[Racal] 6790 On VLF

[Barry Hauser]

Hi Dallas:

Very interesting -- about the standard version with the VLF chips. I just
received the most recent version of the firmware -- partly for LF and also
to get S-meter readability for computer control.

With the varying experiences with low frequency (deafness/noise, etc.),
could another wild card be the optional RF amp?

I'm just getting into these, not too familiar, so please bear with me if I
mis-state something:

My understanding is that the RF amp, which replaces the low pass filter
assembly, provides a 10 DB gain.  However, in a glance at the schematic, it
seems to be just a 3-transistor broadband amp with the rest of the module
consisting largely of the same (or similar) L/C components of the filter
unit it replaces.

I've read that, while it provides the gain, it also introduces noise, and
that it can also be easily bypassed if installed -- but I'd imagine the
jumpering involved would skip over the filters as well.

Do you have that RF amp?  What's the word on it in terms of general
performance as well as affect on LF and VLF?

Barry


----- Original Message -----
From: "Dallas Lankford" <dallas@bayou.com>
To: "Racal Reflector" <racal@mailman.qth.net>
Sent: Thursday, March 21, 2002 7:08 PM
Subject: [Racal] 6790 On VLF


> In my experience the standard 6790 does quite well down to about 100 kHz
with a set of VLF EPROMS.  To refresh my memory, I measured the MF / LF
sensitivity of one of my 6790's again.  For a 5.7 kHz BW (Rockwell Collins
torsion filter) in AM mode using 30% modulation, a 10 dB S+N/N ratio is
obtained with 0.6 uV at 1000, 300, and 200 kHz, and this falls off to 1.1 uV
at 150 kHz.  I haven't listened below 150 kHz.  In CW mode with a 300 Hz
filter, the noise floor (MDS, et al.) is 0.06 uV at 150 kHz.  I use phased
(home built phaser) interference (noise) reducing antennas (short grounded
inverted L's with 9:1 transformer step-down for twin lead feed), and hear
all sorts of non-directional beacons, and some trans-Atlantics from my
location in North Louisiana.
>
> But what I really like about my VLF EPROMS is that they provide tuning
(handy!) steps of 1, 10, and 100 Hz, not the standard (unhandy!) 1, 30, 1000
Hz of the standard EPROMS.  The 100 Hz tuning step is really great for
tuning around with the tuning knob.  But be careful!  There are (at least)
two VLF EPROM sets, one with the 1/10/100 and the other with 1/30/1000.  The
1/10/100 set does not have AGC return (so you can't set up an S-meter with
your remote control software), while the 1/30/1000 does have AGC return
(which is a non-issue in my opinion because I turn my computers off when I
turn my radios on).
>
> These VLF EPROMS are available from:
>
> Allen Bond
> wb4gnt@arrl.net
>
>
>
>
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