[time-nuts] Oscillators and Ovens

[jimlux]

On 11/1/17 10:59 AM, Attila Kinali wrote:
> Hi Jim,
> 
> On Wed, 1 Nov 2017 06:17:31 -0700
> jimlux <jimlux at earthlink.net> wrote:
> 
>> That's why I wish they'd sell OCXOs, cheap, without the oven. Or maybe
>> look for regular XO (no TC).  Those might have a more "pure" (read lower
>> order) freq vs temp characteristic.
> 
> 
> It feels like I have asked this before, but I cannot remember and
> cannot find the mail....So: What do you mean by "OCXO without oven?"
> 


I often have requirements for "good phase noise" but no particular 
requirement for "good temperature stability" or Allan Deviation for tau 
 > 200 seconds - we get frequency knowledge from other sources (e.g. 
just like a GPSDO, or it can be inferred from the measurement)
I often have a system where the environment is pretty benign - a typical 
on-orbit temperature variation might be a degree or two over 90 minutes 
(or longer), if that - we don't know what the temperature will be (in 
advance), but once it's up there, it's pretty stable.  Maybe the temp 
changes a bit due to relative orientation to the sun and similar 
effects, so there's a annual variation.  My current spacecraft is 
expected to change maybe 5-6 degrees over the year.

For instance, in a software defined radio, the input oscillator is often 
fed into some sort of NCO or DDS for tuning - knowledge of the frequency 
is what you really want, because the ultimate requirement is on the 
frequency accuracy at the input or output of the radio - the regulators 
care not a whit what kind of oscillator is inside (well, they DO ask on 
the license application, but it's really not relevant)

THere are also systems like Doppler radars - they need good pulse to 
pulse stability, and low phase noise to avoid reciprocal mixing 
degradation of the noise floor - but they care not what the exact 
frequency is.

In general, OCXOs have crystals with high Q -> low phase noise, 
especially compared to a TCXO, which *can't* have high Q, or the 
temperature compensation circuit can't do it's work.

So, it would be nice to have a *cheap* lowish power packaged part that 
has the Q of an OCXO, but without the power consumption of the oven 
(typically measured in watts).

yeah, I'd be operating it *way* far from the optimum turnover temp, so 
the tempco might be huge (in oscillator terms), but I don't really care 
- in fact, that might give me a way to measure the temperature of the 
system.

I have gotten quotes for OCXOs with the oven disabled-  but that's 
making it a custom part and as Bob has pointed out, the moment you 
deviate from the "catalog part", the cost (and often more importantly, 
the delivery time) goes up.

I think the ideal, of course, is to get an oscillator high performance 
crystal cut with the turnover temp around 20-30C - useless for a OCXO, 
but might be real useful for GPS/temp compensated oscillator where the 
oscillator drives a DDS (or or provides an output giving the estimated 
frequency).