With a 100 Hz 
filter in front of the IF amp,  minimum discernable signal (MDS) was -108 
dBm.  With 100 Hz filters in front of and behind IF amps, the MDS was -138 
dBm.  That's a + 30 dB increase in sensitivity!  With a wider filter, there 
isn't that much difference.  With 1 KHz filters pre and post IF amp, the 
increase was +4 dB.

I ended up using the dual filter setup on the Harris.  It didn't improve S/N 
as much as on the Racal, but still helps.  The Harris is actually more 
sensitive at LF than the Racal, even though the Racal has the factory LF 
option.  The Harris with noise blanker and dual filters is an excellent LF 
receiver.

John Reed

>I forgot to mention that John Reed KA5QEP has done some very good research
> into adding a post I.F. filter board to the Racal 6790 as well as other
> receivers like the RF-590. This involves adding a buffer amp to the output 
> of the
> original I.F. board and running the output to a second board with a few 
> switchable
> narrow I.F. filters - in effect cascading a second I.F. filter to the 
> first.
> He said this gave an outstanding improvement in weak-signal readability 
> when
> switching to narrow filters for weak-signal cw work. He also recommends 
> adding
> an 8KHz wide Roofing filter to the 6790 - that is about the bandwidth of 
> the
> roofing filter in the RA-6790 VLF version 2nd mixer I believe. It limits 
> the
> bandwidth to about 8KHz but the original 16-20KHz wide filters in the 
> 6790's
> I.F. are probably not used that much in most normal mw and shortwave 
> listening. -
> Todd Roberts WD4NGG
>
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