Bruce Griffiths wrote:
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.
My experience says that it also depends on the relative phase... so it
shifts between approx falling saw to approx tri to approx rising saw to
approx falling saw over a 360 degree beat period. Kind of comforting to
see this shift occurring slowly on the scope.
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.
This should not come as a big surprise, as diodes change their
capacitance with applied voltage.
Long term variations in isolation amplifier output due to temperature
variations may be significant.
I haven't heard of AGC being applied to this particular system, but
their use on stabilizing mixer phase detector gain should be known.
Allowing loop gain to change with input signal strength is not always a
good idea.
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.
I've considered that type of arrangement... it could provide some
opportunities not allowed by cabled interconnect. For reasonable
frequencies much simpler transmission models can be used.
Cheers,
Magnus
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
