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

[Bob kb8tq]

Hi

(see below)

> On Jul 9, 2022, at 12:06 PM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> 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

Putting some sort of “gain switch” on the summing amp can help in getting the loop gain to the point it is usable. 

> 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

That sounds right. 

> With the summing opamp it is also possible to add an integrator but this has not been done yet.

Typically a simple roll off cap on the feedback R is about all that is done. 

> 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

Terminating unused devices “at the socket” is often the only way to keep things reasonable.

> 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.

Even a “not so fancy” op amp should do pretty well. The big deal is to get to dual supplies 
with a fairly high voltage on the first stage. 

> The REF voltage output of the OCXO turned out to be rather clean. Much cleaner than a 8705 voltage regulator

The Ref voltage likely also supplies the oscillator. It can be 20 to 40 db “noisier” than the phase 
detector output and have little or no impact on the oscillator performance. Yes, there likely is some
filtering between the regulator and the oscillator ….

Bob

> Erik