[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)

[ed breya]

Erik, it sounds like you have it nearly finished and working - congrats. 
You should be able to greatly reduce those pesky line-interference 
spurs. The first weak link may be the audio signal path, so it's time to 
let the audio box help, if you haven't already optimized it. In the last 
design round, you now have the audio box driven from a coax line with 
its shield connected to your local instrumentation ground, which is 
earth ground there, and the minus end of the 12V supply, right?

How is the other end hooked up to the audio box? I looked at some info 
on the unit, but it doesn't say anything about hookup details - a 
"normal" user (not us) would probably just plug in a mic and go. If it 
is a balanced mic connection, then it is at least some form of 
differential input, and you had been using it this way before, with the 
audio common (low side) connected to your floating signal common. That's 
the way you should connect the audio box still, even though the cable is 
now earth ground referenced at the the instrument box. If it's still set 
up this way anyway, then OK. If you have connected the cable shield to 
the ground at the PC or USB audio box, try removing it. The audio box 
input should have some amount of LF CMR, which should help in this part. 
You can experiment with different arrangements and see what works best. 
The next thing is to try adding common-mode choking with various methods 
- I'll explain later.

 From the very beginning, I think, you have considered the benefits of 
galvanic isolation of the DBM/PD mixer ports, which is good. 
Unfortunately, powering up the OCXOs and getting their signals connected 
leads to some inevitable ground loops anyway, so you can't always take 
full advantage of built-in isolation, but at least you have it. Try to 
use it when you can.

The rest of the line and low frequency ground loop issues are hard to 
predict, depending on the overall powering and operational setups, but 
some general methods can be applied.

Now back to the audio output path, the first thing is to see if there's 
any point to improving its CMR - if the interference is getting in 
elsewhere, it doesn't matter (yet) how good it is. You can experiment 
with the circuit hookup and operation. For instance, in your latest 
post, the first chart shows the operating noise floor and line spurs 
present "without the DUT connected." This can describe a range of 
possibilities, from the RF connector being unplugged, to the DUT being 
totally removed from the experiment, power-wise, ground-wise, and so on. 
In your setup, you have a number of items hooked up, each contributing 
to the ground loop situation.

You can apply the process of elimination to gradually improve it. The 
first and easiest step is to shut everything down except the PC and 
audio analyzer setup, and assess the spurs. If there's little change, 
then the interference is what it is, due to the environment of the 
setup. If the spurs go away, then your analyzer circuit is amplifying 
them from the signal chain, or from the power supplies. Then you would 
short the input of the LNA, power it up and look at the spurs again, and 
so on.

This narrows things down as you go, with various tests to see what 
things make the biggest difference. If the spurs are relatively 
unchanged with power down, then you would unhook things until you're 
left with just the analyzer circuit hanging from the cable to the audio 
box - you systematically delete connections to power supplies, grounds, 
signals etc, until it shows clean. Then you can add back and think about 
the possible ways the interference gets in.

Once you figure out the major interference sources, you'll know what and 
where to make improvements. Keep in mind that within your own project 
circuit, you have lots of options, since you're building it, while on 
most of the other stuff, you'll likely be limited to external fixes like 
CM chokes (prayer beads) and grounding tricks.

Next time I'll talk about some of the methods in detail.

Ed