[time-nuts] Synthesizers for Rb standards (was: Aging 5065A ?)

[ew]

Most Rb's have a filter prior to the multiplier forcing you to make major modifications to the cell.                                                                                                                                                                                                           Bert Kehren                                                                                                                                                                                                                                                                                                                                                       In a message dated 10/13/2020 2:51:53 PM Eastern Standard Time, attila at kinali.ch writes: 
On Wed, 7 Oct 2020 10:22:43 -0400
Bob kb8tq <kb8tq at n1k.org> wrote:

> People have successfully mounted eBay synthesizers in 5065’s. They work 
> and it is one way to take care of that. 

Some time ago, I had a "quick" look on how I would design
a synthesizer for an Rb vapor cell standard. The way I
came up with is pretty much the same as in [1]. There are
several ways to get to 6.9GHz that mostly differ by the
frequency they start from but are otherwise relatively
equivalent. E.g. my favorite is using a 125MHz oscillator,
then multiply *3*2 = 750MHz, multiply that further *8 = 6000MHz. 
Use 750MHz to feed a DDS generating a 41MHz output, mix that
with 125MHz to get 84MHz, mix that with 750MHz to get 834MHz,
mix that with the 6000MHz to get the desired 6834MHz. This has
the slight advantage that the closest harmonic of 41.7MHz
to  6'834.682MHz is an even harmonic (164) while for 34.7MHz,
as used in [1] it's an odd harmonic (197).

The reason for this "complicated" way of getting up to 6.9GHz
is to use the DDS at a relatively low frequency, which gives
the system a higher resolution and also makes the close-in
spurs easier to filter, if you feel like doing so.

As for spurs in DDS, most of them come from the DDS word being
smaller than thee phase accumulator word. This kind of gives
you a wobling, like in a first order delta-sigma modulator
(which it basically is). Most of the DDS I have seen use a
19 to 20 bit phase->sine core (most likely some CORDIC variant).
The reason for this is that the number of transistors used
(and thus the cost and power consumption) goes up with the
square of the number of bits. And at this level, the INL of the
DAC of the DDS starts to dominate the spurs.

I talked a bit with Claudio about the design of [1] and he
told me that the most difficult thing is getting to above
1GHz with low noise. After that it's easy sailing and the
rest is pretty easy to get good enough. Several of the groups
who do research on Rb vapor cells use the devices built
by INRIM and so far I have not seen anyone reporting that
the electronics are limiting them (this includes some emails
I exchanged with people from UniNE).

If anyone wants to use that design, I am pretty sure the
people at INRIM are happy help you with that.

            Attila Kinali


[1] "Simple-design ultra-low phase noise microwave frequency
synthesizers for high-performing Cs and Rb vapor-cell atomic clocks",
by B. François, C. E. Calosso, M. Abdel Hafiz, S. Micalizio,
and R. Boudot, 2015
https://doi.org/10.1063/1.4929384

-- 
<JaberWorky>    The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.

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