Thanks again, I am well out of my comfort and knowldge zone, all I can say is I am told I need to build just the phase meter section of the circuit in the .pdf file, and instead of the meter, (or possibly as well as the meter, unsure of that...) I feed where the meter is / was into the control circuitry. Another poster on a UK forum has commented on the instability potential, when the circuit deviser posted his schematic, by saying:
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."
So I am certainly taking heed of all these warnings!
The UK poster went on to say:
"Actually, looking again at the circuit, it is not necessary to use an opamp in that position to define the mid rail for the comparator reference. Just two equal; value resistors and one electrolytic decoupling the junction to ground (Definitely not two caps to top and bottom in the original configuration shown ... think about it...)
You only need an opamp to define a stiff mid rail voltage where there is likely to be any significant current draw. Or many stages need to be referenced to it - such as a multi stage filter. For single stage configurations, just two resistors and a bit of decoupling is enough".
So I think I will be breadboarding it less the first op-amp bit and less one of the 10uF caps, if you are in agreement?
Thanks again everyone.