Date: Mon, 28 Jan 2008 10:19:50 +0000 (GMT)
From: John Pumford-Green GM4SLV <gm4slv@sighthound.demon.co.uk>
To: rsgb <rsgb_lf_group@blacksheep.org>
In-Reply-To: <88d2415e0801280148r3387041awc60ff337cdbfe549@mail.gmail.com>
Message-ID: <Pine.LNX.4.64.0801280957560.31950@lurcher.twatt.local>
References: <88d2415e0801280148r3387041awc60ff337cdbfe549@mail.gmail.com>
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Subject: Re: LF: Re. GM4SLV Grabber
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Hello Laurie & LF,


On Mon, 28 Jan 2008, lawrence mayhead wrote:

> John,
> I have been looking at your excellent 500kHz Grabber

Thanks!

> and notice that some strong signals are de-sentising
> the receiver and causing other weaker signals to show
> up less clearly. I belive you are using an AOR AR7030
> Rx and I wonder wether you have the AGC on or off ?

The RX (an AR7030) AGC is ON normally. I also notice the de-sensing 
caused by AGC action, usually during the evening, when DI2AM is fairly 
strong.

I tried running with the AGC off. Once the RF gain has been set 
correctly, works well. The downside is that it severely overdrives the 
soundcard when I'm transmitting on 500. With the AGC on the AR7030 can 
wind back its gain, and switch in extra RF attenuation, so the audio 
level to the soundcard doesn't "hit the rails".

The isolation of the antenna c/o relay in the TX is good so the 
receivers aren't muted during transmit (I've measured the level of 
leakage and it was okay, although my notes are at home so can't remember 
the actual measured levels) so I decided to leave the AR7030 AGC on and 
let it cope on its own.

At the moment the 500kHz antenna needs tuning, recent weather has 
rattled the ATU and the variometer needs a tweak, so I've not been 
active on TX for a week or 2. I suppose what I should do is turn the AGC 
off for most of the time and turn it on during operating periods.


> I have noticed this effect with some receivers, even with
> AGC OFF there is still some control of Rx gain, but had
> hoped that the 7030 would not suffer from this defect.

As far as I can tell the AR7030 is pretty well linear with the AGC off. 
I've used it before, AGC off, for 5MHz beacon monitoring, and it worked 
well. I seem to remember doing some checks with signal generator and 
attenuators in the past.


I'll turn the AGC off later and leave it running like that, so you can 
see how it performs.

The 7030 is an excellent RX for this style of operation. It's a pain at 
times when driving it manually and looking for tiny signals. It's very 
capable of hearing stuff, but the user interface, with all the various 
things you want to tweak (RF gain/filter bandwidths/Pass Band Tuning) 
etc all being on different menus. The VFO is also odd, with the smallest 
steps being fractional Hz so it's not possible to tune in exact 
multiples of 1Hz. 

As a stable peice of test equipment it's prety good, as a "tune around 
the band looking for weak signals" it can be a pain. Until I started on 
500kHZ (and also NDB hunting) I always thought it was fine, and couldn't 
understand the antipathy others had for the menu system etc. 

For a "tune and forget" long term monitoring RX it's great.


Cheers,

John