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Date: Fri, 11 Jan 2008 19:11:20 +0000
From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: LF Receiver
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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

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<div>Which are the receivers you have encountered that <u>do</u> suffer this problem of non-switchable AGC?&nbsp; It doesn&#39;t sound like a very healthy trend at all !</div>
<div>&nbsp;</div>
<div>I&#39;d&nbsp;use an SDR-IQ for LF these days.&nbsp;In fact I use&nbsp;one for most listening/monitorinf now.&nbsp;Its better, more adaptable and just generally nicer to use than the RA1792.&nbsp; And infinitley better to tune with point and click on the waterfall display.&nbsp; The only thing it suffers from is not having a&nbsp;clock locked to a master reference.&nbsp;&nbsp;&nbsp;Needs 
66.667MHz and I&#39;ve just never got round to&nbsp;making a locked&nbsp;clock&nbsp;yet.&nbsp; But its probably stable enough for QRSS as it is.</div>
<div>&nbsp;</div>
<div>AGC&nbsp;can be disabled, and is only used for demod anyway, to keep volume constant.&nbsp;&nbsp; The waterfall display operates independently of the demod and is always at&nbsp;fixed gain.</div>
<div>&nbsp;</div>
<div>Andy&nbsp; G4JNT<br><br>&nbsp;</div>
<div><span class="gmail_quote">On 11/01/2008, <b class="gmail_sendername">lawrence mayhead</b> &lt;<a href="mailto:laurie.mayhead@googlemail.com">laurie.mayhead@googlemail.com</a>&gt; wrote:</span>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">I have for a long time used a Racal 1792 reciever for QRSS modes on LF.<br>This receiver failed recently,and I have been looking for a replacement receiver.
<br>This search reveals a problem most modern receivers use DSP techniques <br>for filtering and the AGC is derived from the filtered signal. None of the receivers <br>I have looked at have an AGC OFF&nbsp; facility.This means that if one of these receivers 
<br>is used with ARGO or similar FFT programs a strong signal within the pass band <br>will desensitise the Rx. It seems quite possible that a weak signal could quite<br>disapear. It also appears that some AGC is applied before the digital filter so that 
<br>signals within the roofing filters bandwith will also produce de-sensitisation, and this <br>could be as much as 15kHz away from the tune freq.<br>Any ideas on receivers not using DSP filtering ?<br>Or do I look for another R1792&nbsp; &nbsp; 
<br><br>73 Laurie G3AQC<br></blockquote></div><br><br clear="all"><br>-- <br>Andy&nbsp;&nbsp;G4JNT<br><a href="http://www.scrbg.org/g4jnt">www.scrbg.org/g4jnt</a> 

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