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From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <88d2415e0801111045t1e31ee78vbaf955f4f2a9f3a8@mail.gmail.com> <fc7eccec0801111111l2c7769ccn46aacb57e3d62db2@mail.gmail.com>
Date: Fri, 11 Jan 2008 19:25:34 -0000
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Subject: Re: LF: LF Receiver
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Hi Andy most if not all of the amateur receivers sold recently have not
AGC-off function. As I told Laurie the only one I know is the AOR 7030 which
has performance as good, if not better in some aspects that the 1792 (well
it was British designed and manufactured). The Lowe HF 150, 225 and most of
the Japanese designed receivers seem to rely on have a mode-selected agc
time constant. It means you dont need manual gain controls, cleaner panel,
never mind the performance look at the acreage of chrome etc. Even quite
expensive wideband receiver like the Icom R8500 have to be "bodged" to put
the AGC off. This is despite them having IF output for spectrum monitors.

I am too far away but isnt there anyone that could look at Laurie's RX they
are not really anything out of the ordinary and should be repairable. Full
service gen is available. We cant afford to lose LF stalwarts that easily !!

Alan G3NYK

----- Original Message -----
From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, January 11, 2008 7:11 PM
Subject: Re: LF: LF Receiver


> Which are the receivers you have encountered that *do* suffer this problem
> of non-switchable AGC?  It doesn't sound like a very healthy trend at all
!
>
> I'd use an SDR-IQ for LF these days. In fact I use one for most
> listening/monitorinf now. Its better, more adaptable and just generally
> nicer to use than the RA1792.  And infinitley better to tune with point
and
> click on the waterfall display.  The only thing it suffers from is not
> having a clock locked to a master reference.   Needs 66.667MHz and I've
just
> never got round to making a locked clock yet.  But its probably stable
> enough for QRSS as it is.
>
> AGC can be disabled, and is only used for demod anyway, to keep volume
> constant.   The waterfall display operates independently of the demod and
is
> always at fixed gain.
>
> Andy  G4JNT
>
>
> On 11/01/2008, lawrence mayhead <laurie.mayhead@googlemail.com> wrote:
> >
> > I have for a long time used a Racal 1792 reciever for QRSS modes on LF.
> > This receiver failed recently,and I have been looking for a replacement
> > receiver.
> > This search reveals a problem most modern receivers use DSP techniques
> > for filtering and the AGC is derived from the filtered signal. None of
the
> > receivers
> > I have looked at have an AGC OFF  facility.This means that if one of
these
> > receivers
> > is used with ARGO or similar FFT programs a strong signal within the
pass
> > band
> > will desensitise the Rx. It seems quite possible that a weak signal
could
> > quite
> > disapear. It also appears that some AGC is applied before the digital
> > filter so that
> > signals within the roofing filters bandwith will also produce
> > de-sensitisation, and this
> > could be as much as 15kHz away from the tune freq.
> > Any ideas on receivers not using DSP filtering ?
> > Or do I look for another R1792
> >
> > 73 Laurie G3AQC
> >
>
>
>
> --
> Andy  G4JNT
> www.scrbg.org/g4jnt
>