Re: LF: Automatic variometer circuit

[Andy Talbot <andy.g4jnt@gmail.com>]

Can I warn against one feature shown in the circuit diagram 

The two 10uF electrolytics connected from the output of the first opamp to the supply rails should not be there.   They cause instability as opamps are not designed to drive capacitive loads and frequently go unstable.

If an opamp is used to deliver a mid voltage rail like this, the temptation to add extra decoupling should be avoided.   The opamp output gives a very low source impedance  that will not benefit from any extra decoupling.

If you do not get nay instability using that circuit as it stands, then you are very lucky  - or perhaps it is oscillating and not noticed 

I recently fell into this trap quite accidentally.   It is so "natural" to want to  decouple supply rails with capacitors placed everywhere  that I placed several 100nF caps on a PCB without thinking.  All of them had to be removed to stabilise the resulting circuitry.

Andy  G4JNT

On 29 October 2015 at 21:09, DK7FC <selberdenken@posteo.de> wrote:

Hi Chris,

Am 29.10.2015 21:26, schrieb Chris Wilson:

[...] albeit my loading coil needs manual
tweaking  to  move to the right part of the band. Not sure of the best
way  to  remote  tune it, it would be nice to have the ability to move
about  the  band  without  running  outside in the dark. I may start a
thread about what alternatives there are, thanks again Erwin
   

I'm using a motor driven variometer which uses the phase meter circuit of the M0BMU LF tuning meter. It is working well on LF, where my antenna resonance is so sharp that i have to re-tune when doing QSY by 7 Hz.
See:
https://dl.dropboxusercontent.com/u/19882028/LF/LFtunemeter.pdf
https://dl.dropboxusercontent.com/u/19882028/LF/DK7FC%20Automatic_variometer_tuner%20.jpg
https://dl.dropboxusercontent.com/u/19882028/LF/automatic_variometer_board.jpg
https://dl.dropboxusercontent.com/u/19882028/LF/LF_variometer_drive_engine_arrangement.jpg
https://dl.dropboxusercontent.com/u/19882028/LF/phase_meter.jpg

2011 was a nice LF year for me!! :-)

73, Stefan