Testing a Distribution Amplifier


Below is a copy of my email lab notes to John while testing his new RF distribution amplifier.

0 - Photos


Since this is now on a web page I thought I'd
send you some photos of the setup I'm using.

The first photo shows your board as I'm testing
it. My 5 MHz house reference comes in on the
right as the gold right-angle SMA. For the two
outputs on the left I used high-quality SMA-BNC
adapters to connect to the pair of mil-spec 10
foot cables than run over to the analyzer. I had
a pre-wired red/black molex cable for the power;
how convenient.

The two thin blue wires you see over the board
are the thermocouples I used to keep an eye
on variations of ambient and board temperature.

The second photo shows the board on top of
an Agilent 6612C power supply set to 12.000 V.
In the foreground, unrelated to this test, are a
bunch of MTI oscillators I'm warming up.

The third photo shows the TSC 5110A which
is what I used for most of the testing. It has
a time interval resolution of around 100 fs.
There's some other random T&F gear above
and below it too. In the lower right corner you
can see a glimpse of the blue/purple glow of
a passive H-maser.





1 - Noise output-output


First results are really good. Very clean.

Attached is a plot from a sort of noise floor test
where I put a nice 5 MHz in and then compared
the two outputs against each other.

The first plot is scaled 0.1 ps per division vertical
and one minute per division horizontal. The room
A/C was cycling about every 10 minutes and the
PCB temp varied from 80 to 82 F during the cycles.

The plot shows less than 0.3 ps variation over a
temp change of about 1 C. The second plot is
the same thing with the scale set to 1 ps/div.

So this means the channel-to-channel tempco
is under 1 ps / C.

The next test is input to output noise floor.




2 - Noise input-output


Here are two plots for the input-output case in
the same scale as the previous two plots.

A little more noise, a little more tempco, but
still fantastic.

I may want to work on these tests some more
since this is getting close enough to my lab
noise floor that I need to be careful about a
number of things, like the source, the splitter,
connectors, cables, the airflow, etc.

The ADEV of this thing is 1e-14 at 10 s so
it will work for any frequency standard that
any time-nut wants to through at it. You've
got a winner here.

Next, some load and voltage tests.




3 - Output load effects

Attached is an input-output plot showing the
approximate effect of open, short, and 50R
load on the other output.

The sequence, spread over several minutes,
is open, 50R, open, short, open.

The results are quite stunning. I would have
expected much more effect. Instead there is
just 8 ps total shift from open to short.

You can also see a slight, slow, temporary
drift after a load change; probably thermal.

At 12.00 V the current draw is 37.3 mA (open),
46.6 mA (50R), and 56.9 mA (short).



4 - Supply voltage effects


Attached is an input-output phase plot
showing the approximate effect of supply
voltage. I made a change every minute
or two. For this test the second output
was 50R terminated.

The sequence is 11V, 12V, 13V, 15V, 11V

This is quite nice. I'd say there is no need
for any external regulation. The phase shift
gets very bad down at 10V so 11V is pretty
much the minimum. Above that it's less than
1 ps / V.



5 - Status


All the tests were done at 5 MHz. It's hard to
imagine getting different results at different
frequencies but I'll leave that as an "exercise
for the reader".

I can't test phase noise right now since the
instrument is put away. So it'll wait for later
or someone else can run that test.

Someone else can run it through a network
analyzer and make a nice phase/gain plot.

I didn't experiment with the effect of input
levels at all. The 5 MHz feed was generous;
about 2.2 Vpp (just over 10 dBm, I think).

Now, there are quite a few commercial T&F
RF distribution amps out there. Most seem
much more complicated than yours. So the
question is, why? Do they know something
you don't? Are there other pertinent specs
that you don't address? Is it just because
most of them use parts from 20 years ago?

When I have time, I'd like to run some of these
commercial units through the same tests I just
did on yours. I probably have a dozen different
brands and models to try. A project for later.

Trying larger temperature ranges is something
to check. I'd have to dig out my insulated, one
cubic foot, PID-controlled, Peltier box for that.

It was convenient to measure in my lab today
since the A/C just happened to be cycling every
5 to 10 minutes slowly changing the air flow by
2 F peak to peak.

Anything else you want me to check out tonight?
Otherwise I'll start a 24h run and look for longer
term drift in the board.


6 - Longer-term tempco and drift


Here are two input-output phase plots. The
first is a 30 minute view of the A/C effect on
the board. It doesn't show anything different
from the plots last night but I thought you'd
like this because Stable32 chose to label
the axis in units of femtoseconds and that
looks kind of cool.

