The details about matching (if any) used in the Czech DMTD would be
informative.
To avoid degrading the performance of the DMTD system below that imposed
by the mixers any isolation amps used will need a flicker phase noise
floor below that of the mixers.
Even an opamp based isolation amplifier can be at least 10dB quieter
(for offsets of 10Hz and below) than a typical minicircuits RF amp.
This is still about 10dB or so worse than a good mixer.
A well designed low gain isolation amp built with discrete transistors
can have significantly lower additive phase noise than an opamp.
To reduce the DMTD system noise one can either:
1) Carefully match all ports using series resistors, pads etc as
necessary to achieve the required isolation together with a high output
low flicker phase noise amplifier to drive the splitter
2) Use isolation amplifiers with very low flicker phase noise.
Some isolation between the 2 RF inputs of a DMTD is usually necessary to
avoid injection locking of the 2 sources being compared.
Bruce
Bob Camp wrote:
Hi
Those isolation numbers are *highly* dependent on very good matching at all
ports. That's rarely the case unless you have a bunch of pads running around
the system.
Bob
On Nov 20, 2012, at 5:49 PM, Bruce Griffiths<bruce.griffi...@xtra.co.nz> wrote:
Typical Minicircuits SMT RF amps have a phase noise at best 20dB worse (@10Hz
offset) than the mixer/phase detector.
Their reverse isolation is quite low (<<40dB)
The principle reason that the Czech DMTD has such low internal noise is due to
the absence of any isolation amplifiers.
They use the outputs of a 2 way splitter to drive the LO inputs of the mixers.
A output to output isolation of 40dB or more at 10MHz is possible with some
minicicuits splitters (e.g. SYPS-2-1).
The ZRPD1 has an RF1 - RF2 isolation of around 70dB at 10MHz.
With a channel to channel isolation of around 110dB for a 2x ZRPD1 + Splitter
combination isolation amplifiers may not be necessary.
Bruce
Bruce Griffiths wrote:
Since mixer noise is one of the limiting factors using a mixer with low flicker
noise will help.
NIST found that a custom mixer using diode connected (collector base short)
2N222As had a significantly lower flicker phase noise than either the ZRPD1 or
the 10534A.
They used off the shelf 1:5 impedance ratio transformers (probably from
Minicircuits).
Another issue is the flicker phase noise of any isolation amplifiers used.
This is particularly critical if each mixer uses its own isolation amplifiers.
My current amplifier phase noise measurement setup (for measuring the additive
PN of a pair of well matched amplifiers) has a self noise of around -170dBc/Hz
@ 1Hz offset for a 10MHz input.
Ideally the additive phase noise of any isolation amplifiers should be well
below that of the mixers.
Bruce
ewkeh...@aol.com wrote:
Yes Bruce I have the paper. I am not suggesting to copy it verbatim but if
there is a way to reach reasonable priced 1 E-14 members of the list should
pipe in. I am willing to do an other board. the rest of the systems well
on its way. Einally after three years.
Bert Kehren
In a message dated 11/20/2012 3:28:57 P.M. Eastern Standard Time,
bruce.griffi...@xtra.co.nz writes:
ewkeh...@aol.com wrote:
The D/M is being revisited because of the counter performance. 1 E-13 is
easily attainable but the Czech IREE published a paper and claim 2 E-15.
Do you mean the paper ""optimization of dual-mixer time-difference
multiplier" ?
The ZCD developed in this is a bit of a kludge and is far from optimum.
Reverse engineering the circuit from the description given in the paper
isn't too difficult.
They claim an instrument limited ADEV of ~7E-15 @ 1s.
Do you have a copy of this paper?
Bert Kehren Miami
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