Hi ….. a little elaboration …..
If you *don’t* terminate the main distribution line, it effectively acts as a great big antenna. It floods the lab area (or production floor or building …) with 10 MHz. I unfortunately have a lot of empirical data to back this up. It’s also worth using a “better” grade of coax for the distribution line (as opposed to Home Depot’s lowest cost spool …) for pretty much the same reason. One second ADEV on a typical pod is usually in the 2x10^-11 vicinity. Plenty good enough for driving test gear. Not quite up to acting as your ADEV reference in a DMTD. Bob > On Apr 27, 2019, at 12:25 PM, John Ackermann N8UR <[email protected]> wrote: > > 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. >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
