[time-nuts] Low cost alternate to Dual Mixer/DMTD

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Oct 2 20:30:43 UTC 2009

WarrenS wrote:
> 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,
> Have fun 
> ws

You need to ensure that the isolation between the 2 sources is
sufficient to ensure that locking due to unwanted signal injection
doesn't significantly effect the effective VCO EFC to frequency transfer

This method requires that the one of the 2 sources being compared can be
phase locked to the other.
This isn't always possible, for example, if one wishes to evaluate the
stability of an offset oscillator this technique cant be used.


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