In article , "Richard"
writes:

Dr. A.T. Squeegee wrote:
In article ,
says...

Hi. Anybody developed a nbfm RX project covering the FM band (appx
87.5Mhz-108Mhx).

NBFM? As in narrow band?

What would be the point? Here in the U.S. at least, that entire
band is assigned to FM broadcasting, and it is anything but narrow-band.
Typical deviation from a broadcast station is 75+ kHz.


Maybe I used the wrong term. I think lots of HiFi tuners have very wide
filters much greater than 75 Khz. For DXing it seems then you need no more
than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink)
compared to a regular HiFi tuneer be a narrow bandwidth tuner.

BTW, what would be the result if you used say a 20Khz filter on a FM signal
with 75 Khz deviation? Would you get distortion or a perfectly copyable
signal. I mean is it the analagous to using a 2Khz filter for an AM signal
transmitted at 6Khz wide?

You need to refresh your personal databanks on basic modulation.

In FM the modulation amplitude "swings the frequency up and down
in frequency." [close and simplistic, there's a bit more to it...]
Limiting the bandwidth of the receiver is the same as clipping the
peaks of an amplitude modulation. You WILL get a LOT of
distortion on high-amplitude modulation input at a station.

In a limited-bandwidth AM receiver there is no limit on the amplitude
of an AM signal, just the frequency range of the modulation signal.

In a limited-bandwidth FM receiver there is both a limit on the
amplitude and frequency range of the modulation signal.

Len Anderson
retired (from regular hours) electronic engineer person

"Pete KE9OA" ) writes:
It wouldn't be quite the same. You would be clipping the sidebands, and
experience quite a bit of distortion. A 110kHz filter is about as narrow as
you can go.
I've been meaning to come up with a tuner that would be in the class of a
McIntosh MR78 for the past couple of years, but something has always come
up. Maybe after my current project, I will do this, if there is enough
interest.

Pete

For most people, it makes more sense to simply change the filters
in an existing FM BCB receiver than start from scratch. Indeed,
it seems to be a relatively common practice among people who DX
that band.

Not that building something from scratch wouldn't be interesing,
only that if "narrow bandwidth" is all that's wanted, then there's
no sense in building it all. And there isn't much sense in putting
narrow filters in a mediocre homebuilt FM receiver, which is the
sort of thing you see in construction articles.

I use Delco digitally tuned car radios as my "table radios", running
them off power supplies. For the price, a few dollars at garage sales,
they are pretty good receivers on the FM band. I know it would benefit
from a narrow filter for a few stations I like to listen to. But of course,
a lot of FM receivers aren't that great for distant reception, being too
sensitive, without good overload protection.

In some cases, it might be intriguing to build a single channel FM
BCB receiver. Build it like a ham band converter, with plenty of
tuned circuits at the fronte end, little or no RF amplification,
and a good mixer. Being fixed tuned, one could optimize it for
that frequency, and not worry about tracking, or the problems of
ganging a number of tuned circuits. For the local oscillator, one
could go with a crystal oscillator chain.

Michael VE2BVW


Richard wrote in message
...
Dr. A.T. Squeegee wrote:
In article ,
says...

Hi. Anybody developed a nbfm RX project covering the FM band (appx
87.5Mhz-108Mhx).

NBFM? As in narrow band?

What would be the point? Here in the U.S. at least, that entire
band is assigned to FM broadcasting, and it is anything but narrow-band.
Typical deviation from a broadcast station is 75+ kHz.


Maybe I used the wrong term. I think lots of HiFi tuners have very wide
filters much greater than 75 Khz. For DXing it seems then you need no more
than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink)
compared to a regular HiFi tuneer be a narrow bandwidth tuner.

BTW, what would be the result if you used say a 20Khz filter on a FM
signal
with 75 Khz deviation? Would you get distortion or a perfectly copyable
signal. I mean is it the analagous to using a 2Khz filter for an AM signal
transmitted at 6Khz wide?

