Good work on the project, Julien. I have a couple of the 8140 distribution
amps, one of which I use in my shop. Instead of buying the line taps that
are supposed to be used with the 8140, I just disabled the 12V riding on the
output signal coming out of the amp.
I have another unit as a spart, but not modified (yet). Your pods seem to
be good replacements for the line taps... Do you plan to publish the details
of your pods (schematics & PCB design)? I might be tempted to build a few
for my own use.
Thanks for posting your project. Nice work.
Dave M
Julien Goodwin wrote:
Slightly edited down from my blog, if you want the full thing with
photos see:
https://laptop006.livejournal.com/60862.html
I've mentioned a bunch of times on the time-nuts list that I'm quite
fond of the Spectracom 8140 system for frequency distribution. For
those
not familiar with it, it's simply running a 10MHz signal against a 12v
DC power feed so that line-powered pods can tap off the reference
frequency and use it as an input to either a buffer (10MHz output
pods),
decimation logic (1MHz, 100kHz etc.), or a full synthesizer
(Versa-pods).
It was only in October last year that I got a house frequency standard
going using an old Efratom FRK-LN which now provides the reference;
I'd
use a GPSDO, but I live in a ground floor apartment without a usable
sky
view, this of course makes it hard to test some of the GPS projects
I'm
doing. Despite living in a tiny apartment I have test equipment in two
main places, so the 8140 is a great solution to allow me to lock all
of
them to the house standard.
Another benefit of the 8140 is that many modern pieces of equipment
(such as my [HP/Agilent/]Keysight oscilloscope) have a single
connector
for reference frequency in/out, and should the external frequency ever
go away it will switch back to its internal reference, but also send
that back out the connector, which could lead to other devices sharing
the same signal switching to it. The easy way to avoid that is to use
a
dedicated port from a distribution amplifier for each device like
this,
which works well enough until you have this situation in multiple
locations.
As previously mentioned the 8140 system uses pods to add outputs,
while
these pods are still available quite cheaply used on eBay (as of this
writing, for as low as US$8, but ~US$25/pod has been common for a
while), recently the cost of shipping to Australia has gone up to the
point I started to plan making my own.
By making my own pods I also get to add features that the original
pods
didn't have[1], I started with a quad-output pod with optional
internal
line termination. This allows me to have feeds for multiple devices
with
the annoying behaviour I mentioned earlier. The enclosure is a Pomona
model 4656, with the board designed to slot in, and offer pads for the
BNC pins to solder to for easy assembly.
This pod uses a Linear Technologies (now Analog Devices) LTC6957
buffer
for the input stage replacing a discrete transistor & logic gate
combined input stage in the original devices. The most notable change
is
that this stage works reliably down to -30dBm input (possibly further,
couldn't test beyond that), whereas the original pods stop working
right
around -20dBm.
<Snipped>
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