How can the same FM station appear at two different spots on

[Phil Allison]

Michael Terrell wrote:

Adjacent channel interference like that is caused by the IF bandwidth
and the skirt. The IF transformers aren't brick wall, the amplitude
drops away slowly outside the desired bandwidth.

** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz.


That allows a local

station to be strong enough to cause problems.

** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal.


..... Phil

 

[Phil Allison]

William Sommerwerck wrote:

"Phil Allison" wrote in message
news:1e547992-e524-4256-ae8a-ce0d67815bc8@googlegroups.com...
William Sommerwerck wrote:

Given that the FM band is 20MHz, and twice 10.7 MHz is greater than 20MHz,
if
the LO is above the incoming signal, images would come from stations above
107.9MHz (outside the band).

If the LO were below the incoming signal, you could have in-band images
starting at 98.9MHz.

** Nonsense.
Long as a particular band has less width than double the IF frequency,
no in-band images will occur.

Did you actually read what I wrote? The second sentence says that.

** It says NOTHING the sort - Wanker boy.

In the third sentence, I said "If the LO were //below// the incoming
signal..."

Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8?

** Garbage.

With the LO at 77.4, the image is at 66.7

The FM broadcast band does not suffer from in-band images long as the IF is 10.7MHz of higher.


.... Phil

 

[Phil Allison]

O Ian Jackson wrote:

You don't change the mechanism for the interference, but because the two
TV channel allocations are where they are, if either was used in your
area you would certainly increase the possibility of interference from
them. However, as I've said, I think it's unusual for the LO to be on
the low side (probably for exactly this reason).


** A man who prefers his ignorant opinions to facts is a complete fool:

The 6AQ8 along with the 12AT7 were the most common tubes used for LOs
in FM tuners from the early 1950s onwards.

http://www.r-type.org/exhib/aav0008.htm

They were invariably used as low side oscillators.

The fact that some what might now be considered 'highly desirable
collectibles' had low-side LOs doesn't mean it became a standard.

** Never said it was "standard" - just quite common.

Proves you are wrong - sonny boy.

Were not several TV channels tucked right under the FM band back then ?

Not the present FM band. However, in the USA FM started life between 42
to 50MHz* but this was essentially experimental. After the war, it was
allocated the present band (87.8-107.9 MHz).

** Huh? What is the relevance of that crap ?



ATC (AM, of course) breaking

through on 97.3MHz (at least on my kitchen radio)!
--

** So that is your only case?

Piss off fool.


..... Phil


> Ian

 

[Phil Allison]

William Sommerwerck wrote:

Given that the FM band is 20MHz, and twice 10.7 MHz is greater than
20MHz,
if
the LO is above the incoming signal, images would come from stations
above
107.9MHz (outside the band).

If the LO were below the incoming signal, you could have in-band images
starting at 98.9MHz.

** Nonsense.
Long as a particular band has less width than double the IF frequency,
no in-band images will occur.

Did you actually read what I wrote? The second sentence says that.


** It says NOTHING the sort - Wanker boy.

It says exactly that.

** FFS learn to read you autistic idiot.

MY post say there are NO in-band images at all - neither from high nor low side injection of the LO.

In the third sentence, I said "If the LO were //below// the incoming
signal..."
Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8?

** Garbage.
With the LO at 77.4, the image is at 66.7.

Uh... that's right.

** Wot a thick head.


..... Phil

 

[Phil Allison]

Michael Terrell wrote:

Phil Allison wrote:
Michael Terrell wrote:


Adjacent channel interference like that is caused by the IF bandwidth
and the skirt. The IF transformers aren't brick wall, the amplitude
drops away slowly outside the desired bandwidth.

** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz.

That allows a local
station to be strong enough to cause problems.

** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal.


More Philshit, as always. It's a damned good thing you didn't design
and build deep space telemetry equipment.

** Desperate liars resort to abuse when they have no case.

Even business radios used
expensive crystal filters to reduce adjacent channel interference,

** Not one bit relevant to *wide band* FM broadcast receivers.

instead of 50 cent IF transformers.

** Which, when used in multiples, produce sharp roll ofsf at the skirts of the pass band.


> Digitally tuned FM receivers can still receive an adjacent channel,

** More irrelevance, it matters not how the LO is tuned.

but more than a channel away it
becomes quite distorted.

