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Ok Chris,
Is this the cct ?
I can only think that something in the tuned
cct is a little heat sensitive ? , how high is the 'q' the back to
back diodes could be temperature sensitive ?
what material is the coil former ? some
plastics are very temperature unstable , could the former be expanding ?
Was temperature stability tested on the
original design .. if so is the level scalable with frequency , 100
@ 100K 500 @ 500K , that sort of thing ?
G ..
Sent: Monday, May 04, 2009 10:31 PM
Subject: LF: Field Strength Meter - advice needed
No Graham,
I can swap a flat battery with a new one and the output
hardly moves.
Yes the oscillator and RF stages are regulated and holding
constant.
The problem's before the mixer.
As advised, I
disconnected the RF stage and injected a signal into the
mixer and it gives a
constant output even with a blow heater playing on the insides.
73 Chris
XIZ
From: Graham
<[email protected]>
To:
[email protected]
Sent: Monday, 4 May, 2009
21:31:37
Subject: Re: LF: Field
Strength Meter - advice needed
Chris
Its not the battery is it ... assume
the supply voltage / regulated voltage is holding
constant ?
G ..
Sent: Monday, May 04, 2009 9:23 PM
Subject: LF: Field Strength Meter - advice needed
Thanks again Dave and Alan,
I've spent a few more hours on the F/S
meter today and still have not found the problem.
I can blow with a drinking
straw on any component and
there's very little
output change yet with time there's a slow drift with
temperature - the output goes up
with increased temperature.
I thought
I'd found the problem as by squeezing the ferrite rod 's screening tube
I
could get a fair change in the output (about 10 %).
The tube's metal is not
aluminium but I guess an aluminium alloy.
In any case I changed it for a
proper aluminium tube - no difference !
From the shack's temperature
of 23 degrees and taking the F/S meter outside to an ambient
13
degrees the meter's output changed from 0.48 volts to 0.35 volts
!!
I have run out of ideas now and am thinking maybe it's even the
permeability of the die cast box
that's changing or maybe the perspex end
caps of the ferrite rod. . . . I'm getting desperate !
73 Chris
G3XIZ
From: Dave Pick
<[email protected]>
To:
[email protected]
Sent: Monday, 4 May, 2009
16:10:26
Subject: Re: LF: Field
Strength Meter - advice needed
Chris Have you tried a can
of "freezer" spray to identify where the problem lies? Is it the ferrite aerial
or is it the FET amplifier? If you have enough gain then a little negative
feedback on the FET stage may help, add some un-decoupled source
resistor. Dave 'YXM
2009/5/3 Chris Osborn <[email protected]>
LF,
I've just rebuilt PA0SE's field strength meter (LF
Experimenters' Handbook),
modified for use on the 500 kHz band.
I've
followed the published circuit as near as possible considering available
components,
the differences being:
a) I have used a small variable
capacitor instead of the published varicaps.
b) I used a JFET 2N3819 in the
front end instead of dual gate mosfets.
c) Obviously I now use the MW coil
of the ferrite rod instead of the published LW coil.
I have made a
decent Helmholtz coil which gives a stable magnetic field yet the
field
strength meter's output voltage is very dependent on temperature and can vary
up to
30 %.
I would be most grateful for any advice on curing this
problem as I've spent hours trying
to correct it.
73 Chris
G3XIZ
-- G3YXM IO92BK Birmingham UK
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