"Pete KE9OA" ) writes:
It wouldn't be quite the same. You would be clipping the sidebands, and
experience quite a bit of distortion. A 110kHz filter is about as narrow as
you can go.
I've been meaning to come up with a tuner that would be in the class of a
McIntosh MR78 for the past couple of years, but something has always come
up. Maybe after my current project, I will do this, if there is enough
interest.

Pete

For most people, it makes more sense to simply change the filters
in an existing FM BCB receiver than start from scratch. Indeed,
it seems to be a relatively common practice among people who DX
that band.

Not that building something from scratch wouldn't be interesing,
only that if "narrow bandwidth" is all that's wanted, then there's
no sense in building it all. And there isn't much sense in putting
narrow filters in a mediocre homebuilt FM receiver, which is the
sort of thing you see in construction articles.

I use Delco digitally tuned car radios as my "table radios", running
them off power supplies. For the price, a few dollars at garage sales,
they are pretty good receivers on the FM band. I know it would benefit
from a narrow filter for a few stations I like to listen to. But of course,
a lot of FM receivers aren't that great for distant reception, being too
sensitive, without good overload protection.

In some cases, it might be intriguing to build a single channel FM
BCB receiver. Build it like a ham band converter, with plenty of
tuned circuits at the fronte end, little or no RF amplification,
and a good mixer. Being fixed tuned, one could optimize it for
that frequency, and not worry about tracking, or the problems of
ganging a number of tuned circuits. For the local oscillator, one
could go with a crystal oscillator chain.

Michael VE2BVW


Richard wrote in message
...
Dr. A.T. Squeegee wrote:
In article ,
says...

Hi. Anybody developed a nbfm RX project covering the FM band (appx
87.5Mhz-108Mhx).

NBFM? As in narrow band?

What would be the point? Here in the U.S. at least, that entire
band is assigned to FM broadcasting, and it is anything but narrow-band.
Typical deviation from a broadcast station is 75+ kHz.


Maybe I used the wrong term. I think lots of HiFi tuners have very wide
filters much greater than 75 Khz. For DXing it seems then you need no more
than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink)
compared to a regular HiFi tuneer be a narrow bandwidth tuner.

BTW, what would be the result if you used say a 20Khz filter on a FM
signal
with 75 Khz deviation? Would you get distortion or a perfectly copyable
signal. I mean is it the analagous to using a 2Khz filter for an AM signal
transmitted at 6Khz wide?

Richard wrote:
I hear that FM RX's are pretty complicated affairs. Most FM DXers it seems
just modify commercial sets. Reduce bandwidth from say 230Khz to 110 Khz. I
suppose that going this way has quite a lot of merit. Cheaper probably.

It's certainly the most bang for the buck. Really, a good FM receiver
isn't much more complicated than a good AM receiver -- but a
bare-bones-just-barely-receives-the-strongest-stations FM receiver is
quite a bit more complicated than a bare-bones-..... AM set.

Usually you can reduce the bandwidth of a FM receiver by simply removing
the monolithic ceramic filters and replacing them. I've done that on my
Technics ST-G50; at my location 30 miles outside Nashville, I have
received at least one DX station on every frequency that doesn't have a
local. (yes, that includes the frequencies adjacent to 100,000-watt
locals)

Forget what I paid for the filters - it was definitely less than $5
apiece. My tuner needed two.

=============================

Regarding going to 20KHz bandwidth...

In DX situations with heavy interference, a very narrow bandwidth might
be helpful for identifying DX stations. The programming will be mostly
unintelligible in a 20KHz bandwidth, but maybe it'll be more
intelligible than it would be against the interference from adjacent
channels in a more reasonable bandwidth.

I occasionally use the narrow filters in my TH-F6 HT to DX television
audio. Nobody would dream of listening to that audio for entertainment
but one can identify things they'd never ID on a TV set.

Don't know anyone who's tried putting a 10.7MHz IF 20KHz or similar
bandwidth filter in a FM broadcast tuner. You'd want it to be one of
multiple bandwidths, so you could select something more reasonable for
stronger signals.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

Richard wrote:
I hear that FM RX's are pretty complicated affairs. Most FM DXers it seems
just modify commercial sets. Reduce bandwidth from say 230Khz to 110 Khz. I
suppose that going this way has quite a lot of merit. Cheaper probably.

