Tracking 10Mhz WWV using a spectrum analyzer - Page 1
Forum section: electronics / testgear

author: cncjerry
author_rank: Unknown
timestamp: November 20, 2014, 12:13:20 am
content: I recently picked-up another HP 3457a for what I thought was a good deal. Having two now as well as a few frequency counters from Tektronix, I thought I would check them as the calibrations were old. In addition to the checking the new meter, I was thinking of taking a crack at the ARRL frequency measurement contest as I had an idea I wanted to try.I generated an approximate 1Mhz signal and checked it with two 3457a meters and the counter in my spectrum analyzer and they both were close, +/- the last digit flipping. I then looked at the same signal using my older counters, and they were 17 to 18hz high at 1Mhz So I have two sets of three that seem to be off by 17 to 18hz between the sets at 1Mhz The number is linear in that at 10Mhz I am now off by 180Hz between the two sets, or assume to be as the 3457A only goes up to 3Mhz but the spectrum analyzer and my other counters go higher.For the heck of it I attached a 10Mhz antenna to my SA and dialed-in WWV at 10Mhz. I put the SA on 10Mhz with a 300hz BW and a 10hz VBW then hit a single trigger with the peak tracking on the 10Mhz WWV signal. I then hit the tracking generator so I had an output signal equal to the tracking input. I checked this by using a 1Mhz tracking with the SA TG and it all tracked +/- the last flip.Curiously, while tracking WWV the signal was 180hz higher and mapped against my older gear where I expected the signal to be consistent with my 3457a models, mainly because the 3457a models had previously hit plus/minus last digit with all my scopes and other calibration signals.So the question is more around tracking WWV on an SA and does the modulation and other signals in their transmission put the peak slightly higher, 180hz higher? Or, and I'm not sure what side of the fence I am on with this one, is it all coincidence and justification to purchase one of those GPS based 10Mhz standards? I can see a very well defined peak in WWV and at certain times the sidebands, etc depending on their broadcast. I then up'ed the averaging and it all seems to make sense.Thoughts?

author: retrolefty
author_rank: Super Contributor
timestamp: November 20, 2014, 12:25:30 am
content: Maybe a classic case of man with one watch always knows the time, man with two watches never quite sure?Pick/obtain a reference/standard and live with it.

author: G0HZU
author_rank: Super Contributor
timestamp: November 20, 2014, 01:16:37 am
content: I'd not bother taking the HP3457A too seriously as an accurate counter (it's going to be toytown in terms of resolution and stability) so probably best to turn these OFF.Assuming your other counters are reasonably good quality with reasonable resolution and a reasonable internal reference you should be able to calibrate them to within about 0.2Hz at 10MHz using WWV and a suitable receiver and a dual channel scope. I'm not sure I'd recommend using a spectrum analyser as the receiver though...See W2AEW's youtube video for a practical demo.QuoteThere will be a few people who will recommend this and a few that will recommend a Rubidium standard.But it depends what you want to do. I don't have either GPS or Rb here but I do have some nice spectrum analysers that each have an internal 10MHz OCXO than gives decent performance over time.Often less than 0.05Hz drift at 10MHz in several months. Nearly always less than 0.1Hz drift in 1 year. Easily good enough for my needs.

author: KD0RC
author_rank: Regular Contributor
timestamp: November 20, 2014, 01:20:40 am
content: Hi Jerry,Here is a link to a thread about calibrating the Rigol DSA 815 directly from WWV.https://www.eevblog.com/forum/testgear/tried-a-new-method-for-calibrating-a-rigol-dsa-815/msg396134/#msg396134Here is a link to a YouTube video that I did about ithttp://youtu.be/E82NL086VrEI hope this helps.Len

author: G0HZU
author_rank: Super Contributor
timestamp: November 20, 2014, 01:23:10 am
content: At the other end of the scale, I bought an old Marconi TF2430 counter when I was a student in the 1980s. This counter dates back to the early 1970s and it only uses a basic 10MHz crystal oscillator as a reference.From a cold start it is usually 7 to 10Hz off frequency at 10MHz but after a 30 minute warmup it is within about 1 or 2Hz of 10MHz. It rarely needs adjusting and seems to deliver this level of performance quite consistently year after year.If you want better than this then you can buy a used 10MHz OCXO on ebay for maybe $20 that will improve on the above spec by a factor of 10 (maybe even a factor of 50) after a 30 minute warmup time.

