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.homepages/Bill_Bowden/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)

"Frnak McKenney" <f...@far.from.the.madding.crowd.com> wrote in message 
news:J...@earthlink.com...
>  > 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.
>

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.

 > 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.

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.
>
> 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!>).
>
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.


> Thanks...

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
>
> 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) 

"Tio Pedro"
"Frank McKenney"
>
> > 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.
>
> 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.


** Try actually reading the OP's post  -  pal.

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

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


> 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.)


 **  Where did you get the wacky ides the OP has tube radios ?



....   Phil

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.
> 
> 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.homepages/Bill_Bowden/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!>).


   Can you hear a one HZ signal?


-- 
http://improve-usenet.org/index.html

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

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.

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

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

"Frnak McKenney" <f...@far.from.the.madding.crowd.com> skrev i meddelelsen 
news:J...@earthlink.com...
>
> Before pulling anything apaprt I thought I'd check the math see if
> it was even reasonable.

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

Frithiof Jensen wrote:
> 
> "Frnak McKenney" <f...@far.from.the.madding.crowd.com> skrev i meddelelsen
> news:J...@earthlink.com...
> >
> > Before pulling anything apaprt I thought I'd check the math see if
> > it was even reasonable.
> 
> 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.


-- 
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

Hi, Pete. Thanks for jumping in.

On Sun, 28 Dec 2008 22:52:42 -0500, Tio Pedro <r...@cox.net> wrote:
> 
> "Frnak McKenney" <f...@far.from.the.madding.crowd.com> wrote in message 
> news:J...@earthlink.com...
>>  > 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.
>>
> 
> 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.

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. <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.
> 
> 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.

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

>> 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!>).
>>
> 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.

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.

> 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.

Won't "much tuning range" be controlled by the min-to-max
capacitance ratio? (Okay, its square root.)
 
> An up converter would work, but then you'd be fighting
> AM BCB station blowby.

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". <grin!>

> ...  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.

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)

Hi, Phil.

On Mon, 29 Dec 2008 15:00:11 +1100, Phil Allison <p...@tpg.com.au> wrote:
> "Tio Pedro"
> "Frank McKenney"
>> > 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.
>>
>> 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.
> 
> 
> ** Try actually reading the OP's post  -  pal.
> 
> " I have a couple of AM/FM clock-radios I've accumulated...  "
> 
> These are transistor radios with small transformers isolating the AC mains.

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. <grin!>

>> 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.)
> 
>  **  Where did you get the wacky ides the OP has tube radios ?

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. <grin!>


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)