WWVB Receiver

You can alter the effective permeability by partially saturating the core, but it will spoil the antenna function.

The ferrite stick with coil is just an inductance, and its resonant frequency is dependent of the companion capacitance. The target frequency should not be too far from the design frequency, as the circuit impedance will be far off. Also, the ferrite material works well on the design frequency, and it may show more losses than desired if the frequency is far off the original.

A crystal filter may be a not too good idea, as it is by nature narrow-band with accompanying wild phase/delay characteristic wit even slight frequency differences.

You may be better off with some kind of superheterodyne approach.

How about a giant loop antenna? Use multiconductor shielded cable. Lots of pickup area.

Normally one would ground one end of the shield, but why not resonate the shield?

Thanks for the suggestion, but I don't need a large pickup area. I have a number of WWVB clocks, including two Casio Waveceptor wrist watches. They have a tiny ferrite antenna to fit in the small case, but still have enough signal to synchronize perfectly every night:

I need to compare the phase of the WWVB signal with the GPS to analyze the diurnal time shift affecting both signals. A GPS signal does not provide phase information, so I need to use the 1 PPS signal and compare the time with WWVB. A Ublox LEA-5T-0-003 50-channel 15ns 1pps Timing GPS module will provide the 1 PPS:

Time-Nuts has numerous posts discussing the phase errors caused by resonant circuits in the signal path, so I need a method of tuning the ferrite loop and keeping it at resonance.

Thanks for the help.

If it's too low grind some ferrite off. if it's too high glue some on

Jan Frank wrote: ================

AM radio 101

..... Phil

On a sunny day (Sat, 18 Dec 2021 13:10:35 -0800) it happened snipped-for-privacy@highlandsniptechnology.com wrote in snipped-for-privacy@4ax.com:

Normally one would buy a ferite rod, look up the permeabiliy get a few hundred pF cap calculate L required for frequency, then add that many turns to the rod.

If he has a rod with coil, measure L, calculate C required.

My Casio waveceptor watch has o.a. radio DCF77 reception to automatically set time, the ferrite rod in it is minucule. Always accurate to the second.

On a sunny day (Sun, 19 Dec 2021 05:14:04 -0000 (UTC)) it happened Jan Frank snipped-for-privacy@not.com wrote in <XnsAE052628EBB7idtokenpost@144.76.35.252>:

We have the same watch it seems. Anyways large capacitance varicaps exist (I have used those for very low frequencis) or you could use some normal diode at low reverse bias for that.

You can get pre tuned ferrite loop antennas. You won't need to tune them they are more or less spot on 60kHz +/- small fractions of a ppm.

You should look at the low frequency LBI studies by Duffet-Smith et al in the 1980's. They were disciplining a local Rb clock with the MSF Rugby signal so their local clock was way better frequency stability than the instantaneous signal from Rugby.

ISTR What they found was that the time delays depended critically on the air humidity and amount of dew on the ground at the transmitter site. This is hardly surprising as it affects the time constant of their transmitting antenna. MSF Rugby were interested in the result.

GPS is essentially a better than Rb clock so you should be able to reproduce their results (They were doing it to timestamp interferometer data in a not quite VLBI method at low frequencies).

Looking for the white light fringe in LBI is still a nightmare even when you have precision timestamps - particularly with the hardware of that era.

This isn't the paper I remember but search terms into ADS elude me for the moment. It is related but at 16kHz (submarine comms frequency).

Thanks for the info. I am trying to cancel the diurnal time shift of GPS signals by using phase information from WWVB signals to improve the typical

I have a number of Rb clocks for use in a N-corner hat to compare the GPS and WWVB signals. Ideally the reference should be a hydrogen maser but that is far beyond most people's budgets. However, moving GPS closer would be a valuable improvement at far lower cost.

Interesting comment about humidity. However, there are other signals in the vlf region such as 100KHz that could also be used.

OK. If you have an Rb clock to hand you definitely want the paper that describes the MRAO portable Rb clock synchronisation to MSF & Loran.

If memory serves there was a systematic ~2us lag in the early morning signal compared to light travel time to the remote site which went away as the sun dried the ground.

Not mentioned in the only paper about it I can find at 81.5MHz but the references may well include one with more details of the hardware.

Yes driving a variable direct current thru second coil around the ferrite rod will decrease the permeability. If you set the unsaturated system to resonant slightly low then you could adjust the dc bias to hit resonance. BTW how to you plan to know when the loop is resonant?

