Hell, 60kHz is slow enough that with a fairly primitive mixer you could get down to audio frequencies and do the signal processing on a sound card.
But yea, sampling faster would make a "pure" SW defined radio.
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When I first read your reply I managed to misread it as an attempt at pseudo-Zen humor ("What is the sound of one Hertz clapping, grasshopper?" ). My apologies.
Re-reading the NIST Time Services PDF, the WWVB 60kHz carrier is modulated in a CW-like fashion, that is, the receiver sees narrow (200ms), medium (500ms), and wide (800ms) bursts of 60kHz carrier, at the rate of one pulse per second: one Hz.
If I recall correctly, the ideal AM IF stage has a bandwidth of
10kHz. In other words, it will block the 60kHz WWVB carrier, but (if I "adjust" the RF and LO sections properly) it will pass (and amplify) the modulating pulses, which are now modulating a 455kHz carrier.
But then there's the radio's audio section to think about. I'm not worried about not _hearing_ 1Hz signals, but I have this nasty feeling that the audio section of the typical AM/FM radio might classify a 1Hz signal as either "DC" or "noise", and, on that basis, decide to deliberately block it.
So... let's recap. I'm going to (perhaps) wind a 75-100,000uH antenna coil (which turns out to be on the order of a spool or two of #34 magnet wire), wind an LO tank whose specifics I haven't been able to calculate, and then build my own down-to-DC amplifier to replace the existing (say) 300Hz-10kHz AF section.
You know, it's amazing just how fast "quick and dirty" turns into "really slow, much more complicated than expected, and a pig wouldn't touch it" once you start factoring "reality" into your plans.
( "What is the sound of one hand holding an AM/FM ) ( Clock-Radio striking a nearby head, grasshopper?" )
Frank
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Why bother? Just get an "atomic clock" for about ten bucks, that already has the WWVB receiver in it; open it up and tap into the demodulated signal somewhere inside the thing.
But what would you listen for? Isn't the signal a very low bit-rate digital data stream?
I wonder if NAA is still on the air. They were on 14 KHZ IIRC and at least the were sending CW. I remember an article in QST where someone built a regenerative receiver to pick up their signal using old honeycomb coils and a .001uf variable capacitor. Someone else did the same thing, but used an old tv flyback transformer (minus the ferrite core) for the coil and a three gang TRF capacitor to tune it.
But how long is a 1/2 wave antenna at that frequency?
And only using a few Hz bandwidth. There is a lot of atomospheric interference in that band, and a broad IF isn't what you want. There are also harmonics from TV horizontal sweep and computer equipment. That is why most recievers are TRF. Multiple tuned RF amplifiers or bandpass filters that track a single knob, or is carefully alligned to a single frequency. BTDT about 20 years ago. I built my antenna in copper pipe for a Farady shield and put 40 dB of gain at the antenna. The filters removed over 60dB of in band noise. A coaxial line went to the decoder where there was more gain, and filtering. I had 8 volts peak to peak from the system.
Now, how are you going to amplify a one Hz signal, and what do you plan to use for a speaker?
The modulation is a reduction in signal level, for the time periods you listed. CW is typically on - off keying.
They don't have enough frequency responce, unless it is DC coupled.
Better antenna designs are availible. if you go down to DC you will damage the speaker.
Hit yourself and have someone else record it for you.
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Receivers that are meant to receive CW signals have a BFO (Beat Frequency Oscillator) built in. The BFO is an oscillator that runs close to the receiver's IF frequency. When receiving a CW signal, the BFO beats with the received CW signal and produces an audible beat tone. This tone is what you hear as the keyed CW signal. The BFO frequency is usually adjustable so that the listener can adjust the frequency of the tone to his/her preference.
You will need to build a BFO into your receiver if you want to hear the WWVB signal. The BFO signal can be injected into the final IF amp or directly into the detector. Getting the level of the signal right is likely to be a matter of experiment, depending on the receiver circuit.
