WWVB Receiver

I want to compare the WWVB carrier phase against the GPS 1 pps time signal.

I have a number of Rb's to use in a N-corner hat to analyze the results. The goal is to see if offsetting the GPS 1 pps by changes in the WWVB carrier would help reduce the diurnal shift in GPS time. This could improve the typical GPS accuracy from 1e-12 to possibly 0.1e-12.

Thanks for the paper.

The 10 MHz GPS clock is derived from the 1 PPS. I will use the phase of the WWVB carrier to compare against the GPS signal. I had already planned on putting everything in an oven for stabilization.

That's not how the time code is usually handled. The time code is decoded by looking at the bit timings and pulling out the start of each bit and decoding the value. This value is always late, reporting the time of the minute index mark the message started with. In your case you don't need to consider the data at all. You simply look for the starting edge of each bit on one second intervals. There could be an acquisition mode where the algorithm searches for valid bits without restriction on the timing details. After finding some number of successive bits it can then look for the start of bits only within the window near the 1 second boundaries to help minimize noise impacts. The rising edge time of the pulse will depend on a number of effects on the bandwidth of the entire path. So you can be the judge of the stability of these effects.

To sync to the carrier may have issues with receiving a clean carrier. I've never bothered looking at that detail, so I can't advise on issues there. I suppose with the averaging available this can be mitigated. A simple high-Q resonant circuit may be adequate.

The varicap tempco is not important. It is in a pll inside an oven.

Breakthrough. Paul, W1VLF describes how to make a ferrite rod antenna using around 500pf to resonate at 60KHz:

A MVAM109 will reach 500pf:

This would also allow a 3:1 tuning range covering 60KHz to 180KHz. This includes signals at 100KHz that might also be valuable.

Amidon sells MnZn type 33 material rods for $60 each. Two are required:

- Materials Metal Rod - 1 x 53pcs Red Heat Shrink Tubing, 1 x Storage Box Price: $24.75

You are trying to fine tune the resonance at 60 kHz only, no? So wouldn't it be better to use a low tempco fixed cap at say, 470 pF and a tuning cap to trim the final ~30 pF? I believe the tuning rate drops off quickly as the value drops, so a 500 pF unit would require a higher voltage to get to 30 pF and at that point the sensitivity is low making it less sensitive to noise in the control voltage and the signal on the circuit.

OK. Someone was talking about a varicap to tune the antenna.

On a sunny day (Sun, 19 Dec 2021 13:00:38 -0800 (PST)) it happened Rick C snipped-for-privacy@gmail.com wrote in snipped-for-privacy@googlegroups.com:

All true, even so it is amazing that my Casio Wavecepter watch gets the time / date here in minutes while it is indoors and surrounded by .. 12 wallwarts, most of those switchers, many computers and a big UPS, what not. But indeed if you want only the carrier you need high Q precise tuning. But then the slightest tuning change will cause a phase shift I'd expect. Phase shift at such a low frequency is a lot of time... Temperature stabilization is perhaps a must then.

The trick is to generate a local oscillator derived from a Rb clock and correlate that with the incoming MSF signal over the long term. Provided you do it right and can prevent the LO blinding the MSF receiver you can get a stable local clock that is exactly derived from the mean reference signal at the remote site plus light travel time (which varies with air pressure and humidity enough to be a problem if you are looking for ultimate precision). ISTR it took them a lot of iterations before they were happy that device worked exactly as intended.

As I said they had to defend it against being tricked into drifting when there was dew on the ground plane at the transmitter site - but that only became obvious after they had been using it for a while.

You can be pretty much assured that the carrier will be dirty. Only the long term average of its correlation with a precision tuned local oscillator is any good. The latter being used to discipline the Rb clock. If he can find the hardware paper or one for VLBI synch protocols that ought to provide enough clues on how to do it optimally today.

It relies on the weak law of large numbers to work. More signal is always better but you have to accept that the VLF bands have more than their fair share of noise in them from man made and natural events.

On a sunny day (Mon, 20 Dec 2021 11:28:41 +0000) it happened Martin Brown <'''newspam'''@nonad.co.uk> wrote in <spppda$99r$ snipped-for-privacy@gioia.aioe.org>:

Right, I even did build a lightning detector with a ferrite rod tuned to some VLF frequency. In case of lighting you are going to get periods with no signal (saturation) and all sort of transients, also at other frequencies than the one you are looking for. Lightning happens perhaps more often than you think, on an AM radio just causing cracking noises.

