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Old 24th Oct 2021, 11:18 am   #25
Radio1950
Hexode
 
Join Date: Apr 2017
Location: Buderim, Queensland, Australia.
Posts: 428
Default Re: AD8307 probe-How is it used?

At the risk of being verbose and over gushy.

Reference my older question, the adding of two “unrelated” sine CW signals, the correct algebraic vector sum result will only be apparent on test equipment when the two signals are tending to be “coherent in frequency and phase”.

I know that’s somewhat tautological, but explains the situation.

This is probably what your instinct tells you, and is reproducible in the home workshop.

If I feed the 0 dBm 5 MHz signal, and 0 dBm 6 MHz signal, into an adder (not a mixer) and measure the added output on my AD8307 meter, I “measure” - 6.0 dBm, due to the loss in the adder (the hybrid combiner) and due to the two signals being so far apart in frequency.

The added power algebraic sum is actually occurring, but at a rate of the frequency difference, ie 1 MHz, and is not visible on the meter.

If I then adjust the frequency of the 6 MHz sig gen down, approaching 5 MHz, that rate decreases.

No change in the measurement is observable on the power meter until we get very close to 5 MHz, within a few Hz, when you notice the AD8307 meter starts to vary between a low figure (looks about -20 dBm) and about 0 dBm.

The “low” figure is theoretically much lower, but, visibly, meter and circuit response and hysteresis is at play; similar results with the ‘high”.

I cannot get exact coherence because I have two different sig gens.

If I could have it, and have a zero loss combiner, the power sum should be +3 dBm, 2mW, and if I could change the phase of one sig gen to be exactly 180 deg away, from the other, but on frequency, the power sum should be minus infinity, but in practice will be probably about -60 dBm.

I hope that explains it better.

Conversations tend to make assumptions with those directly involved, to the detriment of casual readers.
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