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Make a BFO (beat frequency oscillator). This would be a 456kHz oscillator with its output injected into the IF. A signal in the center of the IF band will be heard as a 1kHz tone. You'd also hear anything other signals in the RF and IF passband as either tones at different frequencies or noise. For WWVB, there would be short (200mS), medium (500mS) and long (800ms) tones, with the remainder of each second being the same tone at a lowered volume. I forget what it is, I think the 'spaces' are 20 percent the level of the 'tones.'
I noticed the 50-ohm input impedance. A very BIG problem with low-frequency radios is the very high antenna input impedance. That's because the RF wavelength is very long compared to a typical wire antenna. Resonate ferrite-stick antennas are a workaround, since they grab the RF magnetic fields and pull them in as if they were physically much larger. But, for this to work, you need a high Q, and that drives up the impedance of the tank circuit. Z =3D Q 2pi f L. You're talking about a low-noise JFET (or MOSFET, except they're noisy) input preamp stage. Try looking up active-antenna preamp circuits.
It's not that bad, as inductance is (approximately) proportional to the square of the number of turns, it won't take as many extra turns as you might think. An accurate way to measure inductance is probably a good idea for this. My BK 878 wasn't real cheap, but still works great. Failing that, finding the resonant frequency with a known value capacitor and going back through the resonant frequency equation will work.
Poking around, I found this converter with documentation, schematic, sample .wav files of typical signals including WWVH (obviously modulated with a tone 500Hz off). It's a $17 kit with all components. Using 1MHz crystal in this thing would put the output signal into the broadcast band:
For the WWVB receiver shown on the SED page of my website I made an 8" diameter loop antenna, wound inside 1/2" copper pipe (joined with a fiber-insulated fitting to avoid shorted turn).
Worked like a champ even in the presence of fluorescent lighting.
...Jim Thompson
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Do you remember how many turns you used, what its inductance was? I assume you resonated it, what was the Q, do you know? What was your preamp's Zin, 25k or so?
I'll look thru my notes and see if I can remember the details... after
34 years ;-)
I remember that I "slotted" the tubing to allow simple hand-over-hand winding, then squeezed the gap closed with pliers and soldered ;-)
(A friend, Howard Dicken ;-) had a tubing roller to make tubing into a circle... metal detectors in those days ;-)
The things we do as children ;-) ... I was 34+ ;-)
Bipolar... wasn't everything that was analog 34 years ago ?:-)
...Jim Thompson
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On a sunny day (Mon, 29 Dec 2008 17:10:08 -0800 (PST)) it happened Winfield Hill wrote in :
Transistor radios using a ferrite rod antenna usually have a coupling winding of only a few turns to match the first transistor (be it RF amp or mixer). B = f0 / Q , so for 60 kHz a Q of 50 will have 1.2kHz bandwidth... No need for the bandpass curve of a superhet, just overhead. Simply amply the RF. If I was the OP I would leave the transistor in one piece. Winding some turns on a plastic or paper tube, and shifting the ferrite rod to 'tune' it to 60 kHz, with a good zero TC big capacitor would be all you need.
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Okay. In my original vision, I had hoped to use the existing IF stage for amplification and selectivity (overlooking the fact that a 10kHz bandwidth, at 60kHz, is _wide_ band ). Given that, I envisioned keeping LO at RF+455kHz, that is, with the existing separation.
This sounds eminently sane. It also points out a similarity between my concept and L. Frank Baum's story of Nick Chopper (a wood cutter, by trade). To make a long story short, he ran afoul of a witch, got his axe cursed, and wound up losing -- and having to replace -- one body part after another until he was made completely of tin. The advantage of Nick's route was that he was able to live through his metamorphosis; since I don't have that requirement, the VLF converter Dale pointed me at sounds better and better.
Depending on what size coil I wound, I came up with between one and two full spools of #34 magnet wire. Wound around a 1/2"d x 4" rod. Riii-iiight.
Frank
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Thanks for the suggestion. There is, in fact, an article in Circuit Cellar #222 (January 2009) on building a longwave AM receiver using a 500kSps Luminary LM3S811 (ARM) board as the LO/converter stage. I may try that after I have some idea of what's out there to listen to.
Frank
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No. But I think you can roughly calculate from turns and diameter.
Yes.
No.
Yep, That's what I estimate, looking at the schematic.
...Jim Thompson
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