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

Sat Apr 27 17:25:09 UTC 2019

I have a large box of 8140 pods (well, I call them "taps").  They are 
quite handy for stringing distribution points around a large room, 
provided ultimate phase noise performance isn't required.

You *don't* need an 8140 main box to use these.  All the 8140 does is 
put 12V DC bias on its output connectors, and the taps couple their 
power off the coax with an RC choke.  When I was using my setup, I built 
a bias-tee box that was inserted right after the frequency source and 
that worked fine.

However, one note:  for best performance, you want to terminate the end 
of the distribution line in 50 ohms, but since there's a DC voltage 
present, you need to include a DC block (series capacitor) so you're not 
burning 3 watts in the load.

John
----
On 4/27/19 10:17 AM, 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.
> 
> As it turns out, although it can handle lower input signal levels, in
> other ways including power usage it seems very similar. One notable
> downside is the chip tops out at 4v absolute maximum input, so a
> separate regulator is used just to feed this chip. The main regulator
> has also been changed from a 7805 to an LD1117 variant.
> 
> On this version the output stage is the same TI 74S140 dual 4-input NAND
> gate as was used on the original pods, just in SOIC form factor.
> 
> As with the next board there is one error on the board, the wire loop
> that forms the ground connection was intended to fit a U-type pin
> header, however the footprint I used on the boards was just too tight to
> allow the pins through, so I've used some thin bus wire instead.
> 
> The second major variant I designed was a combo version, allowing sine &
> square outputs by just switching a jumper, or isolated[2] or
> line-regenerator (8040TA from Spectracom) versions with a simple
> sub-board containing just an inductor (TA) or 1:1 transformer (isolated).
> 
> This is the second revision of that board, where the 74S140 has been
> replaced by a modern TI 74LVC1G17 buffer. This version of the pod, set
> for sine output, uses almost exactly 30mA of power (since both the old &
> new pods use linear supplies that's the most sensible unit), whereas the
> original pods are right around 33mA. The empty pods at the bottom-left
> are simply placeholders for 2 100 ohm resistors to add 50 ohm line
> termination if desired.
> 
> The board fits into the Pomona 2390 "Size A" enclosures, or for the
> isolated version the Pomona 3239 "Size B". This is the reason the BNC
> connectors have to be extended to reach the board, on the isolated boxes
> the BNC pins reach much deeper into the enclosure.
> 
> If the jumpers were removed, plus the smaller buffer it should be easy
> to fit a pod into the Pomona "Miniature" boxes too.
> 
> I was also due to create some new personal businesscards, so I arranged
> the circuit down to a single layer (the only jumper is the requirement
> to connect both ground pins on the connectors) and merged it with some
> text converted to KiCad footprints to make a nice card on some 0.6mm
> PCBs. The paper on that photo is covering the link to the build
> instructions, which weren't written at the time (they're *mostly* done
> now, I may update this post with the link later).
> 
> Finally, while I was out travelling at the start of April my new (to me)
> HP 4395A arrived so I've finally got some spectrum output. The output is
> very similar between the original and my version, with the major notable
> difference being that my version is 10dB worse at the third harmonic. I
> lack the equipment (and understanding) to properly measure phase noise,
> but if anyone in AU/NZ wants to volunteer their time & equipment for an
> afternoon I'd love an excuse for a field trip.
> 
> 1: Or at least I didn't think they had, I've since found out that there
> was a multi output pod, and one is currently in the post heading to me.
> 2: An option on the standard Spectracom pods, albeit a rare one.
> 
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