I'm guessing you don't need much adjustment. Sliding a ferrite toroid over the rod may well increase inductance a little, a brass ring may do the reverse. Worth a try anyway.

Whistlers are really cool if you build a suitable wideband LF antenna with low noise preamp and feed it into a waterfall display realtime FFT.

The 1PPS is also derived by a PLL locked to the GPS signals, so you're still dependent on the stability of the underlying VCO for low phase noise. The GPS signal is well below the noise floor, so the PLL loop filter is necessarily slow - meaning the VCO phase noise will probably extend to sub-Hz frequencies before the GPS locking signal starts to pull it in. The 10MHz output is no better, of course.

Folk who are enthusiastic about their GPS disciplined 10MHz reference source seem to mostly disregard these issues.

Clifford Heath.

You seem to disregard multipath shortwave propagation and effects of the ionosphere on WWV. And the SNR of GPS is positive when the signal is folded back to its information bandwidth. In the phase noise/short term and mid term you can only rely on your crystal oscillator. Only in the long term the GPS system lends a helping hand. The MTI-260 oven in my GPS receiver is alone by itself not much worse than a rubidium ( that must rely on its crystal osc for the short term, too).

for a glimpse into a qualified GPS receiver.

Cheers, Gerhard

The longwave WWVB ( 60 kHz) is dominated by ionospheric conduction, so multipath is a non-issue for that source (other WWV stations at 5, 10, 15 MHz are less predictable). GPS ought to be less dependent on atmosphere effects, of course 'cuz in the UP direction there's only a few miles of atmosphere.

For best results, you might just want to make an atomic fountain of your own.

You have got it backwards. The 1PPS is from the GPS signal. The 10MHz is from an OCXO locked to the 1PPS.

This is where most people go wrong. There is 20ns to 50ns jitter in the

A conventional phase detector using an RC loop filter will drift during the period between samples, and pass the jitter to the OCXO. Duty cycle, or PWM phase detectors, have huge jitter and suffer from false lock where you can get a non-integer divide ratio. Then you need to filter the error signal with a very long time constant, perhaps days. With care, you can get about

I have designed a zdncpfd: zero deadband noise cancelling phase frequency detector that locks two clocks together but omits the jitter of either clock, such as the jitter in the 1PPS. It bypasses the 20*log(N) of conventional multipliers and dividers. It has many uses outside of GPS and WWVB.

It is a full PFD so it guarantees lock if the locked oscillator has enough range. It is a combination of the first zero deadband PFD patent that I got around 1970, and more recent inventions on noise cancelling high frequency samplers. Yes, such a thing is possible.

It should be noted that Tom Van Baak of Time-Nuts has designed PIC dividers with extemely low jitter that I will use in the countdown. He has also published curves of diurnal phase shift of the WWVB carrier compared to a Hydrogen Maser (how he can afford such a thing is beyound me) and shows the severe shift at sunrise and sunset. Measuring Time is a very absorbing and addictive hobby.

HNY

Ok, we have DCF77 on 77.5 KHz instead

I remember hearing them now & then in Europe

.. and you can remove the Faraday completely if you can carrier lock the (lower) military frequency, too. Don't need no secret polynom for that.

Don't need that; I've made a comparator for a hydrogen maser and a Cesium in space. In zero gravity you don't need a fountain; a fountain is a sorry replacement for zero gravity. The active hydrogen maser has the better phase noise, but cesium is the law, at least in the long run.

Cheers, Gerhard

There are many errors in WWVB phase, primarily the diurnal phase shift at sunrise and sunset. Tom Van Baak of Time-Nuts has published thse errors against a Hydrogen Maser, and they are quite substantial.

GPS has many, many more errors besides a diurnal phase shift. You can get multipath errors from local reflections, errors from the slant range when satellites are near the horizon, even the error from the temperature coefficient of the GPS antenna and coax cable leading to the receiver.

However, with care and special receivers and antennas, GPS is unbeatable. You can get cm level accuracy, which is great for surveying. Getting accurate time is a bit more difficult, but you can get down to 1e-12 accuracy. This is why GPSDO's are so popular for in-house standards.