[time-nuts] Re: FS740 Thoughts?

[Bob kb8tq]

Hi

I have a 740 as do others on the list. The big gotcha is the OCXO in the device.
It has various quirks that limit the performance of the device. The ADEV plot they
show for the OCXO is a “sometimes / maybe / might” sort of thing. One of the 
“get to it eventually” projects here is to see if it can be replaced with something 
that has better ADEV performance. 

Be wary of folks showing up in dirt parking lots in the middle of Montana with one
in the back of their car …. :) Doubly so if the car shows signs of having hauled 
a dozen Cs standards up various mountains….. If they are named Tom, simply 
give up and accept that once you see one you will eventually buy one… 

The process goes sort of like: Gee, that’s a lot of money. Later on, … I need a few
more standard lines at this or that frequency … Then it goes to If only I could come 
up with *that* signal … hmmm … 740 would do that … hmmmm…. off you go to 
get one. 

It’s a handy box to do a variety of things with. The only issue I’ve seen is the OCXO
performance. 

Bob

> On Nov 17, 2021, at 12:50 AM, Matt Huszagh <huszaghmatt at gmail.com> wrote:
> 
> Hi,
> 
> I'm curious to hear people's thoughts on the SRS FS740
> (https://www.thinksrs.com/products/fs740.html) as a lab frequency
> standard. The phase noise performance of the OCXO and Rb options seem
> quite good (the same specs apparently apply during GPS disciplining too)
> 
> | offset |   TCXO   | OCXO/Rb  |
> | (Hz)   | (dBc/Hz) | (dBc/Hz) |
> |--------+----------+----------|
> | 1      |   -90    |   -108   |
> | 10     |   -104   |   -130   |
> | 100    |   -125   |   -144   |
> | 1k     |   -142   |   -143   |
> | 10k    |   -147   |   -148   |
> | 100k   |   -149   |   -149   |
> | 1M     |   -153   |   -153   |
> 
> as does the Allan deviation (during GPS disciplining)
> 
> | time interval (s) | TCXO  | OCXO  |  Rb   |
> |-------------------+-------+-------+-------|
> | 0.01              | 1E-10 | 2E-11 | 2E-11 |
> | 0.1               | 4E-11 | 4E-12 | 4E-12 |
> | 1                 | 3E-11 | 2E-12 | 1E-11 |
> | 10                | 1E-10 | 2E-12 | 6E-12 |
> | 100               | 6E-11 | 2E-12 | 1E-12 |
> | 1k                | 6E-12 | 3E-12 | 1E-12 |
> | 10k               | 6E-13 | 6E-13 | 3E-13 |
> | 100k              | 6E-14 | 6E-14 | 3E-14 |
> | 1M                | 5E-15 | 5E-15 | 3E-15 |
> 
> (I used the typical performance plots for the phase noise and Allan
> deviation data)
> 
> The OCXO does not exhibit the the hump around 2s seen by the rubidium
> oscillator option (PRS10) and therefore could be seen as better in
> applications not requiring the better holdover performance by the Rb.
> 
> It also provides a pulse output with jitter < 50 ps RMS that could be
> used as a PPS signal. It can also provide a low noise 100 MHz output
> signal and can act as a frequency counter and DDS with high resolution
> and accuracy. And it comes with the schematics! But.... it's definitely
> not cheap. Other thoughts/opinions?
> 
> Matt
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