[time-nuts] ECS ECOC-2522 (was GPS Talking Clock)

Nick Sayer nsayer at kfu.com
Mon Mar 5 12:36:01 EST 2018


I’ve built two GPSDO units now with this OCXO. For the first one, I fed the DAC (AD5680) from the oscillator’s reference output. This resulted in a very poor (compared to expected) short term ADEV result (1-2 E-11). There was a lot of noise (something like 5 mV P-P) on the reference output. Also, rather oddly, the reference voltage was something like 2.8v. The datasheet doesn’t say what the expected voltage is, but if you were to use that as your DAC reference, you’d be throwing away the top 20% or so of the tuning range.

For the second unit, I built it with the same circuit I use for the CW OH300, which lacks a reference output. There, instead, I use an NCP51460 precision regulator for a reference. There, I achieved a short term ADEV much closer to expectations. It’s just under 6E-12 at tau 1-5s. ECS claims it ought to be closer to 3, but it’s entirely possible that my reference (Thunderbolt) and/or counter (53220A) are contributing error, or that my design has some other noise contribution I haven’t yet found. Still, it’s at least in the ball park. And the oscillator hasn’t even yet been running for 24 hours, so it may get better with some time (I’m a little skeptical about wear-in helping low-tau ADEV though. Doesn’t that usually operate on longer term drift instead?).

In both cases, the reference was bypassed with a 10 µF and 0.1 µF chip cap adjacent to the DAC. Both power supplies had similarly low levels of noise and ripple. The worst that you could say about either was the amount of blowback from the oscillator itself was far higher than the input noise.

Of course, sample size here is 1 each, so it’s possible that the first one was just a dud. But the theory that the noise on the reference is FMing the output fits the observations.


> On Feb 18, 2018, at 7:54 AM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
> 
> No they don’t. I wrote and asked them and they sent me back some sample data. They were a pretty pleasant surprise.
> 
> <ECOC-2522-10.000-3-F-C Allan Deviation.pdf>
> 
>> On Feb 18, 2018, at 6:04 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
>> 
>> Hi
>> 
>> 
>>> On Feb 17, 2018, at 11:01 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
>>> 
>>> It’s been a while since I’ve posted here, but I’ve had a bunch of irons in the fire. I’m working on adapting my GPSDO to the ECS ECOC-2522, which the manufacturer claims has a short term ADEV in the low -12s, but I haven’t gotten it doing that well yet.
>> 
>> At least on this data sheet: 
>> 
>> https://www.ecsxtal.com/store/pdf/ECOC-2522.pdf
>> 
>> They don’t say much of anything at all about ADEV. OCXO’s in the little packages are rarely super stars when it comes to ADEV.
>> 
>> Bob
>> 
>> 
>> 
>>> 
>>> But one thing that is ready (well, electronically it is - I’m still working on the laser cut case for it) is my GPS Talking Clock.
>>> 
>>> The story is that I called the USNO time number at midnight on New Year’s Day, but the wife noted that it was the wrong time zone. That got me thinking, and I wound up designing a GPS driven simulacrum.
>>> 
>>> It’s an ATXmega32E5 with the usual Venus838 timing module and a µSD card slot. The card is loaded with audio samples that the 32E5 plays back through its DAC. I got double-buffered DMA to work to feed the DAC, so audio playback is a largely background task. I just have to fill the buffer with the next block from the file every ~30 ms or so. The ticks and beeps are generated from an on board 1 kHz source and are turned on by a PPS ISR, so they’re as accurate as possible. The whole thing is basically as accurate as an aural clock can be - the latency induced by the speed of sound has far more impact than anything else.
>>> 
>>> While the audio is turned off, the clock can also do Westminster Quarters (or any other chime you wish to load in).
>>> 
>>> The µSD card is FAT formatted and the audio sample files are easy to make with ‘sox’ (raw, 1 channel, 8 kHz, 16 bit little-endian, unsigned), so there’s no reason you can’t substitute my voice with your own, or make your own chimes.
>>> 
>>> It’s available at https://www.tindie.com/products/nsayer/gps-talking-clock/
>>> 
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