[Lowfer] narrow filters

[Steve Ratzlaff]

Hi Doug,
Veteran NDB DXers such as myself who prefer aural listening for the idents 
rather than using software decoders have been using external narrow audio 
filters for many years, with excellent results. The current commercial 
favorites are the DSP filters, the MFJ 784B with about 30 Hz minimum CW 
bandwidth, and the Timewave 599zx with 18 Hz minimum CW bandwidth. (It shows 
10 Hz bandwidth but that's at -1dB bandwidth, not the normal -3dB bandwidth 
for measuring audio filters.) I currently use a homebrew 4 stage opamp 
filter with 6 Hz bandwidth. This is the best audio filter I've ever used; it 
doesn't ring either. I routinely hear weak signals that are inaudible with 
the dual 250 Hz IF filters in my R75.
73,
Steve AA7U
NE Oregon

----- Original Message ----- 
From: "Douglas D. Williams" <kb4oer at gmail.com>
To: "Discussion of the Lowfer (US, European, &amp,UK) and MedFer bands" 
<lowfer at mailman.qth.net>
Sent: Sunday, October 31, 2010 5:06 AM
Subject: [Lowfer] narrow filters


> Since LF activity really seems to be picking up, I went ahead and
> ordered the 250Hz 455kHz IF filter for my R75. This, combined with my
> current 500Hz, 9MHz IF filter, should give the receiver good
> selectivity. In your opinions, would I see any additional benefit in
> getting an even narrower external audio filter? Some of the DSP units
> can provide selectivity down to 20Hz! I'm not sure if this amount of
> selectivity would help when trying for QRSS signals. I suppose it
> could be of benefit when listening for NDBs. Opinions?
>
> -Doug KB4OER