Bob Camp wrote:
Hi
I don't see anybody arguing that systems work better when there's a
high ADEV than a low ADEV. Most of the papers are heading in the
direction of "it doesn't catch all of the problems I worry about".
Based on what systems need and what ADEV measures, I don't find that
a particularly surprising conclusion. The next step would be for
people to come up with system related measures that do catch what
they are after. Some have tried that, many have not. The next link in
the chain would be getting (paying) vendors to run the tests on
product. As far as I can tell - that's not happening at all. ADEV had
the same issues early on. Until it became a mandatory specification,
not a lot of people paid much attention to it.
Bob
Yes.. and if you read the discussion following that first batch of
papers in the report Magnus linked, you can see the same sort of thing..
everyone had some measure they had developed that was important to their
particular system, but none of them were the same, or even directly
interconvertible.
The problem faced by them (and us at JPL now)is that we're a very, very
low volume customer (a few units every few years).. We *do* actually
pay people to make the measurements, but sometimes, I think the
measurements we ask for aren't necessarily appropriate. It's expensive
and time consuming to do the analysis for a new measurement, and
particularly to validate that it's actually measuring something useful
and relevant, so there's a very strong tendency to "do what we did before"..
Sometimes the previous measurement used something that happened to
depend on an artifact of the system design so that you could test
something you can measure to represent something that's difficult to
measure (e.g. IP3 vs P1dB relationships presume a certain shape to the
nonlinearity). But if you've changed the underlying design, that
artifact may not exist.
This shows up a lot with test procedures/specifications for systems
based on all analog designs that are adopted unchanged for systems with
digital conversions. (look at all the various ways to specify
performance of high speed ADCs). For example, in my world, a common
specification is for performance at "best lock frequency" (BLF, that is,
the frequency at which you can acquire a carrier with the lowest SNR).
In an analog system, this is often where the input corresponds to the
rest frequency of the VCO with everything sort of in the middle of the
range. But a lot of modern systems have no BLF... their performance is
essentially flat over some band, and any small variations are not
indicative of, e.g. minimum loop stress, etc. The time spent to
determine BLF, and any assumptions about performance aren't necessarily
valid.
On the other hand, we don't necessarily engage in a "science project"
for each project to determine performance requirements (and
corresponding test methods) unique to the performance in that system.
There is a need for more generic performance numbers that have
moderately universal understanding.. If I tell you the P1dB for an
amplifier, and I tell you that my signals are 10dB below that, then, in
a short statement, I've actually told you a fair amount about how my
design works and the range over which it's likely to keep working.
That's because you and I have a common understanding of what a P1dB spec
"means"...
Over the past decades, I think a similar understanding has arisen with
phase noise specs and to a lesser extent Allan deviation. That is, given
a phase noise plot, a skilled practitioner can tell whether it's good or
bad in the context of a particular system.
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