[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[Erik Kaashoek]

Getting the simple PNA to lock was a bit difficult due to the overly
simplistic translation of the mixer output to the Vtune of the OCXO
To get some more flexibility I added a summing opamp that summed the mixer
output with the output of the coarse tuning potmeter. As the summing causes
inversion one extra inverting opamp was added. This made the loop gain
constant
To ensure the mixer is in quadrature another opamp was added that amplified
the mixer output into two LEDs. One LED on when below zero ouput from
mixer, the other on when above zero and both dim when zero output. This
made tuning the coarse frequency simple. Turn till the blinking stops and
both LED's light up dim. The fine frequency potmeter was no longer needed
and the frequency counter is also no longer needed to get into lock
With the summing opamp it is also possible to add an integrator but this
has not been done yet.
Shielding is now the biggest problem as any nearby coax connected to a
10MHz source will cause a huge amount of spurs when not at exactly the same
10MHz
Ultra low noise opamps have been ordered to hopefully reduce the internal
noise of the PNA but the reference OCXO may already be the limiting factor.
The REF voltage output of the OCXO turned out to be rather clean. Much
cleaner than a 8705 voltage regulator
Erik