WWVB receiver design advice [closed]

Question

I am completely new to the world of amateur radio and I'm attempting to make a WWVB receiver and antenna. I currently have a large ferrite loopstick antenna built, based on this video. I got a measured inductance of ~10.12 mH and am using ~695pF for my circuit to resonate at 60kHz. I have been researching WWVB receiver designs and have found a few,

https://archive.org/details/73-magazine-1994-01/page/n13/mode/2up page 10

https://www.burningimage.net/clock/sensitive-60khz-receiver/

It seems like using a crystal is popular for highly selective receiver circuits, but I can't seem to find any 60khz crystals online. They also seem a bit pricy and harder to build/design. I was thinking of making a TRF receiver or something similar to that. The first article I linked seems to have a good design and I can follow how it works for the most part, but I wonder if I really need a circuit this complicated. They have automatic gain control which I don't understand at all and doesn't seem necessary for a simple receiver. The second design almost seems to good to be true due to its simplicity. It seems like no one has been able to get it to work besides the designer which worries me, but I wonder what your opinions are on the quality of the design. I think designing my own receiver might be necessary, but don't know that much about rf technology. Which is why I would be interested in your guy's opinions/tips on how I could go about doing this. Below this I wrote down what I know about designing receiver and some of the questions I have.

From my understanding I will need a good preamp stage, utilizing a n channel jfet to boost the signal enough to be used by ICs. Maybe a regenerative amplifier? or just a common source amplifier? I also have a few questions on how careful I need to be with the design of this stage. Do I need a proper ground using a copper plate to prevent noise or could I just use the negative terminal of my battery? Should I use a sampling coil on my ferrite antenna or just hook up the antenna directly to the preamp? Do I also need to keep my receiver circuit in a metal box to minimize outside noise? Does the n channel jfet I use really matter, or can I more or less use anything? I currently have some J112 and 2n5485 s. I also have a good set of 2n2222s 2n3904 2n3906 s etc, but those don't seem to be used in preamp stages because of their lower impedances (I think, please correct me if I am wrong).

So after my pre amp stage it seems like most designs implement a active band pass filter using op amps or transistor equivalents. I understand this part, but it seems like I can't use any opamp. Most of the op amp seem to have jfet input stages?

The next stage seems to be a demodulator which just involves a diode and a low pass filter. Then this signal is squared and sent off to a decoder. This seems straight forward, but if anyone has some helpful insights for this stage I would be all ears. From what I can tell these demodulator circuits can be made more complex for better results?

I ripped this design off another post to try and see if my antenna was even working properly, It seems to work, the output is a amplified 60kHz signal but doesn't seem to be modulated in the WWVB format which is weird. It looks like a 60kHz sine wave with a constant amplitude.

My main questions are,

• Does anyone have some good WWVB receiver designs? Preferably without utilizing crystals.
• How should I ground my circuitry?
• Should I shield my circuitry?
• How exactly should I connect my ferrite antenna to my receiver circuit? Using a sampling coil or just connect the resonant circuit directly? If I use the sampling coil I assume I wouldn't need to connect a capacitor to resonate at 60khz since it would sampling the resonant circuit already.
• What kinds of components should I use, specifically opamps and transistors. What specifications are important?
• Would prototyping on a breadboard cause problems because of the stray inductance and capacitance?
• Does my general idea of a receiver make sense or is there something I am missing?

Sorry for the long post. I have been doing a lot of research, but I'm still confused and trying to find some solutions. If you need me to clarify anything just let me know.

Answer 1

Does anyone have some good WWVB receiver designs? Preferably without utilizing crystals.

OP's first reference circuit is a regenerative amplifier. These simple receivers can work very nicely but are "squirrelly", meaning that frequency drifts easily, and regeneration control is very critical... two qualities that engineers detest. Its simplicity is attractive, but is often frustrating to troubleshoot for a newbie.

How should I ground my circuitry?

When you're working with loop antennas, grounding isn't an important part of the antenna system - an attractive feature of magnetic loops. But grounding may be important for the high-gain amplifier that follows.

Should I shield my circuitry?

When you're dealing with small antenna signals boosted by high-gain amplifiers, you can get feedback from amplifier output back to antenna. A remote antenna helps. A shielded loop antenna may help too. Amplifiers-in-a-metal-box with well-bypassed power connections help prevent feedback - a very good idea.

How exactly should I connect my ferrite antenna to my receiver circuit?

A ferrite-rod antenna should have its inductance resonated with a parallel capacitor or two - one of which is variable. Resonance will be 60 kHz for WWVB. You can feed this directly to a high-impedance preamplifier (like a JFET) mounted close to the rod. It is better to keep the high-gain part of the receiver further away.

What kinds of components should I use, specifically opamps and transistors. What specifications are important?

A J112 JFET should be able to do the preamp. If you use opamps to further amplify the 60 kHz signal, choose a type with higher gain-bandwidth product, at least 10 MHz. A 10MHz GBW will allow an absolute maximum gain of about 100 at 60kHz., if that.

Would prototyping on a breadboard cause problems because of the stray inductance and capacitance?

Yes, you can expect some differences when you re-build a breadboard prototype into its final version. Some tweaking of tuning will also be required.

Does my general idea of a receiver make sense or is there something I am missing?

Yes, the general idea is sound. The diode detector you mention means that WWVB's amplitude modulation will be available, while WWVB's BPSK phase modulation will not be available. Phase modulation requires a different kind of detector.

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OP's preamp preamp circuit is likely oscillating (output is constant-amplitude 60 kHz). This may be due to feedback from breadboard-to-ferrite antenna, or it may be due to feedback from JFET drain to JFET gate through gate-to-drain capacitance.
Taming a tuned-gate + tuned-drain amplifier like this is quite tricky...it may be better to use the JFET as a low-gain buffer instead. A preamp should have high input impedance to mate with the high-impedance loop antenna, and low output impedance to drive the long path (through twisted-pair wires) to the main amplifier. High voltage gain in the preamp isn't desired.

Here's a fragment of a preamp that I use. The loop antenna was wound on a 0.7m hoop with enough wire to be self-resonant, so no parallel tuning capacitor was needed (that's A LOT of wire!). A ferrite-rod antenna tuned with a parallel capacitor could be substituted (some example component values are shown).
The JFET was combined with a PNP transistor that provides some negative feedback, and keep input impedance high. The JFET's drain-to-gate capacitance cannot lead to oscillation, because the signal at the drain is small compared with signal at its gate:

^{simulate this circuit – Schematic created using CircuitLab}
Voltage gain of this preamp is a modest 3.7, so a lot more gain down-stream will be needed. But at least it should be stable, and not oscillate. Since JFETs are notoriously variable one-to-the-next, bias resistor values might require tweaking to achieve the DC bias points shown. Its output impedance is about 1k ohm, so another buffer stage should be added to drive the long wires going to the main amplifier.