** Tuned FM detectors are like that.

Go back to hacking old stereos.

** Go back to washing dunny floors, you pathetic ass.



..... Phil

 

[William Sommerwerck]

"Phil Allison" wrote in message
news:ba56876f-a7d1-4673-996f-6d468e77e692@googlegroups.com...
William Sommerwerck wrote:

"Phil Allison" wrote in message
news:1e547992-e524-4256-ae8a-ce0d67815bc8@googlegroups.com...
William Sommerwerck wrote:

Given that the FM band is 20MHz, and twice 10.7 MHz is greater than
20MHz,
if
the LO is above the incoming signal, images would come from stations
above
107.9MHz (outside the band).

If the LO were below the incoming signal, you could have in-band images
starting at 98.9MHz.

** Nonsense.
Long as a particular band has less width than double the IF frequency,
no in-band images will occur.

Did you actually read what I wrote? The second sentence says that.

** It says NOTHING the sort - Wanker boy.

It says exactly that. It's amazing that someone as intelligent as you can't
understand plain language.

In the third sentence, I said "If the LO were //below// the incoming
signal..."
Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8?

** Garbage.
With the LO at 77.4, the image is at 66.7.

Uh... that's right.

 

[Michael A. Terrell]

Phil Allison wrote:

Michael Terrell wrote:


Adjacent channel interference like that is caused by the IF bandwidth
and the skirt. The IF transformers aren't brick wall, the amplitude
drops away slowly outside the desired bandwidth.

** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz.

That allows a local
station to be strong enough to cause problems.

** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal.

More Philshit, as always. It's a damned good thing you didn't design
and build deep space telemetry equipment. Even business radios used
expensive crystal filters to reduce adjacent channel interference,
instead of 50 cent IF transformers. Digitally tuned FM receivers can
still receive an adjacent channel, but more than a channel away it
becomes quite distorted. Go back to hacking old stereos.

--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

 

[Michael A. Terrell]

Phil Allison wrote:

Michael Terrell wrote:
Phil Allison wrote:
Michael Terrell wrote:


Adjacent channel interference like that is caused by the IF bandwidth
and the skirt. The IF transformers aren't brick wall, the amplitude
drops away slowly outside the desired bandwidth.

** FM receivers have multiple stages of IF band limiting making the falloff very sharp outside the needed 200kHz.

That allows a local
station to be strong enough to cause problems.

** Nope - the FM detector ( ratio or quadrature) is also tuned to the centre of the IF strip and will not demodulate an out of band signal.


More Philshit, as always. It's a damned good thing you didn't design
and build deep space telemetry equipment.

** Desperate liars resort to abuse when they have no case.

DING! DING! DING! Whenever you have no clue you start your 'Angry
Dumbass Dance', and call people a liar. All you do is prove what a fool
you are.

Even business radios used
expensive crystal filters to reduce adjacent channel interference,

** Not one bit relevant to *wide band* FM broadcast receivers.

Only if you have the I.Q. of a rusty doorknob.

instead of 50 cent IF transformers.

** Which, when used in multiples, produce sharp roll ofsf at the skirts of the pass band.

Define sharp. If it is too sharp, it causes distortion in the
recovered signal. Each transformer has an insertion loss in the IF
stages. FM radios use just enough tuned circuits to get barely
acceptable performance. Even the cheap Murata ceramic filters have a
sloppy skirt. The only advantage is that they don't need aligned during
manufacturing.


Cram your bullshit and look at it on a network analyzer. Oh, that's
right. You have no real test equipment, just junk from a '70s TV shop.
You want to talk wideband? One of the Telemetry products we
manufactured had an IF range from 1 KHz to 20 MHz bandwidth at the -3 dB
points. They had to be aligned on a network analyzer, or with a
calibrated sweep generator to achieve the proper skirt.

Digitally tuned FM receivers can still receive an adjacent channel,

** More irrelevance, it matters not how the LO is tuned.

Sigh. More of your stupidity. Digital has no AFC, so it can't be
pulled off of center.

but more than a channel away it
becomes quite distorted.

** Tuned FM detectors are like that.

Go back to hacking old stereos.

