There is a gotcha in the TSC phase data... the architecture introduces a
deliberate phase offset to the phase data so you will always gain (or
lose) phase over time. Hook both inputs to the same source, and you'll
still have a slope in the output data (and the slope and intercept will
be different on each run).
Thus you *cannot* derive the actual frequency or phase offset from the
phase data -- it is only useful for stability and phase noise analysis.
The TSC people told me this was a feature, not a bug. :-) If you want
to get the actual frequency of the DUT, you need to use the fcounter
function. And I've never been sure about the averaging used to derive
the 1/10/100/1000 second values from fcounter, and how best to derive a
longer-term frequency average from that data.
John
On 12/10/2015 1:44 PM, Tom Van Baak wrote:
Lucky you! The 5115A has a thousand times better resolution than a hp5370 or
SR620 counter.
The architecture of the 5110A 5115A and 5120A is described in the manuals.
These are frequency stability analyzers so the initial phase and frequency
offsets are not important. What you're seeing is phase difference data *after*
these initial calibration offsets are removed. This is necessary because the
unit allows a wide-range of frequencies on either port.
That method also keeps the floating point output from losing accuracy over long runs. A
count of raw cycles would grow unbounded and you would lose resolution as you gained
range. So what you're seeing is by design and it doesn't affect the phase noise or ADEV
plots at all. I'm not sure why you call it "useless".
Note the 5110A has a single-DDS option. When both inputs are nominally the same
frequency the streaming data is closer to raw phase. Not sure if the 5115/5120
does also. But either way, none of this matters to ADEV.
Your ADEV plots from raw data should match the ADEV plots on the screen. Watch
out for bandwidth mismatch.
/tvb
----- Original Message -----
From: "Anders Wallin" <anders.e.e.wal...@gmail.com>
To: "Discussion" <time-nuts@febo.com>
Sent: Thursday, December 10, 2015 7:20 AM
Subject: [time-nuts] Data collection from 5115A phase noise test set?
Hi all,
A 5115A phase noise test set landed in our lab and I am wondering about the
data collection.
It has two telnet ports one for commands and one for phase data.
When issuing "start" on the command-port it starts spitting out
phase-difference values on the data port.
However it seems to me that the phase data alone (REF-DUT phase, in units
of the REF period) is almost useless without knowing the internal workings
of the device.
The phase values are clearly not the true phase difference, which would
quickly accumulate to a huge number with e.g. REF=10MHz and DUT=11MHz. My
understanding is that digital downconversion (DDC) is performed on both REF
and DUT signals, and without knowing the LO frequencies for the DDC the raw
phase data alone is almost useless.
The command prompt does have commands for displaying the internally
calculated ADEV, L(f), frequency-counter readings, etc. and these seem to
show correct and OK values, but there seems to be no way to reproduce e.g.
the ADEV or frequency-counter values/statistics from the raw phase values
on the data port??
Is this correct? Any other experiences with the 5115A or higher end 5120A?
thanks,
Anders
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