Have achieved around 120db with 3 cascaded 2N3904 CB amps.
Something like a BFQ18 broadband transistor with its built in emitter
ballasting may be a better choice.
They work well in a pushpull CB Norton transformer feedback amp to boost
the +7dBm output of an OCXO to +19dBm.
Unfortunately Norton CB transformer feedback amplifiers have very little
reverse isolation.
The next most effective step in improving the DMTD would be to
capacitively terminate the mixer IF port and eliminate the cascaded RC
filter chain at the mixer output.
Bruce
Bob Camp wrote:
Hi
A pair of common base amps will get you to what ever your layout will permit. A
single common base at 10 MHz should get you to at least 60 db with a little
care. If you run a reasonable transistor you can run levels that will fry a 10
or a 13 dbm mixer. Again, all the discrete circuits work pretty well. It's the
modular stuff (especially when driven hard) you have to watch out for.
Bob
On Nov 21, 2012, at 1:43 AM, Bruce Griffiths<bruce.griffi...@xtra.co.nz> wrote:
Reverse isolation at 10MHz is around 43dB.
This can be improved significantly by using a Sziklai pair instead of a single
transistor in each amplifier.
However one could also do this in a CB amp.
Since phase detectors require LO and RF input levels of around 10dBm either
deign should suffice.
NIST have shown that high level mixers appear to be somewhat noisier than
either the ZRPD1 and 10534A.
Whilst their custom mixer utilising diode connected 2N222A's has the lowest
flicker noise the increased PCB board complexity may not be worthwhile for this
application.
Reducing the isolation amplifier noise contribution should be more effective
than any other changes to the DMTD design.
Bruce
Bob Camp wrote:
Hi
A few more parts and likely not quite as much isolation. None of that means it
won't do a good job though.
Bob
On Nov 20, 2012, at 11:20 PM, Bruce Griffiths<bruce.griffi...@xtra.co.nz>
wrote:
The attached circuit is somewhat more efficient and a little quieter.
With a little elaboration lower distortion is possible.
Bruce
Bob Camp wrote:
Hi
Given that common practice is to mis-match the IF port on the mixers, it's
probably not realistic to depend on exact match for isolation. Simple / cheap
common base buffers likely are a better approach. Lots of isolation and not
much flicker noise.
Bob
On Nov 20, 2012, at 9:01 PM, Bruce Griffiths<bruce.griffi...@xtra.co.nz>
wrote:
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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