** Go back to washing dunny floors, you pathetic ass.

Whatever the hell that crap means, but I guess that you've heard it
all your life from people around you. Read a damned book on receiver
design. Analog IF bandwidth is specified at the -3 dB points. That
wouldn't be possible without a skirt. Brick wall requires a FIR filter
or another digital filter that processes a digitized input. The design
I worked on digitized to 50 to 890 MHz range for a 70 MHz IF. That was
followed by a pair of FIR filters for the IF and another pair for the
output bandwidth.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

 

[dave]

On 10/28/2014 08:40 AM, Michael Black wrote:

On Tue, 28 Oct 2014, dave wrote:

On 10/27/2014 01:48 PM, whit3rd wrote:
On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote:
In article <v7ss4allgf4oosl9rud3hpt3g3rmr6m58a@4ax.com>,
micky <NONONOmisc07@bigfoot.com> wrote:
How can the same FM station appear at two different spots on the dial?

(3) Intermodulation. If you have two strong stations nearby, their
signals can mix (either in the receiver front end, or elsewhere)
and create "spurious" signals located on either side of their true
locations on the dial. These spur signals will often be noisy and
distorted.

This sounds very likely; if it is due to front-end nonlinearity, it's
possible to test/treat it by inserting an attenuator between the FM
antenna
and the receiver (assuming the receiver has a plug-in antenna).
Lower the signal level, and the spurious response should go away.

Alternately, one can attenuate (filter) either the interfering FM
station or the (presumably
AM) difference-frequency station: this can be done with a lossy
antenna+load
placed near your radio, so can apply without access to antenna
terminals.


I used to work on Radio Row in Houston. One day the FCC came to visit
KILT FM 100.1 because they were causing squeals on the aeronautical
band. It wasn't any of the station's pro gear making the interference;
it was an old console FM receiver in the station lobby. 100.1 + 21.4 =
121.5. Radio row was on the direct approach to Hobby Airport or this
old mis-aligned radio would have never been busted.

I thought that was some of the basis of the ban on electronic devices on
airplanes.

Certainly there is folklore that when AM/FM transistor portables became
cheap and available, suddenly people were using them on airplanes, and
that did or could have caused interference, precisely because the local
oscillator radiated and in the aircraft band.

It's murky whether that was the specific cause of the rule or not, and
probably made murkier since it's been forty years since I read about this.

Michael

Cheap consumer radios are using pretty exotic methods to reduce the mass
and the current draw these days. Any oscillators are deep into the chipset.

 

[dave]

On 10/28/2014 11:11 PM, micky wrote:

On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black <et472@ncf.ca> wrote:

On Tue, 28 Oct 2014, Paul Drahn wrote:

On 10/27/2014 9:32 AM, micky wrote:
How can the same FM station appear at two different spots on the dial?

To the person who complained recently that I was off topic, I'm sorry.
This is not about any repair it would be feasible to make. It's only
about electronics.

Where I live there are two FM radio stations, 88.1 which is only a few
miles away, and 88.5 which is 40 or 50 miles away.

Right now, only my expensive KLM radio plus any car radio gets the
second one well, but I've had some cheap radios that do almost as well.

I lose track of which radios those are, so I'll start tuning at 88.1 and
tune up very gradually. After a period of silence, when I get above
88.5 to what I'd estimate is 88.6 or .7 or .8 I get 88.1 again.

How is that happening? I know about harmonics, but that doesn't apply,
does it?



P.S. This means 88.5 doesn't come in at all. I've tried stretching
out the power cord, which on the cheap radios is usually the antenna.
Sometimes that helps but on most of the radios, 88.5 won't come in at
all.

P.P.S. 88.1 is WYPR Baltimore. 88.5 is WAMU in DC. Sometimes they
play the same thing, like during the top of the hour news, Diane Rehm,
etc. although WAMU is on a 5 or 10 second delay most of the time.
Because the topic and the voices can be the same it means I can't tell
for a while if I've gotten 88.5 or just another 'instance' of 88.1.

If you have the AFC on, the station will pop up at different dial locations
depending on which direction you are tuning. At least my old portable does.

I think I've noticed this too.

But the AFC wasn't on, because that would have made it almost impossible
to get a weak station like 88.5.

Well, I'm calling it weak because most radios won't get it, but Wikip
says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts.
(also ERF? It didnt' say.) So maybe I'm calling it weak because it
farther away, in DC, not Baltimore where I live, but actually, there are
places north of here, farther from DC, the Westminster, Md. area, where
88.5 comes in well and 88.1 barely comes in. A friend moved to
Finksberg and she had to change to 88.5.

