Knowing the magnitude of various effects allows one to make rational
choices based on one's actual or perceived requirements.
Just ignoring effects which may or may not be significant and hoping
that they wont be significant isn't particularly rational.
The aim was a system noise floor of around 1E-13/SQRT(Tau) (for Tau up
to a few hundred seconds at least) or better to ensure that this is well
below the expected stability of sources being compared.
There are also other uses for DMTD systems such as measuring phase
instability of cables etc, that some may wish to pursue.
NIST found a mixer phase shift TC of up to 75ps/K for uncompensated
mixers with 5MHz inputs.
If you carefully analyse your favourite frequency comparison system you
may just discover why this particular technique was quietly dropped as
not being particularly useful for Tau much greater than the PLL inverse
loop bandwidth.
The real problem is that there is no simple way to actually determine
the actual Tau range for which such a system is useful other than
comparing the results obtained with those from a more conventional system.
Bruce
WarrenS wrote:
Bruce
OK, Now what am I missing?
I understand there are 'Freq nuts' and just 'plane NUTS', this sound
like the later.
With a little effort, not hard to hold the temp drift rate change to
1deg / hr (and a couple deg change altogether).
Lets say the Tempco balance of the mixers was as bad a 1ns / deg
That means temp change would add an addition freq error offset term of
half of 3e-13 at tau of 3600 sec (36ns/hr = 1e-11)
Even this gross amount would not effect the results of most of the
things that any of these freq nuts would be testing including probable
the best CS or RB Osc.
AND if they are doing 1e-15 things, they probable want to be hold the
temp to better than 0.01deg for many other reasons as well.
So why even talk about TC phase shifts at the ps / deg level? Who
cares and who needs it?
Does not sound like you are doing the Nuts any service to address
things that are orders of Magnitude below usefulness.
ws
**************
----- Original Message ----- From: "Bruce Griffiths"
<[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Sent: Tuesday, January 26, 2010 1:51 AM
Subject: Re: [time-nuts] DMTD Question
If one carefully the value of a resistor in series with both the
mixer RF and LO ports and adds a 6dB attenuator between the series
resistor and the output of the associated isolation amplifier its
possible to reduce the mixer phase shift tempco by a factor of 10 or
so. The resistor value is chosen to minimise the mixer port VSWR for
the given IF port termination.
At 5Mhz this can reduce the mixer phase shift tempco to less than 5ps/K.
The mixer phase shift tempco also reduces as the input frequency
increases (at least up to 100Mhz or so) by a factor equal to the
ratio of the frequencies for which the phase shift tempco is being
compared.
Thus carefully matching the mixer inputs can reduce the required
mixer temperature stability.
It is believed that the matching reduces the effect of reflections in
the input cables, so a similar effect may be achieved by using an
isolation amplifier located close to each mixer input port.
Bruce
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