Matt

For those GPSDOs which also use the OCXO clock to generate the LO for
the GPS receiver (eg Thunderbolt) the zerocrossing transitions of the
10MHz output are accurately aligned to the leading edge of the 1PPS output.

For other GPSDOs there may be an offset between the 10MHz zero crossing
and the receiver 1 PPS output.

If the PPS output is derived from the OCXO via a divider then the
leading edge of the PPS output will be aligned (within a gate
propagation delay or 2) with the PPS output.

One can always use a dual phase synchroniser to detect the 1Hz
transition using the 10Mhz output.

Bruce

Matt Ettus wrote:

Hi Matt,

All over the map. A well-engineered GPSDO might choose to
align the rising zero crossing of the 10 MHz sinewave with the
leading edge of a very fast risetime 1PPS, because this is
"common sense" but if it isn't in the spec, it's best not to expect
it to happen.

But worst case, if you chose the wrong 10 MHz sinewave edge
you're at most 50 ns off from the 1PPS -- and note this is still
within the +/- 100 ns of UTC accuracy spec that a lot of GPS
receivers claim.

There is often a fixed phase delay between the two outputs. It
may be just a few ns or it may be tens of ns depending on the
number of inverters, buffers, amplifiers in the signal path(s),
the length of internal or external cables, or impedance effects
on risetime (50R vs 1M can make a difference).

One also has to be very careful with the 1PPS pulse itself. Most
GPSDO do not specify the voltage level of the pulse. Now if the
pulse risetime is just a couple of ns this is not a serious problem.
But when the risetime is many ns or many tens of ns then you
need to know what trigger voltage level constitutes the intended
true moment of the 1PPS. See, for example,

http://www.leapsecond.com/pages/gpsdo-rise/

Some of these GPSDO have about a 1 V / 10 ns slew rate which
means if you don't nail down your trigger level you're asking for
very large shifts in 1PPS timing. The zero-crossing of a 10 MHz
sinewave doesn't have the same ambiguity.

/tvb

aceamusements@mchsi.com wrote:

Manufactured in USA during 8th week of 1996

Bruce

The date code is not a production date, necessarily, but the date of that
particular design revision (presumably when it was released to
manufacturing).  It sets a lower bound, but your unit could potentially be
several years newer.  You can get a better idea by looking for the latest
date code on various ICs and capacitors.

-- john, KE5FX

Thanks,

I just purchased this on ebay and was just wondering how bad the tube could be(yrs left)

I have a agilent one now I got new a year ago and its ion pump current keeps rising,the one advertised says its still pretty low @ 2.0uA  ,1450emult..has telcom options installed so I bet it has lots of hrs but may be good for a few yrs?
-------------- Original message from Bruce Griffiths bruce.griffiths@xtra.co.nz: --------------

Are you using terminations or is the end equipment properly terminating
?  Mismatched impedance does weird things to waveforms.  Also check to
make sure you not overdriving on your source.  And a bad power supplies
can induce waveform distortion.

Brian

Matt Ettus wrote:

Matt Ettus skrev:

You can expect anything. Some buffers the PPS from the GPS engine and
then make some more or less elaborate 10 MHz training algorithm. Don't
expect rising edges to coinside for all of them.

Depends, several approaches is possible. A simple approach is just a
dual flip flop approach which is simple and straightforward. There is a
risk for it to switch between 10 MHz cycle, but it may be acceptable for
some applications. You could use any of many forms interpolating
techniques to reduce or remove this risk. Exactly what you can achieve
depends not only on the receiver hardware but also the transmitter side.

The trouble is that the 10 MHz + PPS is a very weekly defined interface
which is not properly standardised. The only standard I know off is an
old US MIL-STD document.

Yes, on a signal not properly terminated. It occurs if you T a signal to
your scope while the signal goes further on... to an open end. Then the
reflex signal will create this type of dwelling.

Cheers,
Magnus