[time-nuts] Re: Phase Noise Measurement

[Erik Kaashoek]

Hi Tobias,
I can not add wisdom on the low noise opamp but I've build a very 
similar PNA, including the VCC/2 shift of the mixer output, and would 
like to suggest some additions to the schematic I found useful.
1: Make the LNA gain switchable. Unity gain for determining the "gain" 
of the PNA, +30 and +60 dB for real measurements.
2: Add one gain stage driven by the output of U1 that drives two leds, 
one for when above VCC/2, one for when below VCC/2. These will show if 
the loop is not locked
3: Inject an zero to VCC offset voltage from a potmeter through a 
resistor into the  min input of U1 to allow pushing the loop into lock.
Without using a negative rail I could measure down to -155 dBm/Hz, 
sufficient for my needs, so I stopped improving.
Erik.

On 24-2-2023 10:00, Pluess, Tobias via time-nuts 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]
>
>
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