EEVblog #459 - Counter Shenanigans - Page 1
Forum section: electronics / general

author: nitro2k01
author_rank: Frequent Contributor
timestamp: April 22, 2013, 02:49:39 am
content: What I expected was that the counter would have a higher frequency PLL (nominally) locked to the 10 MHz reference, which could be slightly out. I assumed it would do this to get a faster than expected update rate in higher digit modes. Of course, I was wrong. I noticed the higher gate times in this video. (D'oh!)

author: vk6zgo
author_rank: Super Contributor
timestamp: April 22, 2013, 03:44:11 am
content: I hate to be a nasty old Greybeard (well,actually I love being one!),but,well,----Duhh!

author: max-bit
author_rank: Frequent Contributor
timestamp: April 22, 2013, 03:56:31 am
content: !And if so compare Rubidium Frequency Standard (GPSD) of cesium frequency standard?Then we come to know the quality of these freq stnd?Maybe men from Agilent something to help?

author: hairykiwi
author_rank: Contributor
timestamp: April 22, 2013, 11:39:53 am
content: Nice follow-up - Cheers Dave.Here'sa request for another, if anyone with a rubidium standard is interested in helping please.

author: kxenos
author_rank: Frequent Contributor
timestamp: April 22, 2013, 11:51:01 am
content: Does anyone have experience with these FE-5680As??http://www.ebay.com/itm/FE-5680A-Rubidium-Atomic-Frequency-Standard-Oscillator-/260839921883?pt=LH_DefaultDomain_0&hash=item3cbb4558dbAre they substandard standards or not?

author: madires
author_rank: Super Contributor
timestamp: April 22, 2013, 12:02:26 pm
content: Another idea to have some fun with the counter would be to feed 1MHz as external reference and input. Does the firmware detect such a bogus reference frequency or will it keep displaying 10MHz happily? How much deviation from 10MHz is possible?

author: grumpydoc
author_rank: Super Contributor
timestamp: April 22, 2013, 12:02:58 pm
content: QuoteI can't see the auction (work block ebay) but there are plenty of resources on the net regarding the 5680A. ISTR that there's even an effort to reverse engineer the whole thing.As to the ebay units I'm certainly happy with mine as a general workshop frequency standard. Currently checking it against a GPDSO and when I checked mid-week the difference between the two was about 3 parts in 1011. I had a quick look last night now thet the GPDSO has had a bit longer to settle in and I couldn't see any relative phase change between the two over about 15 minutes so they are very close. Given that the GPDSO should itself be within about 3 in 1011averaged over 24 hours it's looking like the Rubidium is at least as good as that.However it is a bit of a lottery - you could easily get one with hardly any life left in the bulb or which hasn't been set up correctly. It's also sometimes hard to know which of the variants of the 5680A you'll end up with.

author: EV
author_rank: Frequent Contributor
timestamp: April 22, 2013, 12:04:07 pm
content: Quote from: kxenos on April 22, 2013, 11:51:01 amThis one seems not to have 10 MHz output, only 1 PPS ( 1 pulse per second ).

author: grumpydoc
author_rank: Super Contributor
timestamp: April 22, 2013, 12:07:50 pm
content: QuoteDepends on the counter.The Racal Dana 199x counters have an option which adds an extra board which will accept a 1MHz, 2MHz, 5MHz or 10MHz external standard and locks a PLL at 10MHz to the external ref - in this case it would just work at whatever accuracy the 1MHz standard had.A more naieve implementation might just do everything 10x slower and with all the readings 10x too high.

author: max-bit
author_rank: Frequent Contributor
timestamp: April 22, 2013, 12:09:25 pm
content: Each FE-5680 has an output 10 MHz, the difference is that not everyone can be programmed to any frequency from 1 to 20 MHz

author: EV
author_rank: Frequent Contributor
timestamp: April 22, 2013, 12:13:27 pm
content: Quote from: madires on April 22, 2013, 12:02:26 pmFrom manual:The Agilent 53131A has 1, 5, 10 MHz external reference capability—to match customer’s house standard (however, the Agilent 53132A’s external reference capability is 10 MHz only).

author: EEVblog
author_rank: Unknown
timestamp: April 22, 2013, 12:16:47 pm
content: Quote from: EV on April 22, 2013, 12:04:07 pmStrange, I've never seen one without the 10MHz out. Countless others on ebay with it.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 22, 2013, 12:20:03 pm
content: QuoteWhat does that mean? The FE-5680A has a 10MHz output; not a 1 Hz output. It's also not a pulse generator; it produces a sinusoid

author: EV
author_rank: Frequent Contributor
timestamp: April 22, 2013, 12:26:37 pm
content: Quote from: EEVblog on April 22, 2013, 12:16:47 pmTo get 10 MHz output needs programming the unit. I do not know how, never done it.