It's certainly the most bang for the buck. Really, a good FM receiver
isn't much more complicated than a good AM receiver -- but a
bare-bones-just-barely-receives-the-strongest-stations FM receiver is
quite a bit more complicated than a bare-bones-..... AM set.

Usually you can reduce the bandwidth of a FM receiver by simply removing
the monolithic ceramic filters and replacing them. I've done that on my
Technics ST-G50; at my location 30 miles outside Nashville, I have
received at least one DX station on every frequency that doesn't have a
local. (yes, that includes the frequencies adjacent to 100,000-watt
locals)

Forget what I paid for the filters - it was definitely less than $5
apiece. My tuner needed two.

=============================

Regarding going to 20KHz bandwidth...

In DX situations with heavy interference, a very narrow bandwidth might
be helpful for identifying DX stations. The programming will be mostly
unintelligible in a 20KHz bandwidth, but maybe it'll be more
intelligible than it would be against the interference from adjacent
channels in a more reasonable bandwidth.

I occasionally use the narrow filters in my TH-F6 HT to DX television
audio. Nobody would dream of listening to that audio for entertainment
but one can identify things they'd never ID on a TV set.

Don't know anyone who's tried putting a 10.7MHz IF 20KHz or similar
bandwidth filter in a FM broadcast tuner. You'd want it to be one of
multiple bandwidths, so you could select something more reasonable for
stronger signals.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

Richard wrote:
Maybe I used the wrong term. I think lots of HiFi tuners have very wide
filters much greater than 75 Khz. For DXing it seems then you need no more
than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink)
compared to a regular HiFi tuneer be a narrow bandwidth tuner.

Yes, you used the wrong termgrin.

"NBFM" has a specific technical meaning, IIRC a system where the
modulation index (ratio of peak deviation to maximum modulating
frequency) is less than 1.

For FM broadcast, the peak deviation is 75KHz and the maximum modulating
frequency 15KHz. (OK, I'm ignoring stereo...) So the modulation index
is 5.

For police radio, the peak deviation is roughly 3KHz and so is the
maximum modulating frequency. So the modulation index is 1.

BTW, what would be the result if you used say a 20Khz filter on a FM signal
with 75 Khz deviation? Would you get distortion or a perfectly copyable
signal. I mean is it the analagous to using a 2Khz filter for an AM signal
transmitted at 6Khz wide?

Lots of distortion.

In AM, distance from the center of the channel correlates to modulating
frequency. Restricting the filter bandwidth in the receiver restricts
the frequency response - the high audio frequencies ("treble") are
rolled off. But it has no effect on the range of *amplitudes* that can
be received; a loud sound within the bandpass of the filter will still
be reproduced accurately.

In FM, distance from the center of the channel correlates to modulating
*amplitude*. A loud sound will push the transmitted signal to the outer
edges of the channel. If the receiver's filter is too narrow to pass
that, the signal peaks will be chopped off, resulting in severe
interference.

Remember that the 75KHz peak deviation for FM broadcast is 75KHz *either
side* of center. To get the actual bandwidth required you have to add
the peak modulating frequency to that. 165KHz, not counting stereo.

You can chop some of that off at the expense of some distortion. I've
found in practice, 110KHz filters work fine for DXing though I wouldn't
want to listen to a symphony through them. 75KHz would probably be
reasonably intelligible.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

Richard wrote:
Maybe I used the wrong term. I think lots of HiFi tuners have very wide
filters much greater than 75 Khz. For DXing it seems then you need no more
than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink)
compared to a regular HiFi tuneer be a narrow bandwidth tuner.

Yes, you used the wrong termgrin.

"NBFM" has a specific technical meaning, IIRC a system where the
modulation index (ratio of peak deviation to maximum modulating
frequency) is less than 1.

For FM broadcast, the peak deviation is 75KHz and the maximum modulating
frequency 15KHz. (OK, I'm ignoring stereo...) So the modulation index
is 5.

For police radio, the peak deviation is roughly 3KHz and so is the
maximum modulating frequency. So the modulation index is 1.

