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Julien Goodwin
Building new pods for the Spectracom 8140 using modern components 
28th-Apr-2019 12:07 am
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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.

(The rubidium is in the chunky aluminium chassis underneath the 8140)

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 current (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.

Spectrum with input sourced from my house rubidium (natively a 5MHz unit) via my 8140 line. Note that despite saying "ExtRef" the analyzer is synced to its internal 10811 (which is an optional unit, and uses an external jumper, hence the display note.

Spectrum with input sourced from the analyzer's own 10811, and power from the DC bias generator also from the analyzer.

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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