yes it was 2 cable types.
we use Times Microwave LMR-100A only.
The problem Billy found and fixed was due to some mistakenly
installed RG-174U.
(see the same email discussion of 2007jul18)
On Thu, 4 Mar 2010, David MacMahon wrote:
Hi, Jason,
On Mar 4, 2010, at 14:32 , Jason Manley wrote:
On 04 Mar 2010, at 13:49, Dan Werthimer wrote:
the code resets one of the adc's until the two adc clocks are lined up in
phase.
For future reference, this code gives up trying to sync after a while. I
can demonstrate systems where it doesn't succeed before the timeout and
then gives up. It occurs at PAPER-GB8 regularly.
We've seen this problem at the ATA, too. In our case it turned out to be due
to differences in the ADC clock cables, but not physical length so much as
cable materials (presumably resulting is propagation delay or phase
differences). It turns out that the "phase alignment test" in the ibob
software imposes a fairly stringent requirement on the phase alignment of the
two ADC input clocks. Here's an excerpt from a 2.5 year old message about
this (from the ATA where Fs is 838.8608 MHz)...
On Jul 18, 2007, at 0:09 , David MacMahon wrote:
Here are the details on the ADC clock phase-up process ("Fs" is sampling
frequency, i.e. 100*2^23 Hz == 838.8608 MHz)...
The adc initialization software measures the relative phase of the two
incoming clocks (Fs/4). This is a rather complicated process that I would
rather not explain the inner working of here. Apparently this measurement
is made with +/- 5% precision. If the measured relative phase is
greater/less than +/- 20 degrees (at the FPGA clock frequency, Fs/4), then
the clocks are deemed to be "out of phase", the ADCs are reset, and the
measurement process starts again. If the two clocks are never deemed to be
"in phase", i.e. within +/- 21 degrees (20 degrees plus 5% == 21 degrees)
after 50 attempts, the software finally gives up.
20 degrees at Fs/4 is 80 degrees at Fs, so if the two ADC clocks are more
than 80 degrees (at Fs) apart (give or take a few degrees), then I think
they will never "phase up". 80 degrees at Fs is only 265 ps!
The problem at the ATA was tracked down by Billy Barrott...
On Jul 18, 2007, at 14:03 , William Barott wrote:
I have traced the problem to the cables themselves. Three different stocks
of cables were used in the construction of the RFCB 800 MHz clock feed
lines: The bulk is LMR-100A from the same spool. We also have "RG-174U"
and "High-Quality RG-174A/U."
The RG-174A/U (1 found in system) measures about 35 degrees out of phase
with the LMR-100A.
The RG-174/U (4 found in system) measures about 110 degrees out of phase
with the LMR-100A. It was these RG-174/U cables that were associated with
wildly-off phases and non-syncing iBobs in both chassis.
In another message, Billy reported that all these cables were length matched
to within 1 cm. I believe (but am not 100% sure) that the problem was using
mismatched cable types for an ibob's two ADC clocks rather than one cable
type being unusable.
Back to the present...
On Mar 4, 2010, at 14:32 , Jason Manley wrote:
I think Paul's on the right track: since you cannot calibrate off the sky,
injecting a reference for the calibration is their most reliable option.
Agreed.
Dave
