Hi Here’s the problem:
Spectrum analyzers, synthesizers, signal generatros, and the like all tend to follow a common design approach. They take the 10 MHz reference in and run it through a narrowband PLL. Not every one of them, but 99% of them. The same is true of microwave multipliers and signal sources. What you care very much about is phase noise out to about 100Hz or so. Past that, it simply does not matter. Why? >From the instrument side: If you are headed to microwaves, the 20 log (N) phase noise formula is working against you. 10 MHz multiplied to 100 MHz goes from -170 dbc to -150 dbc. That’s not what you want to see. They long ago came up with the approach of locking up a VHF crystal oscillator to get -170 dbc at 100 MHz. The technique came out a long time ago (as in before I started doing this in the 1970 … or was it the 1790’s …). Cost wise this made sense. They bought a cheap(er) OCXO at low frequency if they needed stability, and just ran a simple circuit with a crystal in it at VHF. >From the distribution side: People expected that if they plugged an HPxxxx into a HPyyyy that it would meet spec. They even expected it to work if the entire chain was not made by HP. Generating -170 level signals is hard enough, distributing them across a building, not so much. The designers made a simple decision, -145 to -155 dbc/Hz phase noise was “good enough” out of a distribution system or out of a master standard. Could they have done better? Probably. Would it have run up costs in that era? Most certainly. >From a system standpoint: The people on both ends of the cable made decisions more or less together. Who knows who went first or what was tried and failed, that’s buried somewhere back in the 1950’s. Did everybody drink the same beverage? I’m sure somebody somewhere didn’t. Every piece of HP gear I’ve ever seen fits the approach above. Every microwave multiplier I’ve ever seen or designed fits it. The Fluke and Comstron gear I’ve worked on or actually seen schematics for works this way. Every distribution amp or distribution system I’ve seen works this way. I have a nasty habit of plugging standard lines into phase noise testers. Each time I do, the data I get supports the decision to do things as shown above. —————— So what’s this mean? A simple distribution board made up for less than $10 should do you just fine for plugging instruments together. There’s no need to go crazy over broadband noise. Bob > On Nov 23, 2014, at 10:50 PM, Bill <b...@hsmicrowave.com> wrote: > > Hi Bob, > > Thanks for your comments. > > The devices in my lab that can benefit from the low phase 10 MHz source are > 1) the spectrum analyzer(s), 2) a Comstron direct synthesizer, 3) the > synthesized signal generators and the test source(s) used to drive microwave > multipliers and signal sources. All these devices will see the 10 MHz phase > noise (improvements) within the narrowest PLL the devices use. > > After spending "bucks" for a low noise 10 MHz source, I can't afford to use > one for each instrument. Besides it would hurt to go through the trouble of > buying a low phase noise 10 MHz reference and lose it in a poor distribution > amplifier(s). Also, the advantages of running all instruments from the same > 10 MHz source are well known. > > So while I was hoping to short circuit some of the design/prototyping effort > in the hopes someone on this thread had been there, I'll just "hit the > books" and do some prototyping and noise testing and see what I come up with. > > Regards...Bill > > -----Original Message----- > From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Bob Camp > Sent: Sunday, November 23, 2014 7:08 PM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Low Additive Phase Noise 10 MHz Amps > > Hi > > While OCXO’s that have -170 dbc/ Hz specs are fairly common, they normally go > deep inside a box of some sort. It’s a rare off the shelf device that takes > in the output of a distribution amp *and* requires that sort of phase noise. > > What’s your target device(s)? > > Why do I ask? Well, a device that has a -170 dbc floor combined with a -170 > dbc oscillator will give you -167. A device with a -200 dbc floor will still > “degrade” a -170 dbc oscillator. That’s a fairly big change in circuit > complexity (and cost) for a 2.9 something db improvement. The list of devices > that might make it worth spending (say) a few hundred dollars a channel > versus under a buck a channel is pretty short. That may put a bound on this. > > One example may help: If you are running phase noise testing, forget about > multi channel distribution amps. They will add a ground loop(s) / pickup > loop(s) that you will be fighting forever and ever. Do that sort of stuff > straight off the oscillator. There is no rational amount of money (ummm …. > errrr … how much do you have?) you can spend to get around this. A second (or > eighth) oscillator is cheaper than even some of the simple approaches that > don’t work very well. The type of OCXO you are talking about is a < $50 item > on eBay. > > Bob > > >> On Nov 23, 2014, at 9:17 PM, Bill <b...@hsmicrowave.com> wrote: >> >> Thanks to all for the response but the distribution amp additive noise can >> be a real problem since the 10 MHz to be distributed is -170 dBC/Hz at 10 >> KHz and needs to be preserved if at all possible. >> >> BTW, the Ettus Octobox doesn't have a spec for additive phase noise, so >> that's out. >> >> Again thanks...Bill >> >> -----Original Message----- >> From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Bob >> Camp >> Sent: Sunday, November 23, 2014 1:09 PM >> To: Discussion of precise time and frequency measurement >> Subject: Re: [time-nuts] Low Additive Phase Noise 10 MHz Amps >> >> Hi >> >> For any “real world” source being distributed, simple high speed CMOS >> buffers will not add enough noise to matter at 10 MHz. That of course also >> assumes that the target gear is the normal bunch of instruments that we all >> play with. >> >> Bob >> >>> On Nov 23, 2014, at 11:47 AM, Bill <b...@hsmicrowave.com> wrote: >>> >>> What's the latest opinion (data) on available low additive phase >>> noise >>> 10 MHz amplifiers for 10 MHz distribution? >>> >>> >>> >>> Regards and thanks.Bill >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