author: G0HZU
author_rank: Super Contributor
timestamp: November 20, 2014, 01:36:32 am
content: QuoteI think a lot depends on the technology used inside the spectrum analyser. Some analysers aren't stable enough to do this type of measurement and even the good ones can typically only resolve to 1Hz with the frequency counter function.My old Advantest analyser actually uses a 'real' internal frequency counter locked to the internal $$$ 10MHz OCXO and it measures the frequency at an intermediate frequency. i.e. you put the marker close to the peak of the wanted signal and the counter will measure that signal. You don't have to accurately peak the marker on the RBW filter response for it to read the signal accurately because it uses a genuine frequency counter for this task. But on other analysers you have to peak the marker right at the peak of the RBW filter because they have a different method of measurement.Overall, it would be best to do it with a regular receiver. eg copy the method used by W2AEW.

author: G0HZU
author_rank: Super Contributor
timestamp: November 20, 2014, 02:20:27 am
content: I tried measuring the frequency of several local AM radio stations here in the UK with my spectrum analyser because I can't get 10MHz WWV here in the UK on my basic antenna.I tried Talksport on 1089kHz and it measured it at 1.089000MHzI tried Absolute Radio on 1215kHz and it measured it at 1.215000MHzI tried an unknown station at 9.57MHz and it read 9.570000MHz.If I feed in a signal at 9.570002MHz from a very accurate lab grade Agilent sig gen (high stability OCXO option fitted) the analyser reads 9.570002MHz which proves it really is measuring down to this 1Hz resolutionIt manages to read these stations fine despite the AM modulation which seems pretty impressive but then it only 'counts' the waveform that passes through its RBW filter. I think it briefly uses zero span mode at the marker frequency and measures the frequency here.Other analysers will use different methods but I can't resolve lower than 1Hz resolution on the counter in this analyser.

author: cncjerry
author_rank: Unknown
timestamp: November 20, 2014, 04:02:13 am
content: I'm going to try some local AM stations to see how they compare.I am using an Advantest SA and it has the built-in counter, etc, etc. It tracks well within the window and I have found it to be pretty accurate down to 1hz. Clearly I am doing something wrong setting the peak.Also, I disagree with the other poster about taking the 3457a meters out of the mix as that is what I was trying to check. I've found my first 3457a and the two 3456a meters I have to be very solid, very repeatable across the board. It doesn't have the range as an FC but it does a good job within its boundaries.I'll report back on the local stations to see what is going on.

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 20, 2014, 04:55:00 am
content: Quote from: cncjerry on November 20, 2014, 12:13:20 amWWV is AM modulated (double sideband). That means that, while there are sidebands on each side of the carrier when audio is present, the center carrier frequency is always at 10.000000 MHz and never offset, certainly not by 180 Hz. The sidebands are offset at equal distances above and below the carrier. The audio tones are modulated at a level of 50%, so the carrier is present during the audio tones.I have a ham radio friend who is into microwave stuff and high precision frequency standards. He doesn't like using WWV as a frequency standard because the path length through the ionosphere is constantly changing, causing the frequency to randomly vary very slightly. It's not enough for most of us mortals to measure (I certainly can't measure it), but he claims he can see the random walk when he compares WWV with his GPS disciplined rubidium oscillator. But even so, it's a very small random variation up and down with a long-term stable average. It's not an offset in one direction, and not by anything approaching 180 Hz.

author: cncjerry
author_rank: Unknown
timestamp: November 20, 2014, 06:27:36 am
content: How about turning on the Freq Cor option in the Cal menu? How come all the experts including me overlooked that little switch?The book is vague about the function of that switch and also shows it in the Off state before calibration which I guess if you think about it would make sense. But nowhere does it say to turn it on after calibration. So I did and my 180hz problem went away. Now it is 10.000 000 depending on the fade but when I have a good signal the peak is tracking right at 10.000 000 Mhz exactly. But I'll be the first to admit it could also have been this round of calibration. My Advantest SA is pretty decent for what I could afford. I would hate for it to develop a problem, though, as I hear they are pretty much undocumented.By the way, I'd did read once that WWV does ISB at some point in the broadcast for the 100hz BCD (IBM wins one and maybe the only over ASCII or was that EBCDIC?) but I don't know if it is on the main or a sub carrier.

author: G0HZU
author_rank: Super Contributor
timestamp: November 20, 2014, 06:18:06 pm
content: QuoteProbably because you hadn't told us what analyser you were using? How can you expect us to know what menus it has or how your analyser operates without knowledge of the make/model?

author: HiTech
author_rank: Regular Contributor
timestamp: November 20, 2014, 06:36:46 pm
content: Quote from: G0HZU on November 20, 2014, 01:23:10 amThe HP 3457a doesn't facilitate the option for an OCXO. In fact it's limited to 1.5MHz max for freq. The freq. counter feature is not what that unit is really about. Personally, I would have both my SA and the FC slaved together from the same OCXO. The HP Option 001 10MHz OCXO can typically drive two units with reliability. Slaving a third might get dicey. This is why I recently purchased a stand alone OCXO along with a distribution amp to interface more than two devices to a single, stabile reference.