BTW, what would be the result if you used say a 20Khz filter on a FM signal
with 75 Khz deviation? Would you get distortion or a perfectly copyable
signal. I mean is it the analagous to using a 2Khz filter for an AM signal
transmitted at 6Khz wide?

Lots of distortion.

In AM, distance from the center of the channel correlates to modulating
frequency. Restricting the filter bandwidth in the receiver restricts
the frequency response - the high audio frequencies ("treble") are
rolled off. But it has no effect on the range of *amplitudes* that can
be received; a loud sound within the bandpass of the filter will still
be reproduced accurately.

In FM, distance from the center of the channel correlates to modulating
*amplitude*. A loud sound will push the transmitted signal to the outer
edges of the channel. If the receiver's filter is too narrow to pass
that, the signal peaks will be chopped off, resulting in severe
interference.

Remember that the 75KHz peak deviation for FM broadcast is 75KHz *either
side* of center. To get the actual bandwidth required you have to add
the peak modulating frequency to that. 165KHz, not counting stereo.

You can chop some of that off at the expense of some distortion. I've
found in practice, 110KHz filters work fine for DXing though I wouldn't
want to listen to a symphony through them. 75KHz would probably be
reasonably intelligible.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

On Fri, 19 Dec 2003 22:11:23 -0000, "Richard"
wrote:

Hi. Anybody developed a nbfm RX project covering the FM band (appx
87.5Mhz-108Mhx).

With digital readout, though I suppose I could just hook up a simple a
frequency counter to read frequency.

TIA. Rich.


If you want to do DX'ing get an existing receiver and put in a
narrower filter. Many of the older tuners are ideal for this. I saw
writeups on this in magazines over 20 years ago and don't remember
what bandwidth filter was recommended.

An interesting project would be mods to the GE SuperRadio II or III.
They are cheap, available and have a tuned RF stage.

73,
Jim

On Fri, 19 Dec 2003 22:11:23 -0000, "Richard"
wrote:

Hi. Anybody developed a nbfm RX project covering the FM band (appx
87.5Mhz-108Mhx).

With digital readout, though I suppose I could just hook up a simple a
frequency counter to read frequency.

TIA. Rich.


If you want to do DX'ing get an existing receiver and put in a
narrower filter. Many of the older tuners are ideal for this. I saw
writeups on this in magazines over 20 years ago and don't remember
what bandwidth filter was recommended.

An interesting project would be mods to the GE SuperRadio II or III.
They are cheap, available and have a tuned RF stage.

73,
Jim

"Jim, N2VX" ) writes:
On Fri, 19 Dec 2003 22:11:23 -0000, "Richard"
wrote:

Hi. Anybody developed a nbfm RX project covering the FM band (appx
87.5Mhz-108Mhx).

With digital readout, though I suppose I could just hook up a simple a
frequency counter to read frequency.

TIA. Rich.


If you want to do DX'ing get an existing receiver and put in a
narrower filter. Many of the older tuners are ideal for this. I saw
writeups on this in magazines over 20 years ago and don't remember
what bandwidth filter was recommended.

An interesting project would be mods to the GE SuperRadio II or III.
They are cheap, available and have a tuned RF stage.

73,
Jim

That's a bit misleading. Most FM BCB receivers, now and in the past,
have an RF stage ahead of the mixer, and it is tuned. I once had
a nice Sony stereo receiver that worked well on FM, and it's lack of
an amplifer ahead of the mixer was pretty uncommon (and likely accounted
for it's good overload resistance).

The "TRF" stage in a Superradio is in reference to the AM band,
where in non-car radios, an RF stage is an exception.

Also, while it does seem a bit of work has been done in the design
for better AM reception, that RF stage and the better than average
loopstick, it does not seem like the FM section is anything to write
home about. I gather it's a fairly generic design.

This is part of the mythology of the Superradio. It's not all that
great, just maybe a little bit better than the average portable or
table radio, but people point to it when the concept of "a better
radio" comes up. Once you start spending the money, better to spend it
on a better design. Or better, buy a car radio that will have good
reception, or some used stereo that did have a bit of extra care in it's
design, so it actually won't overload in an urban environment.

Michael VE2BVW