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[Discuss-gnuradio] Re: New external clock board for USRP

From: Alexander Chemeris
Subject: [Discuss-gnuradio] Re: New external clock board for USRP
Date: Tue, 20 Oct 2009 02:31:48 +0400

Hi all,

As I wrote in my previous mail, we're working on an universal clock
source for USRP (and not only for USRP). It is based on 0.28ppm TCXO
from Connor Winfield [1], National Semiconductor LMX2531 VCO+PLL [2]
and LMK01000 CD [3] for clock generation and Atmel ATUSB for control.
So far we've finished PCB design and working on production of the first
25 units. We'll reserve 5 of them for our own use and testing of
different options, 8 others are requested by community people, so there
are more then 10 left for sale. We plan to finish production at
the beginning of Dec and take units to 26C3. If you plan to attend it,
you have a great chance to see them, play with them and take some of
them with you. ;)

As we promised, units form this experimental batch will be sold out
for only $100 or 66EUR (without shipping). We kindly invite everyone
interested in such unit to get one and try fitting it to your setup.
Also we need your help determining the best feature set for the unit
(read on for the ample list of possible features). We aim at creating
a really flexible and cheap clocking unit, which may used by a broad
GnuRadio community and will best fit its needs. Unit with open source
software and open hardware.

Now, lets get to facts. Board dimensions are 86x44mm (3.4" x 1.7") -
it is designed to work inside of USRP box with RFX boards installed
with no external connections.

Default distribution version includes:
1) Clock board with default options
2) U.FL to SMA cable to connect to USRP
3) Power cable to connect to USRP's fan connector

Default board options:
1) Control from miniUSB or 16-pin connector on USRP daughter boards.
It will be possible to write a GNURadio block to control clocks right
from GNURadio flowgraph!
2) Power from 2-pin connector for connecting to USRP fan power
connector and 2-pin pass-throw to connect fan to. That is board is
connected between USRP and fan.
3) One U.FL clock output with ability to generate frequency in the main
range 2.84-65.83MHz, and additional ranges 65.91-71.82MHz,
72.5-79MHz, and more ranges higher with <=0.44Hz step. Output
levels are CMOS. This means that you can tune your clock precisely
ti whatever frequency you want.
4) Initial frequency calibration 1ppm, temperature stability 0.28ppm,
holdover stability over 24h 0.32ppm [1]. Clock jitter will be measured
when first units arrive to us, I'll post measurement results here.

Pretty simple and flexible isn't it? But what makes this clock unit really
universal is a set of available options. You may solder them by yourself
or request ready-to-use units from us - I will make some notes on price
changes and options compatibility below, mail me for details if you're
interested in particular configuration. We want to be as flexible as
possible to fulfill community need in flexible clock source.

So, basic additional options include:
1) Power may be taken from 2-pin connector, 6V jack input or from USB.
All power options are mutually exclusive.
2) COM-port with RS-232 levels for clock control. This will add about 3.5$
to the price.
3) Up to 5 more additional U.FL outputs (6 outputs altogether), which
share VCO frequency, but may be independently divided in clock
distributor. 5 additional connectors will add 8-9$ to the price.
4*) Output levels may be (a) 4 LVPECL (or CMOS) outputs and 2 LVDS
outputs, (b) 6 LVPECL (or CMOS) outputs, (c) 6 LVDS outputs.
5) SMA connector may be soldered instead of one of U.FL with CMOS
levels. It will add 4.5$ to the price.
6) SMA connector with direct VCO output bypassing clock distributor.
It will add 4.5$ to the price.
7) SMA connector for external clock source. This way onboard
oscillator should be disabled by resistors soldering or should not be
present. It will add 4.5$ to the price.
8*) Oscillator could be changed to 0.5ppm. This will save you 7.5$.
9*) VCO+PLL with clock divider could be changed to clock divider
and multiplier. In this case you won't be able to tune your clock
precisely, but you'll be able to generate, e.g. 13MHz, 26MHz and 62MHz
from a single oscillator.
10*) Frequency range could be extended to 1.87-94.75MHz (and more
ranges higher) at the price of more phase noise.
11*) For nerds only - unit may be used without onboard controller by
direct access to VCO+PLL pins. But this is roughly equivalent to
a VCO starter kit and obviously is not compatible with features like
SPI/USB/RS232 control, and can't be used with power from USRP.
12) 1pps external signal may be used to tame the clock to external
GPS unit. It will be passed to ATMega's interrupt input, so you
should keep in mind that this will need a lot of software work for filtering
out jitter, generated by it. We don't plan to develop this software at
least now, but anyone who need this is welcome to take it.

* These options is not immediately available because of changes in
components list. Some options need testing before we can offer units
with them. Some options available for small orders, some available
only for volume orders. And sure, you can solder them by yourself.
Mail me for details, if you're interested.

There are two big options, which touches a big part of the unit and are
very much experimental. We can't guarantee that they will work,
but we think they will. :)
* TCXO with VCO+PLL could be replaced with VCTCXO with DAC.
DAC can be 12-bit linear or 16-bit delta-sigma. This will make it
about 20$ cheaper then default bundle if we produce it in volume.
The downside of this is that frequency range is much smaller and
more calibration is needed.
* GPS chip could be actually installed right on board to provide
1pps signal. PCB is designed to be used with cheap EB230 GPS
Module and will add about 40$ to the price. But there are some small
limitations - you can't use RS232 output with it, only 5 output channels
are possible. Same notes on software as for 1pps input applies.

We're working on detailed documentation and will make it available
as soon as possible.

1. http://www.conwin.com/datasheets/tx/tx236.pdf
2. http://www.national.com/ds/LM/LMX2531LQ1515E.pdf
3. http://www.national.com/ds/LM/LMK01000.pdf

Alexander Chemeris.

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