[time-nuts] A simple sampling DMTD

Mon Sep 16 22:40:23 UTC 2019

Hi

You are not going to get useful phase noise data out of a normal DMTD ….
The DMTD is targeted at ADEV and similar long term / high accuracy 
measurements. If a DMTD “makes it” to a 10 Hz beat note that is about 
as far as most people take it.

Bob

> On Sep 16, 2019, at 10:41 AM, timeok at timeok.it wrote:
> 
> 
>   Hi Jan,
> 
>   this is precisely the instrument that is lacking at a hobbyist price.
> 
>   It would be excellent to have the possibility of measuring phase noise.
> 
>   Can you anticipate the features of the Sampling DMTD?
> 
>   Can it be used with Timelab?
> 
>   We are waiting for your new ones.
> 
>   Luciano
> 
> 
>   Da "time-nuts" time-nuts-bounces at lists.febo.com
>   A "Discussion of precise time and frequency measurement" time-nuts at lists.febo.com
>   Cc
>   Data Sat, 14 Sep 2019 14:25:48 +0200
>   Oggetto Re: [time-nuts] A simple sampling DMTD
>   Update: I have finished routing the board (placement diagram at
>   http://www.lartmaker.nl/time-nuts/DMTD%20rev1.00%20assembly.pdf ) and
>   ordered a few prototype PCBs.
> 
>   After the earlier discussions on the list I've grown sufficiently concerned
>   about the impact of 1/f converter noise that I have added headers to the
>   board to allow me to replace the D-flipflop sampler with an FPGA-based I/Q
>   downconverter. While the main PCBs are in production I'll draw a simple
>   daughterboard with dual ice40 UltraPlus FPGAs, If the FPGA solution turns
>   out to be necessary (or a noticeable improvement), I'll redraw the main PCB.
> 
>   To be continued,
> 
>   JDB.
> 
>   On Sun, Sep 1, 2019 at 2:09 AM Jan-Derk Bakker <jdbakker at gmail.com> wrote:
> 
>> Dear all,
>> 
>> I've been working on a design for a (relatively) simple, standalone
>> sampling DMTD. Very rough preliminary schematics can be found at
>> http://www.lartmaker.nl/time-nuts/DMTD_rev0.99.pdf .
>> 
>> Design goals are:
>> - ps-level accuracy
>> - comparison of frequencies between at least 10 and 50MHz, preferably
>> between 1 and 100MHz
>> - comparison of (selected) different frequencies (in my case: 10MHz vs
>> 50MHz)
>> - standalone operation, field-portable
>> - option for raw data sampling / (post)processing on a PC
>> - option for generating a tuning voltage to lock the measured oscillator
>> to the reference, so the DMTD can act as a PLL in phase noise test setups
>> 
>> Context: you may remember that a year or two ago I posted to time-nuts
>> about a GPSDO-design geared for mobile applications, which I was working on
>> for an SDR-platform my students are working with. This SDR-platform has now
>> grown to include a 100-channel phased array receiver. To validate the
>> reference clock distribution in this array (amongst other things) I would
>> like to have a DMTD. As the commercial offerings are outside the budget of
>> our lab, I was planning to roll my own.
>> 
>> The core of the system is a transformer-coupled LTC2140-14 dual 14-bit
>> ADC, sampling at an offset frequency of nominally 10MHz+10Hz generated by a
>> VCTCXO (with an option for an OCXO). The ADC was chosen for its large input
>> bandwidth and small aperture jitter. Simulations of a simple software ZCD
>> consisting of a digital filter and least-squares fitting showed that
>> 100ksps would be more than enough to get the desired accuracy. As the ADC
>> design is unable to achieve sample rates lower than 1MSPS, D-flipflops are
>> used to decimate the samples. These DFFs are also used to multiplex the
>> 2x14-bit samples to an 8-bit data bus going into one of the GPIO-ports of
>> an XMega. The XMega runs the ZCD, and generates a tuning voltage for the
>> offset oscillator. Communication to a logging PC is done with a
>> galvanically isolated FT2232H, which has both an ASCII COM-port for the ZCD
>> data and a control interface and an asynchronous FIFO to transfer raw
>> samples. System power comes from the isolated USB bus or a barrel jack; BOM
>> cost in qty10+ is around 100US$.
>> 
>> (The DMTD has a few more power rails than I would have liked. Originally I
>> had planned to use the LTC2295 and have a 3v3-only system, but after
>> re-reading the NIST paper on SDR-as-a-DMTD I concluded that the single
>> clocking path of the 2140 would likely have better aperture jitter
>> correlation between the channels. As a 1.8V/10MHz XMega would only be
>> borderline able to handle the computations, I ended up with this design.
>> LVC logic is used to go from 3.3V->1.8V, LV1T translators for the opposite
>> direction.)
>> 
>> Design decisions and/or non-goals:
>> - I considered putting a small FPGA (specifically a Lattice ice40
>> UltraPlus) between the ADC and the processor. This was rejected because the
>> performance of the decimator appeared to be sufficient, and I wasn't
>> certain that I could get DDR mode + a CORDIC working in this FPGA.
>> - Especially when I found the necessity to move part of the system to 1.8V
>> I considered moving to an ARM. I stuck with the XMega as performance was
>> sufficient, and I am very familiar with both the CPU and the peripherals
>> (particularly time-stamping counters and the Event system) that would ease
>> the ZCD implementation and issues like synchronization between processor
>> and sampling system.
>> - I looked into integrating a phase noise measurement, but could find no
>> easy way that wouldn't degrade DMTD operation in the process. The tuning
>> voltage output is an inexpensive compromise (as I still had a DAC and
>> enough cycles to spare)
>> - The main thing I'm unsure about is the effect of the balun on phase
>> performance wrt temperature and termination matching. I've kept to the
>> baluns as they add less noise than a fully differential amplifier would.
>> 
>> While I've made this design for my own purposes, I would be more than
>> happy to put it under an Open Hardware-license and/or work with TAPR or
>> other parties to get it distributed, should there be interest.
>> 
>> Thoughts?
>> 
>> with kind regards,
>> 
>> Jan-Derk Bakker
>> [planning to start board layout tomorrow; looks like this should
>> definitely fit on a 100x160mm Eurocard inside a Hammond 1455-series box]
>> 
>   _______________________________________________
>   time-nuts mailing list -- time-nuts at lists.febo.com
>   To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>   and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.