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Message-ID: <E23B4ED78E9442C88730357FAEDE3324@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <260286.17766.qm@web28508.mail.ukl.yahoo.com>
In-Reply-To: <260286.17766.qm@web28508.mail.ukl.yahoo.com>
Date: Sun, 3 May 2009 20:23:43 +0100
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Subject: LF: Re: Field Strength Meter - advice needed
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Dear Chris, LF Group,

Looking at the circuit, there are not that many possibilities for varying 
gain with temperature.

The NE/SA 602/612 devices should be quite stable with temperature - the 
Philips data shows less than 1dB variation over 0 - 70 degrees. But if the 
oscillator level is marginal, gain might vary more.

The 741 output amplifier is working at high gain, and low supply voltage, so 
might possibly be marginal. This would be worse if it were driven into 
clipping - unlikely if output V less than 1V RMS or so.

The 2N3819 gain might vary quite a lot with temperature, depending on how it 
is biased, and the particular specimen of 2N3819.

The RC low-pass filter might show significant variation with temperature if 
high-K ceramic capacitors with particularly wild temperature coefficients 
were used.

The ferrite rod antenna is a possibility, especially if the tuning is fixed 
as shown in the circuit diagram. The Q and temperature coefficient of these 
can be quite high, so a temperature change could cause it to drift off 
frequency and change the signal level - with a Q of 200, about 1kHz change 
in frequency would be sufficient.

I would suggest doing a bit of signal tracing - for example, disconnecting 
the RF stage and injecting a signal directly into the mixer would eliminate 
the ferrite rod and 2N3819. Another possibility would be to try heating up 
components individually, e.g. with a fingertip or a soldering iron, to see 
where large changes occur. Or, with the ferrite rod, try re-peaking the 
tuning after changing the temperature to see if there has been a shift in 
the tuning.

Hope this is helpful,

Cheers, Jim Moritz
73 de M0BMU

----- Original Message ----- 
From: "Chris Osborn" <g3xiz@yahoo.co.uk>
To: "LF Group" <rsgb_lf_group@blacksheep.org>
Sent: Sunday, May 03, 2009 6:13 PM
Subject: LF: Field Strength Meter - advice needed