Bob Camp wrote:
Hi
At least from the last time I tried it:
If you use a sine wave input source, it's got to be an amazingly good 10 Hz
sine wave. A normal audio generator will not produce a 10 Hz output with good
enough short term stability / noise to give you useful data. Audio generators
may be out there that will do the job, but I certainly don't have one, and have
never come across one.
Since the output of the mixer is basically a triangle wave, it makes sense to
use that as your test source. A triangle wave also has the nice property that
it's easy on the math. You don't have any approximation issues with the
integers going into the DAC. That shoves the inevitable digital crud higher in
frequency.
When both the RF and LO ports are saturated, the mixer output waveform
depends on how the IF port is terminated.
The output is indeed approximately triangular with your IF port
termination method when both the RF and LO ports are saturated.
With the IF port terminated in a capacitor when both RF and LO ports are
saturated the output waveform is quasi trapezoidal.
When only the LO port is saturated the IF output is sinusoidal.
Another nice thing about a pure digital approach is that it provides a clean trigger for
the "start" channel of the counter you are testing things with. You can even
set up the DAC to put out square waves to see just how good various bits of the chain
are. Tough to do that with anything other than another arbitrary function generator.
I agree that the reference is going to be an issue and that a LED stack may be
the way to go. No matter how you generate the test tone, power supply noise
will be an issue.
The output amplifier on the DAC is my biggest worry. I could go with a current
out DAC and something like an OP-27. That won't give me 1nV/Hz either, but it
will at least be within shouting distance of it. Sigma deltas might be a third
option. I have no idea what their low frequency flicker noise looks like.
Producing a high amplitude (eg 20V pp) output and attenuating it down to
say 2V pp or so typical of a mixer will significantly reduce the noise
due to the output amplifier.
So, other than the noise issue (which obviously needs to be analyzed / tested /
pounded on) any other issues with the approach?
--------
At least from what I have seen in the past, level sensitivity on the inputs shows up
pretty fast in the output "beat note" as you vary the input signals that are
supposed to be saturating the mixer. If they are doing their job, a 2 db level change
produces a very small change in the output. If you have something amiss in that
department, you will see it pretty fast. On that I'm pretty much in agreement with
Rubiola's stuff.
Yes but NIST used a saturated mixer and still found that the mixer phase
shift depended on how hard you drive the diodes.
Long term variations in isolation amplifier output due to temperature
variations may be significant.
Since I intend to mate the isolation amps up directly on the same board as the
mixer, there is no real need for a 50 ohm interface between them. If the mixer looks
like 18.26 ohms, the amp output can be transformed to that level rather than 50
ohms. Everything is matched (over a 1/8" trace) and you don't burn up power in
a bunch of resistors. How well that idea works - time will tell. It's easy to put
the resistors in if it flunks out.
So many things to try ....
Bob
You could also try driving the mixer ports from a highe impedance source
(eg transistor collector).
One early NIST paper advocated this.
Bruce
On Feb 1, 2010, at 9:48 PM, Bruce Griffiths wrote:
Bob Camp wrote:
Hi
Ok, next up on the dual mixer stuff is checking the limiter chain. To do that
with any chance of the results meaning anything you need a good triangle wave.
You certainly can build some pretty complex gizmos to make them. There also
appears to be a fairly simple approach.
If I take a fairly good 16 bit DAC that will accept a clock a bit above 1 MHz, I
can feed a simple count up / count down into it. That should give me a triangle
wave at (clock rate) / 2^32. Simply put, 1.3 MHz data gives me a 10 Hz triangle
wave. The digital crud should be almost entirely up around the clock rate or
higher and> 90 db down. That assumes that the DAC is a low clock feed through
version and that it's got good linearity.
A reasonable dual mixer or heterodyne system should have some kind of low pass
filter built into it. Even a 150 Hz lowpass should knock the digital stuff down
into a -160 noise floor.
The gotcha seems to be flicker noise out of the DAC. There's no guarantee that
the gizmo will have a 1nV/Hz class noise floor. The same sort of audio spectrum
analyzers used for phase noise should be able to measure the noise coming out
under various conditions.
The nice thing about this gizmo is that it does not have to *only* put out a
triangle wave. If you drive it with a micro, you can tell it to do all sorts of
things. You might try a number of DC levels as you check for noise. You might
also try various triangle wave levels to see how everything matches up. Slew
rate limited square waves also sound interesting.
There are a couple of other details like DC level shifting and driving it all
with a decent clock. Both need to be done properly, but they don't appear to be
the limiting factors in this kind of setup.
I suspect this approach has been tried before. Any record of it out there?
Bob
_______________________________________________
Bob
You arent going to find a DAC with a 1nV/rtHz noise floor off the shelf due to
the reference noise.
Heroic filtering measures will be necessary to reduce the reference noise, then
you will have to deal with the DAC component noise which will almost invariably
be greater than 1nV/rtHz.
If it has an external reference capability you could try using a series stack
of leds as in this application the reference tempco shouldnt be too important.
A Josephson junction stack would work as a DAC well with very low noise.
NIST uses such JJ stacks as sigma delta DACs to calibrate the Johnson noise
thermometers.
Why can't you just use a sinewave test source?
Only the part near the zero crossings is of any importance.
Another effect to consider with diode mixers/phase detectors is that at 10MHz
the amplitude sensitivity may be as high as 256ps/dB with both inputs ports
saturated.
Reducing the input port VSWR with a series resistor and attenuator pad can
reduce this effect by a factor of 10 or more.
Bruce
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