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What does 92.5 half to do with Channel 12?

J

jras20

I got a small problem, on my Tuner, if I tune to 92.5, its a station, but when I'm on channel 12, it blanks it all out, it just shows a big white screen and then blanks out to blue (what my vcr does on a blank channel) I think it does that on 11 & 13 also but not as bad. My tuner is right by the TV which may be one of the cause, but my FM antenna is on a seperate pole than the tv, and is pointed the other direction.
Thanks,,,
J
<P ID="signature">______________
jras20</P>
 
> I got a small problem, on my Tuner, if I tune to 92.5, its a
> station, but when I'm on channel 12, it blanks it all out,
> it just shows a big white screen and then blanks out to blue
> (what my vcr does on a blank channel) I think it does that
> on 11 & 13 also but not as bad. My tuner is right by the TV
> which may be one of the cause, but my FM antenna is on a
> seperate pole than the tv, and is pointed the other
> direction.

When you tune your radio to 92.5, a "local oscillator" circuit inside that radio is tuned to 103.2. The 92.5 and 103.2 signals are mixed, creating (among other signals) a signal on 10.7MHz which carries the 92.5 programming. This signal is amplified in a fixed-tuned "IF amplifier" and then "demodulated" and sent to the speaker.

(if you were to change stations to 93.3, the local oscillator would move to 104.0 - causing the difference to continue to be 10.7. By using this "superhetrodyne" scheme, the local oscillator is the only thing that has to be tuned -- the "IF amplifier" can always operate on 10.7 and never needs to be adjusted.)

This 103.2 signal from the local oscillator is not perfect. It has "harmonics" - signals on multiples of the desired 103.2.

103.2*2 = 206.4;
103.2*3 = 309.6;
103.2*4 = 412.8;
etc...

TV channel 12 is 204-210MHz. The video carrier - the part of the channel most susceptible to interference - is at 205.25MHz. 206.4 is plenty close enough to cause severe proplems.

I don't know that there's a whole lot you can do about it, other than to turn the radio off when you're trying to watch channel 12, or at least listen to a different station!
 
Thanks for the info! I just didnt understand why it blanks out like that, I have a place that is in a fringe area on most TV stations & radio, theres only 3 that I am in a "local" area of the stations, TV theres only 1 right now that I can get a 64db gain on, but the rest are fringe. etc. ch.4 ch.5 ch.9 ch.12, Channel 25 is the only "local" one which even that isnt that clear I do have direct tv down there as well, but its not as fun to use that as it is dx'n tv :)

thanks again,,
<P ID="signature">______________
jras20</P>
 
You might want to check your tuner - FCC regulations for receivers usually preclude leakage like you are describing. If you have advanced test gear, I bet the oscillator section is clipping - perhaps an aluminum electrolytic bypass cap has opened up or become leaky. If you fix it - you probably will get better reception on your tuner as well - as all those harmonics probably show up as noise in your receiver and limit effective sensitivity.
 
> You might want to check your tuner - FCC regulations for
> receivers usually preclude leakage like you are describing.
> If you have advanced test gear, I bet the oscillator section
> is clipping - perhaps an aluminum electrolytic bypass cap
> has opened up or become leaky. If you fix it - you probably
> will get better reception on your tuner as well - as all
> those harmonics probably show up as noise in your receiver
> and limit effective sensitivity.

I won't say it's particularly common for 2nd harmonic problems to affect TV signals, but on the fundamental frequency (no x2 involved) local oscillator radiation from FM radios is pretty common. If I'm listening to a weaker station above 99MHz, it's pretty common for it to be "sucked out" at the next stoplight. Do a bit of math, and one usually finds there's a popular local station 10.6 or 10.8MHz lower. (by watching traffic when the light goes green, you can often also figure out which car is listening to which station!)

Indeed, to some degree the FCC recognizes that, avoiding assigning stations 10.6 or 10.8MHz apart when possible.

Never underestimate the corner-cutting of radio manufacturers!
 
Thanks I baught that tuner off of ebay, so you never know! If I get a chance over the weekend I'll lookinto it.
<P ID="signature">______________
jras20</P>
 
> I got a small problem, on my Tuner, if I tune to 92.5, its a
> station, but when I'm on channel 12, it blanks it all out,
> it just shows a big white screen and then blanks out to blue
> (what my vcr does on a blank channel) I think it does that
> on 11 & 13 also but not as bad. My tuner is right by the TV
> which may be one of the cause, but my FM antenna is on a
> seperate pole than the tv, and is pointed the other
> direction.
> Thanks,,,
> J
>
Funny you should mention that. When I was a kid I used to listen to a local AC station on 92.5 (WQST Forrest, MS) If someone were watching channel 12 in the next the room picture would break up. Ocassionally if my parents were hogging the TV and wouldn't let me watch something on the other station (thats back in the old days when people had only one set) I would go back to my room and put it on 92.5 and mess up the TV. I used to laugh myself silly hearing them fussing about the TV going out. They would be twisting on the fine tuning knob (remember those?) and hollering for me to come fix it.
 
> I bet the oscillator section is clipping

Bruce, that is the operational state of all
oscillators.

It is what establishes their ultimate power or voltage
'output' level.

That can be easily seen when running a SPICE circuit
simulation, from a cold start - where one can see
the oscillation 'build' and finally stabiize at
some designed value.

At the point the oscillator transistor will be alternately
in the 'cutoff' and saturated states, this is normal.

No single cap such as an aluminum low-frequency
power supply filter or audio bypassing 'cap' is going
to have any effect at RF frequencies - that is what those
.01 uF ceramic discs are used for!

Today, of course, 0603 and 0402 chip caps are used in
place of those leaded, through-hole mount 'disc' style
ceramic caps.

Take a spectrum analyzer, connect it up to the antenna
jack on any gicen receiver and you would be surprised
on how much LO (local oscillator) signal leaks out,
sufficient to affect weak TV signals if other factors
are just right.
 
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