I wonder since you are comparing against the 1PS GPS timekeeping can you simply go by the 1PS WWVB time code instead of measuring WWVB carrier frequency?

piglet

At 60 KHz, 1 ns resolution is 0.02 degrees. I don't think a ferrite rod antenna will be stable to numbers like that. A loop or solenoid air-core antenna might.

Maybe a solenoid in an oven! With tuning/amplifiers as needed. May as well put your GPS in the oven too. (I'm thinking ovens because we're controlling some optical stuff to a few mK just now.)

There are diodes specifically designed for capacitance tuning. Although they are largely obsolete and hard to find, they still available. I don't recall the capacitance required to tune a ferrite loop to 60 kHz, but some hundreds of pF should suffice for fine tuning. No? I think the trick would be dealing with the temperature characteristic of the fixed capacitor already installed, but I think you said you would control the temperature. Even so, you might want to replace that with your own capacitor with a more stable temperature coefficient.

So is this feasible or am I missing something?

Why does the ferrite rod need to be highly tuned if there is plenty of signal? An alternative approach might be to use a broadly tuned ferrite antenna with just enough selectivity to avoid overloading subsequent stages with other signals and then digitise the 60kHz WWVB signal with a wide-band 24-bit audio ADC, perhaps sampling at around 192ksa/s. This would be fine for acquiring a 60kHz signal. You could then process the ADC output with a linear-phase digital filter to get the final selectivity you need. John

Varicaps have ghastly tempcos that change with bias.

I think Jan is looking for high time resolution, nanoseconds maybe, so varicaps and ferrites are out of the game.

If he's looking for a diurnal time shift, that could be overwhelmmed by the diurnal temperature cycle whacking all the gear.

That is a point. Although you get 60,000 times more points to average with the carrier phase. In the end the detail that is being used for timing with the modulation are the edges of the modulation changes. These will be nowhere near as precise as the carrier phase due to the bandwidth limitations of the transmitter and receiver and even possibly the atmospheric conditions.

If you don't care about the absolute time, but only relative changes, I think measuring carrier phase is the way to go.

It may well be the GPS side that is limiting. The 1 PPS signal is often specified with a significant jitter, probably a result of being produced in a clocked digital system. The 17 us carrier is three orders of magnitude from the GPS digital jitter that may be 20 ns to 50 ns, so it will require a great deal of averaging of both the GPS and the WWVB signals to improve on that precision.

I wonder if it would be just as effective and perhaps easier to implement a disciplined local oscillator that simply references the GPS signal? The characteristic of the WWVB signal that is being used is the *average* accuracy. I'm sure the GPS signal has this same property, so in reality the WWVB signal is redundant. In fact, the GPS signal probably has superior short term stability, but the generation of the 1 PPS signal mitigates that. Other frequency signals can also be emitted by most GPS devices, 1 kHz, 10 kHz, etc. They will have the same jitter spec most likely, but provide more points to average over.

What am I missing about using WWVB to discipline a local oscillator that would make it better than the GPS disciplined local oscillator? One downside is that WWVB is *not* a strong signal in many locations so that it is not available during the day.

What is being measured are the temporary deviations in the GPS signal which can be just as well compared to the *average* GPS timing rather than to an external reference.

What phase shift do you want to measure, the GPS or the WWVB?

You can get a cheap surplus rubidium standard that will drift nanoseconds per day. That could be the reference that you measure the other against.

It's fun to trigger a scope from one rubidium and zoom the rising edge from another at ns per division. It looks internally triggered! Check it occasionally and see it creep across the screen.

The ferrite rod needs to be highly tuned because unless you are very close to the transmitter, the signal gets swamped by extraneous interferers. The tuning is actually how a loop antenna works. The effective height of a typical small loop antenna is small, but is multiplied by the Q to determine the output voltage. It is the high Q that makes a sensitive loop.

Even with the high Q of a tuned loop antenna additional filtering is important as interferers are often strong.

The ferrite or capacitance adjustment is to mitigate a source of error in the measurement. It may be within the project's error budget to have some error in the error compensation, a secondary impact on the measurement itself. We can only know what is an acceptable solution if we had detailed requirements. So we brainstorm and propose ideas that may well work depending on the details such as the quality of the temperature stabilization.

With temperature control of the electronics, this could provide a very stable reference.