Modding an AM BCB radio for VLF reception

Your front end (tuned antenna coil) will have a high enough Q and enough arithmetic selectivity to limit the bandwidth to probably much less than 10 kHz. The antenna will tune sharper than the IF bandwith.

Don't forget most of these are AC/DC "hot chassis" designs with one side of the line connected to the chassis or to a floating common buss.

Tuning caps were typically 365 pF for the antenna, and 140 pF for the LO. The LO coils are tapped, and used in a Hartley oscillator configuration (usually a 6BE6 converter stage.) There are several on-line calculators for determing resonance. You won't have much tuning range with a 365 pF cap. Getting the stages to track will be even harder if you want to tune either side of 60 kHz that is... And the antenna stage tracking will be sharp and very critical.

An up converter would work, but then you'd be fighting AM BCB station blowby. I'd redesign the set for fixed single frequency on 60 kHz using new LC values for the LO and antenna. Forget using the existing LO and Antenna coils, they won't contribute much reactance to total values you'll need.

Pete

"Tio Pedro" "Frank McKenney"

" I have a couple of AM/FM clock-radios I've accumulated... "

These are transistor radios with small transformers isolating the AC mains.

.... Phil

Can you hear a one HZ signal?

You really have two choices: either up-convert or rebuild the front end. Just adding an inductor is going to reduce the sensitivity of the ferrite rod aerial ("antenna" - U.S.) and would have significant implications for the stability of the LO, so is unlikely to be successful. Up-conversion is probably the preferred option.

My VLF up-converter just consists of three dual-gate FETs (I have a boxful of BF900) - one for the LO, one as a mixer, and one as a gain-controlled RF amplifier. The hardest part of the whole job was winding the coils!

Bob

May be a moot point if you're just doing this for the heckuvit but there's nothing to listen to per se at WWVB since the time is encoded with pulse width modulation, ie no voice.

GL, Bill

"Frnak McKenney" skrev i meddelelsen news:JsidnbYp05epoMXUnZ2dnUVZ snipped-for-privacy@earthlink.com...

Superhet seems like massive overkill to me. Would it not be better to use a some kind of direct conversion receiver?

My design was TRF.

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Ah. Hadn't considered that.

Although... the Q of the loopstick+tuning_cap might be high, but I wonder about the effect of (loopstick+RF_choke) on the L part of LC. Especially when L(choke):L(antenna) is > 100:1. But I guess I'll find that out along the way.

Even those with power transformers can have exposed 115VAC "hot spots" once one opens up the box. Thanks for the reminder.

I've been using Scilab. I started off using OpenOffice's spreadsheets, but there's something about the way a spreadsheet works that doesn't feel comfortable to me when I'm playing around.

Won't "much tuning range" be controlled by the min-to-max capacitance ratio? (Okay, its square root.)

Right. The upconverter is, in essence, a rebroadcaster... sort of like when one of my nieces gets an e-mail message and replies by "texting".

Agreement on the "won't contribute much", since we're talking about neighborhoods of 600uH vs. 75000uH. As to the fixed-frequency part, I may have to fall back on that, but I'm kind of curious about what the entire 50-150kHz spectrum "sounds" like from an AM receiver's point of view.

Frank McKenney

-- The world is divided into two groups of people: Those to whom "No user serviceable parts inside" is a warning, and those to whom it is an open challenge and provocation!

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

I _think_ so, but I also seem to recall seeing some recent boxes that were trasnformerless. In any case, it's always good to remember that it all _starts_ as 115VAC.

Pete may have been looking at some of the stuff from the bottom layer. I won't swear that there _isn't_ a 50C5-based radio in there _some_where; I know I ran across a somewhat moldy tube-based guitar amp a few weeks back.

Frank

-- An education isn't how much you have committed to memory, or even how much you know. It's being able to differentiate between what you do know and what you don't.ÿ -- Anatole France

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

Bob,

Thanks for the feedback.

