[time-nuts] 5065A phase-noise

[Magnus Danielson]

Fellow time-nuts,

It was brought to my attention that I should have some illustrative
plot, so therefore I attach it here.

Let me walk you through the plot.

First of all, I use the cross-correlation technique of the TimePod, so
the Notes contains list of which references I used. While you can
measure under the references, further down the harder it is to get away
from the coloring and get decent precision. That is why I have been
measuring through potential sources to come up suitable sources. Which
is best depends on what measure one uses, so for ADEV stability other
sources may be better. For phase-noise measures, I currently prefer the
OSA8600 and OSA8601 pair that I've used before. Both being AT-cut BVAs.
The EFOS10 match up to these when pulling the serial link.

You find the OSA8600 trace with EFOS10 and OSA8601 as reference as the
green trace. It is in the plot as reference and you see that it is
quieter most of the times, except in the far out noise where it is about
the same, so we are not really challenging the measurment there.

I have three 5065A, so I denote them 5065-1 and 5065-2 just to keep them
apart. The 5065-1 has the 10811 based 05065-6097 retrofit, while the
5065-2 has the traditional 00105-6013 oscillator.

The 5065-1 trace is the blue trace. As we walk it from the high
frequencies you see that it has unusually high white noise floor, which
I noted my posting. It stays flat until we come down to about 20 Hz, at
which times it turns upwards and then goes into a white frequency
modulation as it gets captured by the control-loop of the rubidium, we
would otherwise expect a flicker frequency modulation there, more on
that further down.

The 5065-2 trace is the pink trace. As we walk it from the high
frequencies it has an OK far out noise, about the same as the BVA. At 1
kHz it folds up into the flicker phase modulation which continues at
about 10 Hz into a white frequency modulation.

The 5065-2OL trace is the red trace. This is the 5065-2 turned into
open-loop, at which time you have the EFC locked and the oscillator and
output buffers is what you measure. As we walk it from the high
frequencies, we have exactly the same properties as when the loop is
closed until we come down to 10 Hz where you see the traces separate and
the OL variant goes into flicker frequency modulation.

The 5065-2OL trace is an illustration of Leeson model, and it seems like
the Leeson knee is about 10 Hz. The 5065 loop bandwidth also seems to be
about 10 Hz. However, the slope is confusing as it is kind of halfway
between flicker phase modulation and white frequency modulation.

Now, while these measures is performed not on the oscillator itself but
oscillator and output buffers, unless something is wrong with output
buffers on either box, the output buffer noise seems to support good
enough flicker and white noise to support the output of both
oscillators. Then running with that assumption it seems like the
00105-6013 has excess flicker noise issue while the 05065-6097 has
excess white noise.

The 05065-6097 has a really crap square squaring that may be the
explanation for the white noise behavior. I'll see if I can't tap the
pure 10 MHz and measure that, it should be better.

So, I hope this illustration was helpful.

Cheers,
Magnus

On 2020-01-12 04:15, Magnus Danielson wrote:
> Fellow time-nuts,
>
> As I have been measuring this, I've been surprised by the behavior
> differences of my 5065A rubidiums.
>
> One of them has a modernization so the 00105-6013 oscillator is replaced
> by a 10811 oscillator with associated support-board. This retrofit is
> known as 05065-6097 [1].
>
> Now, the phase-noise plots show two main things:
>
> For the 00105-6013 assembly, the far out noise-floor is around -154 to
> -155 dBc. This fine. However, at around 1 kHz is the 1/f flicker noise
> corner, providing considerable noise. I have measured both with closed
> and open-loop, but the noise is the same, so this comes from the
> oscillator. The loop opens just at the EFC input of the oscillator. The
> transition to 1/f^3 corner comes further down, but not really bad
> location, the flicker phase sticks out.
>
> So, then for the replacement kit, the 10811 is really good, now isn't
> it? Well... turns out that the wide noise is around 12-13 dB worse than
> the old 00105-6013, it's only as one gets to the closed in noise that it
> raises up, and it is quite respectable. I've not measured this open-loop
> yet.
>
> So, none of them is stellar in the phase-noise department.
>
> I've noticed that the 0565-6097 has a rather crude adaptation of the
> sine into the TTL input of a 7474 flip-flop setup to act as divide by 2.
> You have a series inductor and capacitor into a voltage divider-chain
> that sets the midpoint. I am not quite sure that this provides good
> noise properties as it squares up.
>
> I've not spotted likely reasons for problems yet in the 00105
> oscillator, most transistors seems to have pretty OK emitter
> degeneration which should help to keed flicker noise down.
>
> Have a look at ponder over these things.
>
> Cheers,
> Magnus
>
> [1]
> http://www.ko4bb.com/manuals/HP_5065A_A10_quartz_oscillator_replacement_kit_1981_restored.pdf
>
>
>
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