Modding an AM BCB radio for VLF reception
 

I'd like to listen in on NIST's 60kHz time broadcasts on WWVB
"directly", but I like building things, and besides, I'm too chea...
er, "thrifty" to go out and buy a VLF-specific receiver.

Down in the basement (a.k.a. The Gorge of Eternal Electronic Peril)
I have a couple of AM/FM clock-radios I've accumulated (yard sale?
local thrift shop? I forget). Unlike today's Walkman-alike radios,
these tend to have large, discrete, and easy-to-reach components, so
I wondered if it would be possible to modify one of these to tune in
RF in the WWVB and LF-RFID (125kHz) neighborhoods.

Before pulling anything apaprt I thought I'd check the math see if
it was even reasonable. I assume these units are standard superhet
designs with a local oscillator and a 455kHz IF section, such as the
circuit at:

http://ourworld.compuserve.com.homep...wden/radio.htm

AM BCB: 540-1600 kHz
Tuning capacitor: 140- 15 pF (RF)
60- 10 pF (LO)
0- 12 pF (trimmers)

From Ye Olde Traditional Formula:

F = 1/(2 * %pi * sqrt(L * C) )

it follows that, for a fixed L, F varies with 1/sqrt(C). The
capacitance of the RF gang of the tuning capacitor has a 10:1 range,
so it can control a frequency range of 1:sqrt(10), or 1:3.2.
Similarly, the LO section provides a frequency range of 1:sqrt(6),
or 1:2.4.

Working this backwards, it looks like an antenna loop designed for
this capacitor would be between 620uH and 660uH; choosing 640uH and
working forward the RF section would tune from 532 to 1624 kHz.

This means the LO will run from (532+455) or 987kHz to (1624+455) or
2079kHz kHz, a range of 2.1. Calculating from the low end the
associated inductor (LO tank coil) would need to be 428uH yielding
an LO range of 993-2433kHz; working from the high end we get 586uH,
an LO range of 849-2079kHz. Even if I choose a "middle" value of
500uH, my LO calculations imply that the LO will be as much as
200kHz "off" from the ideal mixing frequency of (RF+455kHz) at
either end of the dial; since radios appear (in general) to work,
I'll have to assume there's a problem with my LO math.

But ignoring that for now (famous last words grin!), that says
that if I stick a 75000uH choke in series with the radio's ferrite
loopstick, giving a total inductance of 75640uH, the _RF_ section
will (theoretically) have a tuning range of 49-149kHz. _IF_ I can
get a LO that tunes in sync with this, that is, 504-604kHz, then the
IF section will accept and amplify any AM signal from 49 to 149kHz,
envelope-detect it, and feed the result through the receiver's
amplifier.

I can see a couple of problems, even if I figure out why my LO math
doesn't work out:

1) My desired LO range is 1:1.2, well below what the LO section of
the tuning capacitor is designed for (1:2.4). However, it looks
like slapping a 100pF disk cap in parallel with the LO section
will reduce its range to around 1:1.2, and I'd need to find (or
wind) a new tank coil anyway.

2) By putting a (relatively huge) RF choke in series with the
loopstick antenna, I'm creating a fairly unbalanced "tapped
inductor". I have no idea what problems might result from this.

3) I'll be limited to broadcast-alike (10kHz AM) or similar
signals unless I want to add a BFO. Since curiosity is what's
prompting me, I can live with this restriction.

4) Something Completely Different that I haven't even considered.
grin!

Has anyone tried this? I looked around the 'web a bit, and did find
some "upconverter kits" which appear to turn a superhet receiver
into a super-superhet receiver grin!, but no one attempting to
directly modify an AM receiver.

I have a couple of other items with higher priority (e.g. my
"Federal and State 2008 Final Tax Exam") I have to get out of the
way before I'll have time to solder anything, but I would like to
hear from anyone who caes to comment (especially on my math grin!).

Thanks...


Frank McKenney
--
All scientific men were formerly accused of practicing magic.
And no wonder, for each said to himself, "I have carried human
intelligence as far as it will go, and yet So-and-so has gone
further than I. Ergo, he has taken to sorcery."
-- Baron de Montesquieu
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

Modding an AM BCB radio for VLF reception
 

Frnak McKenney wrote:
I'd like to listen in on NIST's 60kHz time broadcasts on WWVB
"directly", but I like building things, and besides, I'm too chea...
er, "thrifty" to go out and buy a VLF-specific receiver.

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

Modding an AM BCB radio for VLF reception
 

Bill,

Thanks for the comments.

On Mon, 29 Dec 2008 07:17:39 -0400, Bill M wrote:
Frnak McKenney wrote:
I'd like to listen in on NIST's 60kHz time broadcasts on WWVB
"directly", but I like building things, and besides, I'm too chea...
er, "thrifty" to go out and buy a VLF-specific receiver.

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.