But maybe the FCC makes them arrange their antennas so that in the city
of Baltimore and its populous suburbs, 88.5 doesn't overpower 88.1.
But the frequencies are different, and there's no Baltimore 88.5, so why
would 88.5's antennas have to avoid the populous part of Baltimore, or
any part?

I was thinking along that line, except thinking of pointing out that for
whatever reasons, not great selectivity or a noisy synthesizer, a station
can be heard on more than one frequency. But, I can't recall that
happening when there's an adjacent station, then the first station being
received further up.

If that second station wasn't there, AFC is a good suggestion, and
something we might not think of much anymore, with so many fm receivers
digitally tuned. But I'd think it would "lock" to the statino further up,
that presumably is stronger at that point than the first station.

Michael

The fcc.info link I sent lets you search the FCC database in a friendly
gui fashion

 

[dave]

On 10/29/2014 02:30 AM, Arfa Daily wrote:

snip



Well, I'm calling it weak because most radios won't get it, but Wikip
says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts.
(also ERF? It didnt' say.)


ERP - Effective Radiated Power ? The 'real' transmitter output
multiplied by the 'gain' of the transmitting antenna.


So maybe I'm calling it weak because it
farther away, in DC, not Baltimore where I live,


There are many many factors that affect the propagation of a VHF signal
over a lower frequency one, some of which will degrade that signal, and
others of which can, under the right conditions, enhance it. VHF signal
reception is a lottery, once you are outside the designed service area
of the station.


but actually, there are
places north of here, farther from DC, the Westminster, Md. area, where
88.5 comes in well and 88.1 barely comes in. A friend moved to
Finksberg and she had to change to 88.5.

But maybe the FCC makes them arrange their antennas so that in the city
of Baltimore and its populous suburbs, 88.5 doesn't overpower 88.1.
But the frequencies are different, and there's no Baltimore 88.5, so why
would 88.5's antennas have to avoid the populous part of Baltimore, or
any part?


Without seeing a published map of the station's service area, it's
impossible to say. However, something as simple as a tall building in
the direction of the transmitting site, can be enough to cast a 'radio
shadow' across a large swathe of territory on the 'downstream' side

Arfa


Michael

An obstruction in the energy field creates Fresnel Zones on the side of
the obstruction opposite the antenna. These are alternating peaks higher
than normal and dips lower than normal. The distance between the peaks
and nulls (Fresnel Zones) is determined by the distance between the
antenna and the obstruction.

 

[Phil Allison]

David Platt wrote:

Commercial FM is generally allowed a +/- 75 kHz carrier deviation.
Due to the way FM works, and due to the fact that the station is
transmitting a stereo subcarrier (centered on 38 kHz, with its own
sidebands going out as much as 15 kHz on either side),

** It's worth pointing out that there is only ONE carrier for an FM broadcast signal. At any instant in time, there is only one frequency to deal with and one signal voltage coming from the detector.

With FM stereo, the detector's output includes supersonic signals up to 50 KHz or so. The supersonic stuff provides the L-R difference signal.

A lot of FM radios/receivers have fairly "broad" intermediate-
frequency filters... e.g. one or two crystal filters with 220 kHz or
even 250 kHz bandwidth.

** Crystal filters are a tad expensive for an FM radio - so designers make do with tuned transformers and Ceramic filters in the 10.7MHz amplifier stages.

Even a budget FM tuner will have at least one or two of each along with a tuned RF stage to provide good out of band and nearby signal rejection for normal use. FM DXing is NOT normal use.

Such broad receptivity lets almost all of the
"desired" station's signal in... and that's good for low-distortion
stereo reception since you get the whole stereo subcarrier.
Unfortunately, if there's a strong signal on the "alternate" channel
(200 kHz away), that signal's outer sidebands will end up getting
through the filter, and will probably affect the stereo subcarrier and
increase distortion or "break through" into audibility.

** There are never two, strong FM signals separated by 200KHz - authorities govern frequency allocations on the band so as to prevent this.

If you're
trying to tune in a weak, distant signal that's on an "adjacent"
channel to a strong local (100 kHz away) the problem is even worse.

** Only mad FM DXers have that issue.

There are ways to work around this:

- Use an FM tuner which has a narrower IF bandwidth. Better tuners
often have a wide/narrow switch setting, with the narrow setting
using different (or more) crystal filters with reduced bandwidth -
200, 180, 150, or even 110 kHz.