author: EV
author_rank: Frequent Contributor
timestamp: April 22, 2013, 12:46:54 pm
content: Quote from: olsenn on April 22, 2013, 12:20:03 pmHere is data from an other FE-5680A:FeaturesExtremely low silhouette: less than 1.0 inchStability over temperature: 5x10-11Frequency: 1 Hz to 20 MHzFast Warm-up: <5 min.Stability: To 5x10-12/ to 2x10-10/year

author: N2IXK
author_rank: Frequent Contributor
timestamp: April 22, 2013, 01:16:07 pm
content: There are MANY variants of the FE-5680, most of which are poorly documented (proprietary). The most desirable are the 1-20 MHz programmable ones. The most common are the fixed 10 MHz sinewave output. Most but not all of these have the 1PPS output, as well. Some units are only 1PPS, some output unusual frequencies for specific applications. There are also variants with or without separate SMA RF output jacks, and units that only need the +15 power, with the +5V supply developed internally.Some people have come across units that need 24V or 48V power, as well as units with an analog C-field trim input (and no serial communications option).

author: kxenos
author_rank: Frequent Contributor
timestamp: April 22, 2013, 02:12:56 pm
content: The thing is, if someone finds a suitable, working FE-5680A and adds some video amps he probably will have a decent hobby lab reference, right?

author: c4757p
author_rank: Super Contributor
timestamp: April 22, 2013, 02:20:11 pm
content: Quote from: kxenos on April 22, 2013, 02:12:56 pmI'd say so. I did my own board instead of using a commercial amp - amplified and buffered both 10MHz and 1ppm, but also integrated the power supplies onto that board. Works great.

author: ddavidebor
author_rank: Super Contributor
timestamp: April 22, 2013, 02:36:05 pm
content: suggestion for what video amp choose?

author: EV
author_rank: Frequent Contributor
timestamp: April 22, 2013, 03:34:13 pm
content: Quote from: c4757p on April 22, 2013, 02:20:11 pmWhat commercial amp?

author: kxenos
author_rank: Frequent Contributor
timestamp: April 22, 2013, 03:43:35 pm
content: He probably means something like thishttp://www.google.gr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CC8QFjAA&url=http%3A%2F%2Fwww.stable32.com%2FAn%2520Octal%252010%2520MHz%2520Distribution%2520Amplifier.pdf&ei=BFl1Uda-CK-b1AWUlYHgBw&usg=AFQjCNHD-g8bb6JcRYhlB96BuuOYzxIVdA&sig2=IJGDaRh3QgxhG4UNaj77gw&bvm=bv.45512109,d.d2k&cad=rjaor thishttp://www.google.gr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDQQFjAB&url=http%3A%2F%2Fwww.ese-web.com%2F210.htm&ei=BFl1Uda-CK-b1AWUlYHgBw&usg=AFQjCNFiTaHeoeKO-QClBZjJpLVRm7CgeQ&sig2=FF_9ZagtsMUafRbqCz_i2A&bvm=bv.45512109,d.d2k&cad=rjabut there are pretty nice solutions like tf.boulder.nist.gov/general/pdf/498.pdf from NIST for example.

author: Joules
author_rank: Contributor
timestamp: April 22, 2013, 03:49:19 pm
content: Dave, how accurate are the Rigol 10Mhz out compared to your Rubidium standard. ?

author: JackOfVA
author_rank: Unknown
timestamp: April 22, 2013, 10:34:28 pm
content: Quote from: max-bit on April 22, 2013, 03:56:31 amI posted a comparison of an HP Rb versus an HP Cs standard athttps://www.eevblog.com/forum/blog/eevblog-457-oscillator-calibration-followup/msg220911/#msg220911

author: EEVblog
author_rank: Unknown
timestamp: April 22, 2013, 11:12:56 pm
content: Quote from: Joules on April 22, 2013, 03:49:19 pmWithout even testing it, I can tell you it's crap. Same as the internal crystal in the Agilent counter.

