[time-nuts] Low cost alternate to Dual Mixer/DMTD
warrensjmail-one at yahoo.com
Fri Oct 2 17:39:06 UTC 2009
I have been using a simple low cost, high performance alternate solution to the standard Dual Mixer/DMTD.
The idea is based on an analog version of NIST's "Tight Phase-Lock Loop Method of measuring Freq stability".
http://tf.nist.gov/phase/Properties/one.htm#oneone fig 1.7
By replacing the "Voltage to freq converter, Freq counter & Printer with a PC data logging DVM,
It was simple enough to be up and running from scratch in minutes,
and the best part, it cost me nothing because I already had the four main parts that are needed.
When a high resolution data logging DVM is used you don't need the offset voltage.
To get better performance which seems to exceed most DMTD for low tau numbers, it takes a little more work
and the use of a higher speed oversampling ADC data logger and a good offset voltage.
I'll also add that this is not a popular solution,
but as far as I've been able to determine it is the BEST SIMPLE configuration,
IF you know analog and have an HP 10811 osc to use for the reference.
I've attached a Basic modified NIST Block Diagram showing what I made:
The NIST paper sums it up quite nicely:
Using this configuration, it is not difficult to achieve a sensitivity of a part in 1e-14 per Hz resolution of the frequency counter,
so one has excellent precision capabilities with this system.
(I'm achieving well under 0.1 ps Phase resolution, and 1e-12 at Freq resolution with 0.1 TC, limited by my noisy reference)
Note that the logged data is in Frequency and not Phase.
I have found Ulrich's Plotter program great for doing the ADEV graphs
As always, Negative criticism welcome,
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