Skimming the Meinberg manual finds reference to outputs that
are TTL levels into 50ohms, TTL levels without load specified, and
2.5V TTL into 75 ohms. I didn't study it enough to understand which
exact output has which drive spec. Meinberg's technical support should be
able
to directly answer questions about the drive circuits and the
loads they can drive.
An old and simple 16way 1PPS driver design, schematics and Gerbers
included, may be found at
https://casper.berkeley.edu/wiki/16_way_1PPS_driver
The HERA project plans to use the White Rabbit Switch and
WR-LEN Endpoint from Seven Solutions for its 1PPS and 10MHz
distribution.
http://sevensols.com/index.php/timing-products/
Matt
PS.
Our lab has a collection of random synthesizers like
https://www.valonrf.com/frequency-synthesizer-6ghz.html
https://www.dsinstruments.com/store/products/category/rf-signal-generators/
SG6000E
OLED display ADF4350 signal generator RF source from eBay
On Fri, 19 Oct 2018, Nitish Ragoomundun wrote:
Date: Fri, 19 Oct 2018 13:45:34 +0400
From: Nitish Ragoomundun <[email protected]>
Reply-To: [email protected]
To: [email protected]
Subject: Re: [casper] SNAP Board clocking for a low-frequency array
Many thanks for your replies, Bob and Prof Inggs. Now we have a better idea how
to tackle the issue.
Cheers.
On Fri, Oct 19, 2018 at 12:37 PM Michael Inggs <[email protected]> wrote:
Hi Nitish
I am sure the Meinberg TTL will battle to drive 20 mA into the SNAP. It is
probably rated at a
few TTL loads.
For multiple boards, you will need to buffer the output of the Meinberg into a
clock
distribution network i.e. multiple output gate / driver leading to the number
of boards you aim
to drive as separate TTL channels. This is a bit pedantic, but just linking the
Meinberg in
parallel to a number of TTL inputs (check the Meinberg drive capability in TTL
loads in the
first place) will lead to multiple reflections and some jitter on the leading
edge.
For our NeXtRAD multistatic radar that uses GPS synchronisation between nodes,
we have
implemented an ubiquitous Frequency Distribution Unit (FDU) that can swallow
almost any 10 MHz
or 100 MHz clock, at all the weird logic levels around, and then providing 10
and 100 MHz in 6
ports per voltage standard (5V, 3V3, sinewave). There is an optional 10x
multiplier (low phase
noise) for converting from 10 MHz to 100 MHz. Block diagram attached. You can
implement any
subset of it. Can provide board layouts etc.
We also have a home made GPS Disciplined Oscillator that is proved to provide
less that 4 ns rms
jitter between nodes with common view satellites. It provides 1 pps, 10 and 100
MHz outputs (at
various levels). Stephan Sandenbergh is the father of this, and has implemented
some cunning
fast lock mechanisms. Otherwise you will be there for hours if the power goes
off.
Stephan also built a test jig for comparing two sources and plotting the Allen
Variance. Be
careful of low cost GPSDOs as they are designed for NTP applications in
networks, where 15 ms is
all that is required.
We also have a great deal of experience and hardware (with results) of using
White Rabbit to
synchronise receivers.
Regards
On Fri, 19 Oct 2018 at 09:04, Nitish Ragoomundun <[email protected]>
wrote:
Hi all,
We are building a low-frequency array for the observation of the deuterium
hyperfine line
at 327.4 MHz with a bandwidth of 250 kHz. We intend to use SNAPs for
acquisition. The
boards will operate at full 12 channels input, thus the ADCs at 250 MSps. We
will
subsequently decimate the data rate, as our working bandwidth is narrow.
Concerning the clock input for the SNAP, https://casper.berkeley.edu/wiki/SNAP
states the
following:
Digital 1 PPS: 50 ohm single-ended LVTTL logic levels
* SMATP13
* Vin-high 2.0 to 3.3 Volts. Low current drive sources, such as typical LVTTL
or CMOS
gates, probably can not supply the 40mA required to supply the 2.0V into
the 50ohm
load.
* Vin-low 0.0 to 0.8 Volts
Actually we considered buying the GPS-disciplined Meinberg M500
LANTIME(https://www.meinbergglobal.com/english/products/modular-railmount-ntp-server-ieee-1588-solution.htm)
, which is advertised to output 1 PPS TTL levels. Now, our first question is
whether the
SNAP can work with this? Also, the note about low current drive sources is a
little
confusing, can anyone shed some light here.
Secondly, we would like suggestions from anyone who has experience with
clocking the SNAP.
You see, we run a very tight budget and the M500 LANTIME clock is expensive. We
would like
to know if there is a less costly way to clock the SNAP and synchronise several
boards.
Thanks.
Best regards,
Nitish Ragoomundun
Department of Physics
University of Mauritius
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10 Devon Street, Simon's Town, South Africa. Tel: +27 21 786 1723 Fax: +27 21
786 1151 Skype:
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