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

WarrenS warrensjmail-one at yahoo.com
Fri Oct 2 22:13:35 UTC 2009


Yes, there are a few disadvantages using this simple low cost configuration
along with all of its advantages.
You can not get everything for nothing, but you can get higher speed, better
resolution and less noise from this.

> This can not be used to evaluate the stability of an offset oscillator.
Correct,  It can not test osc frequencies that are much different than the
ref osc such as an offset Osc,
BUT one of the many advantages of this configuration is that an offset Osc
is not needed.
This will give freq offset data down to under 1 ms (>1 kHz) without Any
offset osc.

>You need to ensure that the isolation between the 2 sources is sufficient
I have no problem with signal injection at 1e-12 resolution levels, even
with 10811s without buffers
using 10db attenuators and ground Isolation transformers at each Osc output.
Maybe due to the fact that they are locked.
This is verified by adding a slow low level freq modulation on the DUT osc
and plotting the freq change on the reference's EFC.
The phase error can also be changed by adding large or small offsets
at the Loop amp's input to check for any pulling tendency.

From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
> Warren
> 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
> function.
> 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.
> Bruce
> 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

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