[time-nuts] Sub-ps delay line

[Magnus Danielson]

Hi,

My first thought would be to use a pair of couplers before and after the 
delay line and bring it into a mixer to serve as a phase detector such 
that you can create a control loop to stabilize delay. This way you get 
a handle on the temperature variations.

There is trombone delays that can be used. They seem to reach that level 
of resolution.

There is microstepper boxes, but usually they operate on 5 or 10 MHz.

There is multiple ways to design delays for CW signals, microsteppers 
uses various forms of gear-boxes and programmable generators. Chips 
either use gate delays or programmable comparator vs. ramp of some form.
Ensuring temperature stability and drift limits is however always an issue.

Delay loop oscillator for calibration can be done, but biases can be 
problematic, so a number of different setups needs to be done to build 
confidence. It's a combinatorial exercise which is quite interesting.

Cheers,
Magnus

On 02/07/2017 05:13 PM, Mattia Rizzi wrote:
> Hello,
> I'm looking/designing a sub-ps delay line with very high stability.
> Basically it has microwave requirements on phase matching.
> The main features that such delay line should have are:
> - sub-ps resolution and about 1 ns range
> - High stability, must not drift more than 2ps/year, preferably 1ps/year
> - Temperature coefficient (tempco) below 1 ps/celsius
> - Low phase noise floor, target random jitter below 100 fs RSM from 100Hz
> to 1MHz.
> - flicker noise below -90dBc at 1Hz (100MHz carrier)
> - cheap (below 50 euros) and PCB integrable
> - optional: autocalibration or a way to check calibration health over time
> (checking the oscillation frequency of the delay line connected as loop?)
>
> Operating conditions: The delay line will be used for RF distribution,
> where the clock signals (100-200MHz) must stay in +/- 10 ps error window.
> Since timing jitter (wander) is 1.6ps RMS, the delay line must be very
> accurate, with maximum of +/- 3ps of delay uncertainty. The delay line is
> used to phase-match the  clock outputs at factory, hence will not be
> anymore modified (or for only fine corrections, tens of picoseconds). The
> factory calibration compensates for the delay line and PCB
> process/production variations. The boards will operate at almost same
> temperature and humidity levels over years of continuous running.
>
> Circuit constrains: The delay line is fed with an AC-coupled LVPECL clock
> output (only P output used) and should provides a single-ended AC clock
> output signal.
>
> Indeed, no commercial chip fits into these requirements.
>
> My idea is to use an RC filter to delay the input clock signal and then to
> restore the clock edges with a LTC6957-1 (LVPECL outputs).
> The RC filter would be realized using a varactor (Skyworks SMVA1470-004LF)
> and a 16-bit DAC to control the voltage bias (+ stable voltage reference).
> I already checked the values, and sub-ps resolution seems easily
> achievable. The solution requires a factory calibration due to the
> non-linear behavior of the varactor, but since I only need small
> adjustments, this is not a problem.
>
> The problem is to guarantee the calibration over years of operation.
> Since a femtofarad parasitic capacitance can change the delay, I already
> thought about protecting the delay line with some kind of resin (Epoxy?)
> and/or a RF cage to protect it from dirt and moisture.
> One of the issue is aging. I derived a typical varactor aging from VCTCXO
> oscillators (no varactor manufacturer knows the effects of aging on its
> products, apparently) and it's still good. But the aging of LTC6957 is not
> known.
> Is the PCB fabrication using microwave requirements on the dielectric fine?
>
> Based on your experience, do you think that such delay line can respect the
> requirements listed above, especially stability?
> Am I missing something?
> Thank you!
>
> cheers,
> Mattia Rizzi
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