[time-nuts] Sub-ps delay line

Mattia Rizzi mattia.rizzi at gmail.com
Tue Feb 7 11:13:43 EST 2017

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
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!

Mattia Rizzi

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