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
60kHz WWVB receiver, but one of the problems one runs into with
an idea like that is that "debugging" can be exremely frustrating
without some piece of equipment that can offer a "second opinion".

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? grin!


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)

Modding an AM BCB radio for VLF reception
 

"Frnak McKenney" wrote in message
m...

Bill,

Thanks for the comments.

On Mon, 29 Dec 2008 07:17:39 -0400, Bill M wrote:
Frnak McKenney wrote:
I'd like to listen in on NIST's 60kHz time broadcasts on WWVB
"directly", but I like building things, and besides, I'm too chea...
er, "thrifty" to go out and buy a VLF-specific receiver.

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.

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
60kHz WWVB receiver, but one of the problems one runs into with
an idea like that is that "debugging" can be exremely frustrating
without some piece of equipment that can offer a "second opinion".


For $14 you can buy a decent converter- and put it on a decent receiver.
http://jacksonharbor.home.att.net/lfconv.htm

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

Modding an AM BCB radio for VLF reception
 

Frnak McKenney wrote:
Bill,

Thanks for the comments.

On Mon, 29 Dec 2008 07:17:39 -0400, Bill M wrote:
Frnak McKenney wrote:
I'd like to listen in on NIST's 60kHz time broadcasts on WWVB
"directly", but I like building things, and besides, I'm too chea...
er, "thrifty" to go out and buy a VLF-specific receiver.
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.

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
60kHz WWVB receiver, but one of the problems one runs into with
an idea like that is that "debugging" can be exremely frustrating
without some piece of equipment that can offer a "second opinion".

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? grin!


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 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?

Modding an AM BCB radio for VLF reception
 

"Frnak McKenney" wrote in message
m...
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
60kHz WWVB receiver, but one of the problems one runs into with
an idea like that is that "debugging" can be exremely frustrating
without some piece of equipment that can offer a "second opinion".


I suggest you consult http://tf.nist.gov/stations/wwvbcoverage.htm. This
shows the predicted coverage of WWVB at two hour intervals. Quoting from
NIST's text:

"The coverage maps shown below are computer-generated estimates of WWVB
signal strength under typical conditions. They show estimated signal
coverage at two-hour intervals over the course of a day with WWVB operating
at 50 kW radiated power. The shaded areas indicate those areas where signal
levels are 100 microvolts per meter or greater. Note that the coverage area
contracts during daylight hours and expands during nighttime hours."

It appears that, except for Florida, the middle and northern East Coast,
and selected portions of the Northwest, the continental United States
coverage is pretty solid. With the cost of commercial WWVB clocks being as
low as $10.00, I'd buy one just to see if it can synchronize where you
live.

QST and Ham Radio magazines have published several low frequency converter
designs that may be useful for study:

Apr 2002 QST, Frank Gentges and Steve Ratzlaff: AMRAD Low Frequency
Upconverter (correction Oct 2002)
Jun 1977 QST, Doug DeMaw & Jay Rusgrove: A High-Performance Low-Frequency
Converter
Nov 1976 Ham Radio, Guenter Ruehr:Tuned Very Low-Frequency Converter
Apr 1964 QST, G. Holmes Wilson: A Simple Low-Frequency Converter
and for the Boatanchor enthusiast
Sep 1940 QST, Raymond Woodward, A Low-Frequency Converter

73, Dr. Barry L. Ornitz WA4VZQ

Modding an AM BCB radio for VLF reception
 

On Mon, 29 Dec 2008 15:29:58 GMT, Dale Parfitt wrote:

"Frnak McKenney" wrote in message
m...
--snip--
On Mon, 29 Dec 2008 07:17:39 -0400, Bill M wrote:
Frnak McKenney wrote:
I'd like to listen in on NIST's 60kHz time broadcasts on WWVB
"directly", but I like building things, and besides, I'm too chea...
er, "thrifty" to go out and buy a VLF-specific receiver.

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.

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
60kHz WWVB receiver, but one of the problems one runs into with
an idea like that is that "debugging" can be exremely frustrating
without some piece of equipment that can offer a "second opinion".

For $14 you can buy a decent converter- and put it on a decent receiver.
http://jacksonharbor.home.att.net/lfconv.htm

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,

Thanks for the pointer. I apparently overlooked this one. Now all I
have to do is find "a decent receiver". grin!

Actually, it's a good excuse to fire up the old Heathkit Mohican
shortwave set (with SIX -- count 'em! SIX transistors!) my father
and I built some years back as a brithday gift for my mother. I
replaced the power supply electrolytics a few years ago so I could
hear static and background hiss over the hum (yes, it was that bad
grin!), but the insides could use a good dusting as well. Tighten
up the dial cord. That sort of thing.

And $14 vs. several hours with a 'scope and a signal generator
trying to tune my "hacked" RF and LO sections sounds like a good
choice.

Thanks.


Frank
--
Once we believe in ourselves, we can risk curiosity, wonder,
spontaneous delight, or any experience that reveals the human
spirit. -- e e cummings
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)