author: JackOfVA
author_rank: Unknown
timestamp: April 23, 2013, 12:42:54 am
content: Quote from: EEVblog on April 22, 2013, 11:12:56 pmThere's another issue with the Rigol time base that surfaces in the function generator. I have a DG-4102 and found that there's a lot of phase noise and/or jitter on the output when using the internal time base. But the output cleans up very nicely when the DG-4102 is clocked with a high quality external time base.The two plots below show the difference. Test setup is to put the DG-4102 to Sine output at 10.050 MHz, and connect that to a frequency mixer (in this case, I used a Mini-Circuits mixer) and the second mixer input to a known high quality clean crystal oscillator (in this case, an HP oven 10 MHz oscillator). The difference signal, centered at 50 KHz, is piped into an HP 3562A dynamic signal analyzer, which for the purposes of this discussion can be thought of as a very low noise FFT spectrum analyzer operating over the range 0-100 KHz. Mixing the DG-4102 output with a clean 10 MHz signal lets you magnify, in effect, the phase noise by an instrument with very narrow bandwidth operating at 50 KHz. The 3562A frequency span is set at 20 Hz, so each horizontal division is 2 Hz.The first plot shows phase noise with the DG-4102's stock time base. Second plot shows the phase noise with the DG-4102 external time base input connected to an HP 3816A GPSDO. (I say phase noise, could be considered jitter as well.)Quite the difference.Rigol should have taken a leaf from Agilent's book and made an extra cost option of a high quality time base.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 23, 2013, 02:49:03 am
content: Quote from: ddavidebor on April 22, 2013, 02:36:05 pmSome TV VDAs go out to 10Mhz or so,but others roll off a bit more sharply.Don't get aVCA,as the clamp circuit might mangle your signal-----it relies on the composite video signal for its timing.

author: ddavidebor
author_rank: Super Contributor
timestamp: April 23, 2013, 05:02:34 am
content: Thanks.And another thing, my clock give me a square wave, because a lot of instrument accept square wave (i'm right?) is ok if i use digital buffer of rthe right speed for it?

author: Joules
author_rank: Contributor
timestamp: April 23, 2013, 05:39:14 am
content: Thanks JackOfVA, that was just the data I was looking for.

author: JackOfVA
author_rank: Unknown
timestamp: April 23, 2013, 11:15:09 pm
content: Out of curiosity, I connected a Racal 1992 frequency counter to the 10 MHz time base output of a Rigol DG-4102 function generator and looked at the frequency change from a cold start. The 1992 counter time base is supplied by an HP Z3816A GPSDO. 1992 set to read frequency to 0.01 Hz increments, with a 1 second gate. (Some clever work by the lads at Racal.) Data collected via the IEEE-488 bus.Plot below shows results from a cold start (last time it was powered up was several days ago) with approximately 3.5 hours of data collected. I'll let it run overnight and see how it looks tomorrow morning.Leaving aside the questionable short term jitter, the overall frequency accuracy and stability isn't all that bad. After a rather short warm up period, it's stabilized about 0.5 Hz high at 10 MHz or 0.05 ppm.The DG-4102 is in my basement shop, so it's not exposed to large temperature swings, which should help considerably.It's about a year old and I have not adjusted the time base, by the way.

author: Teneyes
author_rank: Frequent Contributor
timestamp: April 24, 2013, 03:04:26 am
content: Quote from: JackOfVA on April 23, 2013, 11:15:09 pmIt is not correct to say that the error is 0.05 PPMAnd the combined jigger and short term stabilty within +- 0.003 PPM.It would be nice to see your same setup with the Rigol DS4102 generating 100MHz when also externally locked to the GPSDO. ( to reudce the phase jitter), Please

author: JackOfVA
author_rank: Unknown
timestamp: April 24, 2013, 12:19:19 pm
content: Quote from: Teneyes on April 24, 2013, 03:04:26 amPlot below shows 17 hours of data.By an error of 0.05 ppm, I meant the offset error. Short term excursions are considerably less as you note.Don't understand which data set you want to see with the Rigol at 100 MHz - a frequency count or the spectrum display showing close in noise and jitter?Frequency count is simple enough, but in order to measure close in noise over a few Hz span requires a clean, stable, source of 100 MHz to mix with the Rigol. At 10 MHz I can do this easily with an HP precision crystal oscillator with excellent phase noise performance. Unfortunately I don't have such a reference source for 100 MHz. All my 100 MHz signal generators are synthesized so they will be the limiting factor. (I suppose my elderly HP 8640B would be the cleanest of the various signal generators I have, but even with the frequency lock enabled, it moves around a few Hz over the short term at 100 MHz.)