[time-nuts] NCOCXO anyone?
nsayer at kfu.com
Thu Jul 21 19:56:08 EDT 2016
Oh my. That’s a bit more than I was originally considering… What I had in mind was adding a DAC front end to an OCXO so that you could tune the EFC with serial commands rather than analog and calling that a product.
A simple version of what you seem to be describing, however, *sounds* to me something like this:
The microcontroller has the same phase discriminator system the GPSDO has, except that instead of the reference signal coming from a PPS, it comes from an input reference. The microcontroller can get a phase difference reading between the oscillator output and the reference and in software can tune the oscillator DAC output to arrange for a certain rate-of-change, adjustable via serial commands.
Does that sound about right?
Perhaps a more traditional PLL approach - using the 4046 PC2 output with an RC and simply allowing the controller to sample that makes some sense (calibrating it may be painful). I’ll have to do some more thinking about it. :)
> On Jul 21, 2016, at 3:39 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
> Hi Nick,
> There may be several threads in the time-nuts archives related to your question. The greater concept is a phase microstepper (aka microphase stepper). Imagine a small board that takes =10 MHz in and puts ~10 MHz out. Using RS232 (or SPI or I2C) you control the phase, or even the phase over time, which is to say, the frequency offset. Maybe do it with analog (EFC). Maybe do it with digital (DDS).
> There are highly-prized commercial instruments that do this. But no amateur has tried yet. You should be the first. If you think about what we do with steering oscillators -- for GPSDO, or for dual-mixers, or for home time scales, or even sidereal or mars time -- having a device that cleanly steers phase and frequency with simple RS232 would be very useful. For example, it would allow anyone to steer a Rb or Cs standard, even though many of these lab instruments do not have analog EFC or digital tuning options.
> The possibility of this at the amateur level occurred to me when I played with Bert's 9913:
> Read especially about the "programmable modulus mode" which can be used to avoid truncation errors and achieve perfect long-term phase; kind of like the difference between PLL and FLL in a GPSDO's 1PPS.
> Look also at how the amazing FE-405 oscillator works:
> And the idea of [mis]using a DDS as a high-resolution phase measurement technique was confirmed with the PicoPak project:
> So, yes, please take the bait and play with all aspects of your NCOCXO idea.
> ----- Original Message -----
> From: "Nick Sayer via time-nuts" <time-nuts at febo.com>
> To: "Chris Arnold via time-nuts" <time-nuts at febo.com>
> Sent: Thursday, July 21, 2016 10:05 AM
> Subject: [time-nuts] NCOCXO anyone?
>> Would anyone see any value in a board that had an OH300 with a serial interface for tuning?
>> I had a thought perhaps to make one starting with my GPSDO and just ditching the GPS part and possibly adding an RS-232 level converter. I could conceivably bring it all out to a DB9 and emulate an FE-5680 (obviously it's long term stability would be poorer without some discipline) or just make my own protocol up.
>> Sent from my iPhone
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