I can believe you on the sensitivity question, but I don't think I follow the part about destabilizing the LO, which will have its own tank coil. This is separate from the aerial-RF-Choke combination. Or did I miss something? (I realize the LO tank coil/transformer will need to be replaced... once I can figure out what inductance it ought to have. These calculations would be so much _easier_ if it weren't for all the silly multiplication and division stuff. )

Yes. I strongly suspect that I'll get much better results by re-winding the aerial ("mermaid" -- Walt Disney ) than by simply adding a 75,000uH inductor in series with the existing one, but I haven't goten around to calculating how many turns of #26 magnet wire that would take. "A bunch", I suspect.

Frank

-- If a little knowledge is dangerous, where is the man who has so much as to be out of danger? -- Thomas Henry Huxley, 1877

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

Bill,

Thanks for the comments.

I'm hoping that what I build will at least give me some idea of the WWVB signal strength. I started out being curious about the various "Atomic clocks" and thought about building a dedicated

It's a question of "navigating" towards a solution. Without some sort of compass -- or at least some recognizable waypoints -- you have no idea if you're getting closer to a working design, going in circles, or moving further and further away. VOMs, 'scopes, signal generators, spectrum analyzers, PRINT/printf(), and even (Ack! Phlbbbbt!) 16" tall fanfold core dumps are all tools we have created for helping us find our way through mazes where our everyday senses are useless.

And, as you say, I'm just curious: if WWVB is out there, unheard (by me, so far), I wonder what else might be?

Frank

-- If you are not being criticized, you may not be doing much. -- Donald Rumsfeld

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

I did this chip design some 34 years ago...

It may offer some clues.

...Jim Thompson

Could be. The trouble is, when one is starting out on a trip into unfamiliar territory, it's hard to know whether skis, a canoe, or a Mercedes is more likely to help one reach one's destination.

It has been a long time since I wired any RF circuitry, and while I know _about_ various receiver designs, this is my first attempt to _create_ one. Since I lack the experience to base my judgement on, I need to gain some -- probably with a lot of pain and frustration, since that's the usual way it is gained. My hope is that, by the time I finish making this "converted" receiver work

-- or decide to abandon it -- I will have learned enough to do a better job on my next attempt.

Attempting to modify an existing design this way is, for me, an attempt to refresh my understanding of RF design principles -- including the math -- by testing what I think _ought_ to work against what _does_ work. And I believe that I will learn as much from hacking a $5 radio as I might by ordering $50 worth of parts from Digi-Key and starting from scratch.

But we'll see.

Frank

-- Computers, like leftovers, make you feel smart TWICE: Once, when you carefully select all the best equipment and components, and again when you manage to get SOMETHING working despite the fact that none of the new stuff does what you thought it would. -- Dr. E.A. McKenney

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

For $14 you can buy a decent converter- and put it on a decent receiver.

WWVB may be strong, but you may never know with your idea, - not knowing your converted receiver's sensitivity, immunity to overload, images and the ability to have a decent antenna.

Dale W4OP

Many transistor radios use a single transistor converter stage, that both oscillates at the LO frequency and mixes the RF down to the IF. So the antenna circuit tuning interacts with the LO tuning circuit. If you allow the separation between RF and LO to get too small then the interaction becomes stronger, impacting stability.

I would take the converter stage out, or convert it to a low-gain preamplifier. Then I'd put in a front end circuit featuring separate LO and mixers. I'd probably try some combination of padding capacitors on the LO to get the right range while keeping impedances more or less the same as they are in the current radio, but I'd probably also plan on putting a separate antenna fine-tune capacitor, if for no other reason than to allow me to use a nice narrow RF circuit.

Yup.

For just WWV TRF would probably be better. This would allow you amplification at two frequencies (60kHz and baseband), and as long as you don't want to change frequencies you don't have to muck about with tracking.

For the OP's desired 60 - 125kHz, a superhet would probably provide the best quality, although DC may provide an economical "production" solution.

The WWVB receiver design on the SED page of my website is TRF and uses the AGC voltage as the method of signal detection.

...Jim Thompson

Never mind that. I just want to know who named their kid Frnak.

-- Paul Hovnanian snipped-for-privacy@hovnanian.com

----------------------------------------------------------------------- Have gnu, will travel.

Too cheap? Don't bother reading further.

Like to build things? This would seem to be a great project for a software defined radio.