[time-nuts] Building new pods for the Spectracom 8140 using modern components

[Dave M]

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>