FM BCB Rcvr. design...
 

Hello fellow radionuts (radionauts?). For some reason, I've decided
to design and build an FM tuner. Just to see if I can make a good one
I guess. A few questions have presented themselves:

-PLL or quadrature (IC) decoding?


-Stereo decoding: single IC or "discrete"?


-Throw in SCA! Why not? PLL?


-Anyone in S Ontario (KW) area?


-I'm still having a hard time figuring out AFC. I can see how I might
do it with a PLL - the DC error voltage being used to pull the LO to
the correct frequency. How was it done in the old days? ;-) If I
could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres cool.



All thoughts greatly appreciated.

Michael Dunn
VA3SSP
mdunn @at@ cantares.on.ca
Tech museums: http://www.cantares.on.ca/museums.htm
Design portfolio: http://www.cantares.on.ca/portfolio/

Michael Dunn wrote:

Hello fellow radionuts (radionauts?). For some reason, I've decided
to design and build an FM tuner. Just to see if I can make a good one
I guess. A few questions have presented themselves:

-PLL or quadrature (IC) decoding?


-Stereo decoding: single IC or "discrete"?


-Throw in SCA! Why not? PLL?


-Anyone in S Ontario (KW) area?


-I'm still having a hard time figuring out AFC. I can see how I might
do it with a PLL - the DC error voltage being used to pull the LO to
the correct frequency. How was it done in the old days? ;-) If I
could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres cool.



All thoughts greatly appreciated.

Michael Dunn
VA3SSP
mdunn @at@ cantares.on.ca
Tech museums: http://www.cantares.on.ca/museums.htm
Design portfolio: http://www.cantares.on.ca/portfolio/

You can also use the signal from a quadrature decoder, whether IC's or
tube -- and there's a clever game you can play to make a gyrator out of
a pin-toad, which is how you pull the LO around if you're doing it with
tubes, etc.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

In article ,
says...
Hello fellow radionuts (radionauts?). For some reason, I've decided
to design and build an FM tuner. Just to see if I can make a good one
I guess. A few questions have presented themselves:

-PLL or quadrature (IC) decoding?

I can't speak to your own needs and interests, but for my part, coming
off a big all-mode receiver project, I would say that my next one will
definitely be... DSP-based.

I used the Philips SA604AD in my receiver for both WBFM (broadcast) and
NBFM decoding, the former at 10.7 MHz, the latter at 455 kHz. It is
quadrature-coil based, easy to use and not too hard to find. All of the
coils and filters needed for all FM modes are stock parts at DigiKey.
But the SA604AD doesn't decode stereo directly, so that may be an issue
for you.

-Stereo decoding: single IC or "discrete"?

Again, the next time I go through all this rigamarole, the final IF
output will go straight into an ADC.


-Throw in SCA! Why not? PLL?

In a non-DSP solution, the choice of PLL-versus-quadrature techniques is
usually driven by the need for stereo decoding. You're more likely to
want a PLL to regenerate the 19 kHz pilot tone for stereo decoding.
This is not necessarily the same stage (chip) that does the FM
demodulation. Stereo MPX decoding is strictly a baseband process,
regardless of implementation.

I didn't care about stereo support in my application, so a quadrature
solution was fine. I don't know much about the mechanics of stereo
pilot carrier recovery and channel separation, never having implemented
it or researched it in earnest. Google returned
http://transmitters.tripod.com/stereo.htm, which at first glance looks
like a good summary of the process.

-I'm still having a hard time figuring out AFC. I can see how I might
do it with a PLL - the DC error voltage being used to pull the LO to
the correct frequency. How was it done in the old days? ;-)

Almost the same way. You could think of AFC as a crude FLL (frequency
locked-loop). The output of the ratio detectors in the old-school FM
sets had a DC bias that corresponded to tuning error. When the radio
was exactly on-frequency, the average carrier frequency deviation was
zero, and the discriminator's DC output was centered. Drift would cause
the development of a non-zero DC bias in one direction or the other,
which was fed back to the 1st LO to steer it back on frequency. (Prior
to the widespread use of varicap diodes, this was a rather kludgy
process.)

There was no phase-locking effect in an AFC system, because ratio
detectors can only be used to compare a single incoming frequency
against a fixed tank circuit's center frequency. For a true PLL effect,
you need a way to generate an error voltage or current on the basis of
the phase difference between *two* signals.

If I
could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres cool.

My suspicion is that you can dispense with the preselector and just use
a strong (high-IP3) front end with a bandpass filter. 88-108 MHz is
considered a narrow tuning range these days. :)

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

"Michael Dunn" wrote in message
.. .
Hello fellow radionuts (radionauts?). For some reason, I've decided
to design and build an FM tuner. Just to see if I can make a good one
I guess. A few questions have presented themselves:

-PLL or quadrature (IC) decoding?

DSP would be best.

-I'm still having a hard time figuring out AFC.

You don't need it if you have a synthesised oscillator.