** Yep - mad FM DXers sometimes do that.

- Use a directional FM antenna, and aim it in the direction which
gives the best results. This may be "aimed towards the desired
station" (increasing its relative strength), or "aimed at an angle
away from the undesired station" (to put the interfering station in
a "null" in the antenna's reception pattern).

** Sure - an antenna rotator with a high gain Yagi on top is what every home needs. Pure bollocks.

The most common FM antenna is the TV antenna on the roof or maybe a dipole parked inside the roof cavity.




.... Phil

 

[chuck]

On Thu, 30 Oct 2014 12:01:48 -0700, dplatt@coop.radagast.org (David
Platt) wrote:

I had a local situation at 94.5MHz interfering with 94.3MHz. The
94.3MHz station is an out of town station and signal strength is weaker.
The interference was on all my radios. I called the Radio station
engineer and he suggested the engineer from the out of town station
probably put me up to making the call, this was not true. From the
conversation, I think he had got a lot of calls about the interference,
but he assured my the station was in compliance with FCC Reg's. It was
Hip Hop vs O'Reilly back then. It went on that way for years until the
station changed from Hip Hop to some other format, then the interference
went away.

The station engineer might have been telling the strict truth... it
would have taken a spectrum analyzer or modulation meter to be sure.

Commercial FM is generally allowed a +/- 75 kHz carrier deviation.
Due to the way FM works, and due to the fact that the station is
transmitting a stereo subcarrier (centered on 38 kHz, with its own
sidebands going out as much as 15 kHz on either side), the FM
station's actual RF "footprint" can easily have significant energy 120
kHz on either side of its nominal carrier frequency. That's more than
half-way out to the "alternate" channel center, 200 kHz away. If the
station tends to run "loud" (highly compressed audio, cranked all the
way up) then the "wide footprint" is likely to be present much or most
of the time.

Things can be even worse these days, since many stations are also
transmitting in-band/on-channel digital subcarriers which go out even
further.

A lot of FM radios/receivers have fairly "broad" intermediate-
frequency filters... e.g. one or two crystal filters with 220 kHz or
even 250 kHz bandwidth. Such broad receptivity lets almost all of the
"desired" station's signal in... and that's good for low-distortion
stereo reception since you get the whole stereo subcarrier.
Unfortunately, if there's a strong signal on the "alternate" channel
(200 kHz away), that signal's outer sidebands will end up getting
through the filter, and will probably affect the stereo subcarrier and
increase distortion or "break through" into audibility. If you're
trying to tune in a weak, distant signal that's on an "adjacent"
channel to a strong local (100 kHz away) the problem is even worse.

There are ways to work around this:

- Use an FM tuner which has a narrower IF bandwidth. Better tuners
often have a wide/narrow switch setting, with the narrow setting
using different (or more) crystal filters with reduced bandwidth -
200, 180, 150, or even 110 kHz.

The narrower filters can eliminate a lot of adjacent- and
alternate-channel bleedover. The price is higher distortion
(especially in stereo) since the outer FM sidebands of the desired
station are also eliminated by the narrower filters.

- Use a directional FM antenna, and aim it in the direction which
gives the best results. This may be "aimed towards the desired
station" (increasing its relative strength), or "aimed at an angle
away from the undesired station" (to put the interfering station in
a "null" in the antenna's reception pattern).

I have a Denon tuner where the narrow ceramic filters were not narrow
enough to weed out the station I wanted to eliminate. I bought
narrower filters from Digikey which resolved my problem. Chuck

 

[Tim R]

On Wednesday, October 29, 2014 10:23:21 PM UTC-4, micky wrote:

On Wed, 29 Oct 2014 22:09:28 -0400, micky <NONONOmisc07@bigfoot.com
wrote:


** I keep buying radios from the 60's and 70's at hamfests, looking for
one that will get 88.1, 88.5 and 101.1


Not 101.1. 90.1, C-Span radio, which I guess I've lost interest in.
It's boring as all get out during the committee hearings, and the 7AM
program used to be great, but it's been discovered by the wackos.

I can't stand C-span though they have some good programming. The problem is that fake synthesized brass keyboard they use for the theme music. How hard would it have been to buy a CD of a real brass ensemble playing it? That piece has been recorded many times.