[time-nuts] Re: Phase Noise Measurement

[Bob Camp]

Hi

It would be a good idea to add a bit of L/C based lowpass filtering between
the mixer output and the op-amp input. It does not have to be anything exotic.
Having RF get to the op-amp can create unpredictable results. 

Since you are looking for noise at the < 1nV / sqrt(Hz) level, decoupling the
bias supply will be a challenge. Getting this done at low offsets ( 10Hz …. 1Hz …)
likely will be very difficult. 

The classic way to check noise floor on this sort of setup is to use a hybrid
network to split one oscillator output into two quadrature signals. They then
go into the two inputs of the mixer. 

If you find your noise floor isn’t quite low enough, any of the mixers in the RPD-1
family will improve things a bit. 

When you go to calibrate the setup with a beat note, You will need a way to 
reduce the gain on your AD797. ( = you don’t want it saturating …).

Bob

> On Feb 24, 2023, at 4:00 AM, Pluess, Tobias via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Dear all,
> 
> for a long time I had this project on my todo list. I recently started with
> building my own phase noise measurement equipment.
> I have a batch of brand new UCT 108663 Oscillators (new old stock
> possibly?) and had the possibility to measure them a while ago on a
> Keysight E5052B analyser. I now want to make my own PLL + LNA to measure
> the oscillators on my own spectrum analyser.
> For this, I built the circuit shown in the image. A SRA-1 mixer is used as
> phase detector. Since I want to avoid a negative voltage rail, I connected
> the IF centre tap to a reference voltage, such that the IF output signal
> swings around VCC/2 and not around ground.
> The amplifier and integrator are configured in the same way. So far, the
> PLL works very nicely. By changing the resistor R3 in the feedback path of
> the first amplifier, I can easily adjust the loop bandwidth. Oscillators
> stay nicely in quadrature. OK, so far this works.
> Now comes the interesting part. The LNA. I would like to use AD797 or
> similar (ADA4897 maybe?). I wonder whether it is, from a low noise
> viewpoint, possible to use the schematic as shown? I have decoupled VCC/2
> with several different capacitors and in theory, this configuration of the
> amplifier should work. But will it in practice yield an acceptable result?
> 
> I would like to avoid a negative voltage rail if possible, because the
> tuning voltage output will need to be positive anyways, and I would like to
> avoid any switching converters.
> 
> thanks for any hints,
> best
> Tobias
> 
> 
> [image: image.png]
> <image.png>_______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe send an email to time-nuts-leave at lists.febo.com