author: G0HZU
author_rank: Super Contributor
timestamp: November 20, 2014, 06:53:37 pm
content: Quote from: HiTech on November 20, 2014, 06:36:46 pmI agree and that's why I suggested turning the HP3457A meters off and forgetting about the frequency counter mode they haveIt's a toy feature on the DMM that is probably referenced to an internal clock that has maybe 10-100ppm stability over time/temperature. Best to stick with a regular frequency counter.

author: Co6aka
author_rank: Unknown
timestamp: November 20, 2014, 06:57:01 pm
content: RF amp, to 10 MHz crystal filter, to IF-amp, to 10 MHz crystal filter, to hard-limiter, to buffer, to counter or whatever.

author: German_EE
author_rank: Super Contributor
timestamp: November 20, 2014, 07:46:44 pm
content: Rather than WWV I calibrate to DCF77 which is about 30 Km away from me but the principle of this method is the same:1) Receive, filter and amplify the signal until you can display it as a sine wave on a scope. Feed this signal into channel 1 and trigger on it.2) Feed your 10 MHz oscillator signal into channel 2. In my case I need to use a DDS to generate 77,50 KHz from 10 MHz but as you are using WWV you can use the raw 10 MHz signal.3) The signal on channel 2 will drift left or right. Alter the frequency of your oscillator until there is zero movement, you are now within fractions of a Hertz.4) Use this 10.000000 MHz signal to calibrate all your counters and analyzers.

author: VK5RC
author_rank: Unknown
timestamp: November 20, 2014, 11:44:11 pm
content: Have you seen K5CMs website, he gives some good advice on fmt without breaking the budget? Also some devices even with a frequency reference input don't use it for all of the rf chain eg HP line level meters 3586, a mod for this unit is available if you want to lock the IF stage.Have fun!

author: cncjerry
author_rank: Unknown
timestamp: November 21, 2014, 01:21:49 am
content: Quote from: German_EE on November 20, 2014, 07:46:44 pmI'm missing something here so please help as though it doesn't apply to this project I am now curious. In step 1 above, how do you receive the signal? Broadband filters only like the old TRF radios? You can't use an IF without corrupting the signal, right?So I could make a few 10Mhz crystal filters, put a tuned amp on the front and back of the filter and hook it to my 10Mhz antenna, I guess. That will give me the 10Mhz WWV carrier with modulation. I'll have to look into the format of the signal as I thought there was an ISB 100hz component in it. But if I just filter and amplify without any IF, I should be able to capture the 10Mhz signal, no? From there it is easy.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2014, 01:38:36 pm
content: Quote from: cncjerry on November 21, 2014, 01:21:49 amUsing a tuned receiver with no IF works fine:http://www.w4cwg.com/wwv10rx.htmlYou could also use the same local oscillator to shift WWV and the reference to be adjusted which the page at the link above mentions. I am not sure why you could not use the same local oscillator to mix the signal twice to recover the reference. Beware of high Q filters because they can suffer from enough phase shift change over temperature to affect accuracy.NIST says that WWV/WWVH are only good to 1x10^-7 (0.1 PPM or 1 Hz for a 10 MHz reference) do to propagation changes. Just an oscilloscope can easily be used to make comparisons at a level 10 times better than that. An OCXO could be 200 to 1000 times better over 24 hours. Averaging WWV/WWVH over 24 hours can improve this to a range of 5x10^-9 (0.005 PPM or 0.05 Hz for a 10 MHz reference) to 1x10^-9 (0.001 PPM or 0.01 Hz for a 10 MHz reference).http://www.nist.gov/calibrations/upload/sp250-67.pdfhttp://www.timeok.it/files/ocxo_comparative_table.pdfhttp://tf.nist.gov/service/pdf/calibrations.pdfGround wave propagation from WWVB (60 kHz) is much more accurate if you can get it and is within an order of magnitude of GPS.

author: German_EE
author_rank: Super Contributor
timestamp: November 21, 2014, 06:21:38 pm
content: My DCF77 receiver is in two parts. The head end contains a large ferrite rod antenna and capacitor designed to resonate at 77.5 KHz, there is then a three stage amplifier with a balanced output that also contains some LC filtering. The bench box contains some more amplification, a 77.5 KHz crystal filter and a fixed DDS chip that takes a 10 MHz signal and generates 77.50000106 KHz (close enough) for adjustment.