Modding an AM BCB radio for VLF reception

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?

Thanks, Rich

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?

Have you ever seen the actual signal demodulated?

That is idea for 5 KHz Am signals.

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.

Got a junk drawer full of parts? Access to cheap surplus parts?

How about an up-converter?:

Regards, JS

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.

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

se

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

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

..

noise

The LF356 is at least 34YO. ;-) I was using them in '74. Kinda=20 noisy, as Winfield indicates, though.

--=20 Keith

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.

--snip--

Thanks, Jim.

Frank

-- A very common attitude among scientists (and to be fair, most people) is that those who disagree with their judgments lack the knowledge to make the correct choice. -- Pete Shanks / Human Genetic Engineering

-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

Just

Tim,

Thanks for the comments.

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

-- Hanlon's Razor(amended): ÿNever attribute to malice that which can be adequately explained by ignorance, indifference, or stupidity.ÿ

-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

Paul,

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

-- Character is not made in a crisis, it is only exhibited

-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

--snip--

Only in the NIST documents. The structure of the WWVB signal looks a lot like the WWV data stream.

Thanks for the warnings and hints.

I'd need to tap the "envelope" at some point ahead of the speaker and feed it into something else (probably a microprocessor).

My language was admittedly inexact.

Yup.

Fair enough. Plus, trying to distinguish 200ms vs. 500ms vs. 800ms "clicks" wouldn't be fun for very long.

I'll add that to mu To-Do- list.

Thanks, Michael.

Frank

-- A person's character and their garden both reflect the amount of weeding that was done in the growing season.

-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

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

Just

--snip--

Ben,

Thanks for joining in.

Hm. Yes, I have a 2MHz signal generator around here somewhere I could use with a SS VOM.

Thanks. That's the one Dale pointed me at, and I think it's the best way to get started with VLF. Once I've wandered around the "terrain" a bit I'll have a much better idea of what I'll want to do next.

Frank

-- "...each new generation born is in effect an invasion of civilization by little barbarians, who must be civilized before it is too late." -- Thomas Sowell

-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)