Leon

Michael Dunn ) writes:

-I'm still having a hard time figuring out AFC. I can see how I might
do it with a PLL - the DC error voltage being used to pull the LO to
the correct frequency. How was it done in the old days? ;-) If I
could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres cool.


Michael,
Get a copy of the "Radio Designer's Handbook" by F. Langford-Smith. It's
all about tube technology but pages 1150-1160 in the 1953 edition
(revised '67) give a very good explanation of AFC and how it was done then.

Good luck and have fun.
.... Martin VE3OAT

Thanks to all so far for the various comments. I guess i should have
explained my design philosophy for this a bit.

I don't want to do anything fancy, like DSP or even synthesized tuning.
But, I don't want to go for a minimalist six-transistor design either!
;-) And I certainly don't want to use a radio-on-a-chip! I do want
something of good quality in the end.

I'm looking at the middle road. An LNA. A mixer/LO chip. What comes
next is more up for grabs. Maybe I'll protoboard several approaches in
parallel to gain the most experience and see which works best. A
1-chip solution (quadrature; perhaps use a better delay than a simple
LC). A PLL chip + a stereo chip. And if I get ambitious, maybe a
"discrete" stereo decoder design... Fun wow.

Michael
VA3SSP

In article ,
Michael Dunn wrote:
Hello fellow radionuts (radionauts?). For some reason, I've decided
to design and build an FM tuner. Just to see if I can make a good one
I guess. A few questions have presented themselves:

-PLL or quadrature (IC) decoding?

Quadrature. Probably double tuned. (One of the old, now obselete,
demodulator IC's had a good description on double tuned quad coils,
but damned if I can remember which one. I searched for it, as there
was a newsgroup discussion of them going on at the time).

PLLs are feedback loops. Feedback loops are driven by error. In this
case error would show up as distortion.

You could also mix down to a lower IF than 10.7 and use a frequency to
voltage converter. (A precision one shot triggered on each cycle).

-Stereo decoding: single IC or "discrete"?

-Throw in SCA! Why not? PLL?

Narrowband IF ICs like the MC3361 make a good SCA demodulator. Basically
a superhet using the onboard oscillator and mixer that runs a 455 kHz
IF and a quadrature demodulator.

I did one with the MC3359, and I think it's on my ftp pages
at ftp://ftp.eskimo.com/u/m/mzenier .

-Anyone in S Ontario (KW) area?


-I'm still having a hard time figuring out AFC. I can see how I might
do it with a PLL - the DC error voltage being used to pull the LO to
the correct frequency. How was it done in the old days? ;-) If I

Just about all FM detectors (ratio det, discriminator, quad)
output down to DC (from the offset from mistuning) , which can
then be integrated/lowpassed to create the AFC voltage. On
frequency may be at 1/2 the power voltage for a quad detector, not
zero volts.

could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres cool.

Signal strength is a different signal from AFC. AFC comes after the
signal has been limited, so that's sort of information is lost.

Random comment: Elektor Electronics magazine, the european project
magazine, had a fancy FM tuner project about a half a dozen years ago.

Mark Zenier Washington State resident

In article ,
John Miles wrote:

-Throw in SCA! Why not? PLL?

In a non-DSP solution, the choice of PLL-versus-quadrature techniques is
usually driven by the need for stereo decoding. You're more likely to
want a PLL to regenerate the 19 kHz pilot tone for stereo decoding.
This is not necessarily the same stage (chip) that does the FM
demodulation. Stereo MPX decoding is strictly a baseband process,
regardless of implementation.

SCA signals are the Muzak, ethnic radio, and library for the blind
signals hidden on narrowband FM subcarriers. 67, 75, and 92 kHz.
There's also digital stuff, pagers mostly.

Mark Zenier Washington State resident

Behold, Michael Dunn signalled from keyed 4-1000A filament:

Thanks to all so far for the various comments. I guess i should have
explained my design philosophy for this a bit.

I don't want to do anything fancy, like DSP or even synthesized tuning.
But, I don't want to go for a minimalist six-transistor design either!
;-) And I certainly don't want to use a radio-on-a-chip! I do want
something of good quality in the end.

Hmmmm, some 12AT7's, a 6U8 mixer..... ;-)

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca

Michael Dunn wrote:

Hello fellow radionuts (radionauts?). For some reason, I've decided
to design and build an FM tuner. Just to see if I can make a good one
I guess. A few questions have presented themselves:

-PLL or quadrature (IC) decoding?

-Stereo decoding: single IC or "discrete"?

-Throw in SCA! Why not? PLL?

-Anyone in S Ontario (KW) area?

-I'm still having a hard time figuring out AFC. I can see how I might
do it with a PLL - the DC error voltage being used to pull the LO to
the correct frequency. How was it done in the old days? ;-) If I
could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres cool.

All thoughts greatly appreciated.

