Re: LF: Re: Field Strength Meter - advice needed

[Chris Osborn <g3xiz@yahoo.co.uk>]

Many thanks for all advice Jim,

I'll have another happy day experimenting and implementing your suggestions.
I've blown on the ferrite rod through the slot in the screening tube
using a drinking straw but the output seems quite steady as a result of this exercise.

The oscillator output seems stable in both frequency and ampitude.

Re the clipping of the AF stages, this seems not to be the problem and even a flattish
PP3 battery  doesn't cause any significant change in output.

Thanks again for your help.

73 Chris G3XIZ

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From: James Moritz <james.moritz@btopenworld.com>
To: rsgb_lf_group@blacksheep.org
Sent: Sunday, 3 May, 2009 20:23:43
Subject: LF: Re: Field Strength Meter - advice needed

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