The second is an 8 hour run last night with
two things to note. First, there is a drift on
the order of 5 or 6 ps during that period. The
previously measured tempco of 1 ps/C
suggests this overnight drift is not likely
temperature related. It could be cables or
mechanical stresses reaching equilibrium
in BNC connectors. Or humidity slowly
affecting cable dialectric. Another likely
cause is new components settling in. I
really have no idea. If I were to look into
this it would take days or weeks of data
to look for trends, etc. But you should also
know this is quite common; lots of weird
stuff happens at the ps level in a home lab.
And the rate of drift, say 20 ps a day, puts
this "noise" down in the in the -16's so not
to worry.

The other thing to note in the plot are the
spikes. This is less common, and often the
result of less than perfect connectors, or
cables, or solder, or kids playing next door,
or whatever. I didn't see this on my shorter
runs earlier. I normally would solve these
problems, or re-run the data, or just toss
the data out, but in this case I wanted you
to at least see it just to get a feel for life in
the picosecond world.




7 - Latency measurement


One more result for you -- the latency of the
dist amp at 5 MHz is about 1 ns.

I got this by measuring A, measuring B, and
then replacing the board with a 1 cm SMA
male-male adapter but keeping everything
else the same.

Phase plots attached. The raw data is:

mean phase, channel A: -30.718 ns
mean phase, channel B: -30.675 ns
mean phase, jumper: -29.772 ns

So the board delay at 5 MHz is about 0.9 ns.

It also looks like the channel-channel phase
difference is on the order of 50 ps.





8 - Jitter animated GIF


I also realized that the previous plots can be
used to graphically show the added jitter of
the dist amp. So attached is a 3 frame 1 fps 
animated GIF showing the reference signal
going through channel A, channel B, and the
board-bypass shunt.

I set the scale to be the same in each case;
2e-13 = 0.2 ps / div vertical, which means it's
1.6 ps full-scale.

The message here is that this board is really
low noise.

The plot also serves as a verification that the
test rig itself exhibits some noise and tempco
but that its noise and tempco are about 10x
less than the dist amp. This gives me some
confidence that the measurements I've been
making the past day are valid.



9 - Precise tempco measurement


I wanted to measure the tempco a little more
precisely so I used a logging 0.01 C resolution
thermometer placed near and above the board.

MJD timestamped readings were made every
10 seconds and these are plotted below in red.

TSC 5110A time interval readings were averaged
every 10 seconds and these are plotted in blue. 

The lab thermostat and summer weather cause
the A/C to cycle every 5 or 10 minutes. Below is
a one hour plot showing both the board latency
and the ambient temperature tracing sinusoidal
curves; the correlation between the two is very high.

For this particular plot both scales were manually
adjusted until both curves were close to the same
size, peak to peak. From this we can conclude the
board tempco is +1.0 ps / 0.4 C which is:

     +2.5 ps / C (1.4 ps / F).



10 - Tempco and ADEV plot

To clarify, a tempco of 2.5 ps / C means you get
an unwanted phase shift of 2.5 ps for a 1 C change
in ambient temperature. You can't directly relate
that to frequency shift unless it is specified how
long it takes for said temperature change to occur.

Phase shift is just phase shift; but phase shift
over time is frequency shift.

To illustrate, as an extreme example, if the 1 C
ambient temperature change were to occur over
a time span of just one second (think freezing
to boiling in 100 seconds!) you would see the
board phase shift by 2.5 ps per second; which is
equivalent to a frequency offset of 2.5e-12 (during
that one second).

On the other hand, as a more practical example,
when the ambient temperature changes by 1 C
over, say 500 seconds (about 8 minutes), then
2.5 ps per 500 seconds is a frequency offset of
5e-15. You aren't going to see this.

So you see how temperature stable the dist amp
really is. I wouldn't change anything. The carbon
comp resistors are suspect, but even as-is this
tempco will be in the noise for everyone. I can't
wait to test the 5087A to see how it compares.

Below is an ADEV plot of your distribution amp.
It represents a noise floor for any measurement
made using a signal from the distribution amp.
As you can see, it is orders of magnitude below
any OCXO, cesium, or GPS ADEV plot.

Even without an enclosure, and with my lab A/C
cycling the way it is this summer, the only effect
you can see in the plot is a slight hump around
tau 100 seconds.

Another view of the tempco can be observed in
a Stable32 power spectrum. I've circled a spike
around 3e-3 Hz, which is the "frequency" of the
A/C -- its on/off cycle, and thus the period of the
slow sinewave thermal cycle, is on the order of
300 to 400 seconds, so frequency is 0.003 Hz.






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