Michael Dunn
VA3SSP
mdunn @at@ cantares.on.ca
Tech museums: http://www.cantares.on.ca/museums.htm
Design portfolio: http://www.cantares.on.ca/portfolio/

Here's a design I used quite successfully:
www.web-ee.com/Schematics/FM_RCVR/FM_RCVR.pdf

Bob
K4QQK

Michael Dunn wrote:
Thanks to all so far for the various comments. I guess i should have
explained my design philosophy for this a bit.

What is your reception goal for this receiver?

- High fidelity?

- High sensitivity?

- Adjacent-channel rejection?

- Overload tolerance?

- Low cost?

(or of course, some combination of the above?)

I think the best way to do it will be VERY dependent on what you're
trying to accomplish.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

In article ,
says...

PLLs are feedback loops. Feedback loops are driven by error. In this
case error would show up as distortion.


Well, that's a pretty extreme oversimplification.

Error above the loop bandwidth shows up as distortion. To use a PLL to
recover FM, you use a wide bandwidth that covers the entire baseband
spectrum of interest. That way, the error *is* the signal. Very
standard technique.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

In article , Doug Smith W9WI
wrote:
What is your reception goal for this receiver?

- High fidelity?

- High sensitivity?

- Adjacent-channel rejection?

- Overload tolerance?


Pretty much all of the above. Cost is secondary. Though I do like
simple elegance, and I'm not going to go as far as the SUPRX mentioned
by Bob, though it must be a fine rcvr.

Mark mentioned:
could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres
cool.

Signal strength is a different signal from AFC. AFC comes after the
signal has been limited, so that's sort of information is lost.

Yes, I know. I was just conceptually linking AFC to "AAT" and
"APS". The latter two would be a lot trickier I think since we're not
dealing with a monotonic function, but one with a peak. Might need a
uP or some overly complex analog circuits...


Speaking of quadrature demods, wouldn't best performance be had by
using a 23.3ns ) wideband delay instead of a single or
double-tuned LC. I just found one with 30MHz BW...

Michael

Speaking of quadrature demods, wouldn't best performance be had by
using a 23.3ns ) wideband delay instead of a single or
double-tuned LC. I just found one with 30MHz BW...

Probably not, unless there's something I'm missing. You don't want a
wideband circuit here -- the quadrature detector takes advantage of
phase shift through the passband. Basically it uses a mixer to compare
a signal with a phase-shifted version of itself, with the output being
proportional to the actual phase difference. With a single-tuned
circuit, the 3 dB point corresponds to 45 degrees of phase shift, and
that's where you center the quadrature coil.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

"Michael Dunn" wrote in message
.. .
In article , Doug Smith W9WI
wrote:
What is your reception goal for this receiver?

- High fidelity?

- High sensitivity?

- Adjacent-channel rejection?

- Overload tolerance?


Pretty much all of the above. Cost is secondary. Though I do like
simple elegance, and I'm not going to go as far as the SUPRX mentioned
by Bob, though it must be a fine rcvr.

Mark mentioned:
could design not only AFC, but also automatic tuning of an RF
preselector and perhaps even an antenna tuner, that would be tres
cool.

All of my older FM tuners have a three-gang "bread slicer", A.K.A. variable
capacitor. It tunes (1) the LO, (2) mixer input, (3) RF stage input. Since
the OP wasn't interested in a synthesized LO, a multigang capacitor would
seem to make sense.

With some imagination, one could arrange a scheme so that the LO would lock
to
a synthesized signal every 200 kHz step. Thus one-knob tuning of multiple
stages, yet with synthesizer stability. Details are left as a student
exercise.

73 de bob w3otc

"R J Carpenter" wrote in message ...

All of my older FM tuners have a three-gang "bread slicer", A.K.A. variable
capacitor. It tunes (1) the LO, (2) mixer input, (3) RF stage input. Since
the OP wasn't interested in a synthesized LO, a multigang capacitor would
seem to make sense.

With some imagination, one could arrange a scheme so that the LO would lock
to
a synthesized signal every 200 kHz step. Thus one-knob tuning of multiple
stages, yet with synthesizer stability. Details are left as a student
exercise.

73 de bob w3otc

You'd also probably get higher Q with air var. cap.
But everything else is more complicated with this approach

SioL

In article , John
Miles wrote:

Speaking of quadrature demods, wouldn't best performance be had by
using a 23.3ns ) wideband delay instead of a single or
double-tuned LC. I just found one with 30MHz BW...

Probably not, unless there's something I'm missing. You don't want a
wideband circuit here -- the quadrature detector takes advantage of
phase shift through the passband. Basically it uses a mixer to compare
a signal with a phase-shifted version of itself, with the output being
proportional to the actual phase difference. With a single-tuned
circuit, the 3 dB point corresponds to 45 degrees of phase shift, and
that's where you center the quadrature coil.

Phase shift = delay for a given frequency of course. 90deg at
10.7MHz is 23.3ns. Being wideband means the delay and amplitude change
over a 150kHz BW are MUCH less than with a single LC. Seems to me that
would give the best performance and lowest distortion...

Michael