Hi,
Won't the U70 output, when divided by 2 by R450 and R452, need
to match the output signal from U71?
That suggests something like 2*(3.0 to 3.3V) =
6.0 to 6.6V at U70 pin 6.
I can readily guess the designer wanted at least a few
outputs from the Roach that would emmulate COTS 1PPS sources and
those often drive TTL or 5V CMOS levels when terminated
in 50ohms.
I would distribute a signal from a 50 ohm source such as at J11
using 50 ohm coax and terminate into a 50 ohm load. This way
clean transmission with minimal unwanted reflections that could
cause multiple edges or other signal integrity features.
If the ultimate receiving circuit was high impedance, as in
a standard TTL or CMOS logic gate, I'd place inline at the
receiving end of the 50 ohm coax cable a 50ohm feed-through
terminator.
Matt
On Thu, 18 May 2017, Zhu, Yan wrote:
Date: Thu, 18 May 2017 08:31:33 +0000
From: "Zhu, Yan" <zhu...@nao.cas.cn>
To: Michael D'Cruze <michael.dcr...@postgrad.manchester.ac.uk>,
"casper@lists.berkeley.edu" <casper@lists.berkeley.edu>
Subject: Re: [casper] ROACH2 sync_out
Hi Michael and all,
I'm also intended to use sync_out or other GPIO to output square wave to
control noise source,
I measured 6V for sync_out and GPIO pin 1.5V.
After digging into ROACH2 schematics(page 25 in
roach2_rev2_schematics.pdf),
I found the differential signal is first converted into single ended
by SN65LVDT2 than
buffered out by THS3091. The THS3091 is given a 12V supply.
After a quick look at THS3091 datasheet, it will output about 12V at
50Ohm load with 15V VS
and 3V with 5V VS, so 6~7V output is reasonable for 12V power supply(a
little lower than expect?).
Can anyone remember why pull the sync_out that high instead of normal TTL
or CMOS level?
If I want to use it to drive a load which expect TTL or CMOS level, how
should I connect them?
Thanks
Yan
------ Original Message ------
From: "Michael D'Cruze" <michael.dcr...@postgrad.manchester.ac.uk>
To: "casper@lists.berkeley.edu" <casper@lists.berkeley.edu>
Sent: 2017-05-12 02:10:26
Subject: [casper] ROACH2 sync_out
Dear all,
I’m planning to use a 0.5Hz square wave, generated from the
FPGA and output via sync_out, to eventually fire our cal
diode (via much cabling). A quick hardware test today shows
the sync_out port driving at circa 7V (!). This is a bit
higher than I was expecting. Does this venture as a whole
seem like a particularly bad idea to anyone with experience
using sync_out? Is this output voltage roughly as expected?
Thanks a lot,
Michael
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