borderblaster said:
That's what they are called, obviously not visual. a harmonic is a multiple of the fundamental frequency (2 times 700kHz is 1400 kHz).
This post will be longer than I'd like, but I don't think the terminology can be adequately explained any other way...
"Superheterodyne circuit": Edwin Armstrong's invention, which is used in nearly every radio.
In this circuit, most of the signal processing is done at an "intermediate frequency" -- usually 455KHz in an AM radio. (usually 10.7MHz for FM)
The incoming signals are mixed with a "dead air" signal from a "local oscillator". The results of the mixing process are the sum of the two frequencies, and their difference. The frequency of the local oscillator signal is adjusted so the difference is always 455. For example:
Station frequency local oscillator sum difference
WTMJ 620 1075 1695 455
WNOV 860 1315 2175 455
WOKY 920 1375 2295 455
WISN 1130 1585 2715 455
WEMP 1250 1705 2955 455
WZUU 1290 1745 3035 455
WRIT 1340 1795 3135 455
(yes, I realize these are old calls. I'm too lazy to look up the new ones...)
The advantage of Armstrong's circuit is that you only have to tune one circuit -- the local oscillator -- to change stations. Previously, you had to adjust 3-4-5 different circuits every time you changed stations. The downside is that certain combinations make it possible to receive stations you aren't tuned to -- and that's what's being discussed here.
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Images:
Note above that when you tune to WTMJ, you mix the WTMJ signal on 620 with a dead-air signal on 1075. The difference between the two is 455, the "intermediate frequency" signal you want.
But what happens if there is, along with the WTMJ signal on 620, another signal on 1530. The signal on 1530 also mixes with the dead air signal on 1075. You get the sum (1530+1075=2605), and you get the difference. (1530-1075=455)
Yep, there's that magic 455 again. Your intermediate frequency circuits can't tell the difference between the 455 obtained from 620-1075, and the 455 obtained from 1530-1075. You'll hear
both stations. (or, maybe more relevant to this discussion, if WTMJ is off the air, you'll hear the 1530 station even though your radio is tuned to 620)
Better radios contain a "preselector" circuit, to keep 1530 from getting to the mixer when you're tuned to 620. This costs money, so some radios omit it and others use the cheapest preselector possible.
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Local Oscillator Harmonics:
We're assuming above that the "dead air" signal from the local oscillator is "pure" -- is only on the desired frequency. In fact, no oscillator is perfect. It generates some signal on multiples of the desired frequency --
harmonics. The 2nd harmonic is twice the desired frequency; 3rd harmonic is 3 times; 4th harmonic is 4 times; etc...
(I suppose you could technically call the desired frequency the "first harmonic", but in fact the term "fundamental" is used)
So let's say we're still tuned to WTMJ. Again, the local oscillator is on 1075 -- but because it's not perfect -- it has harmonics -- the local oscillator is also on 1075*2=2150, 1075*3=3225, 1075*4=4300, 1075*5=5375, etc... Harmonics usually get weaker with increasing "order". (the 4th harmonic is weaker than the 3rd, which is weaker than the 2nd, which is weaker than the fundamental, etc.. Usually you can ignore anything past the 4th, except with REALLY CHEAP radios...)
Let's consider that second harmonic on 2150. Let's say that, along with WTMJ on 620, there's a powerful expanded-band station on 1700. It mixes with the second harmonic of the local oscillator. You get the sum (1700+2150=3850), and you get the difference. (1700-2150=450, again you can ignore the minus sign)
450 isn't exactly 455, but it's close enough. You'll hear a distorted version of the expanded-band station along with WTMJ. (or again, if WTMJ is off you'll hear the expanded-band station at 615 on your dial, a frequency that's supposed to be empty)
Again, a decent preselector should prevent this, but many radios "cheap out" on this circuit. Also, better-quality local oscillators have weaker harmonics and so are less susceptible to this. Digitally-tuned radios are more likely to have local oscillator harmonics, because of the way the local oscillator's frequency is controlled.
This issue often causes reception of 6MHz shortwave broadcasts on receivers tuned to the upper part of the expanded band.
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Spurious responses:
This is a generic term for any situation where a receiver, tuned to a given frequency, receives a signal that is not actually being transmitted on that frequency. It includes images and local oscillator harmonics. There are other things that can go wrong in a receiver to cause these as well.
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Transmitter faults
It's relatively rare for spurious signals to be the result of problems at the transmitter. But it does happen:
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Harmonics:
Harmonics can also happen at the transmitter.
All transmitters radiate some harmonics. The FCC requires (73.44) that harmonics be at least 80dB weaker* than the main signal. Sometimes, filters fail and stations temporarily fail to meet this standard.
Again, transmitter harmonics happen at multiples of the station's frequency. If a station is broadcasting on 1350, its harmonics will be on 1350*2=2700, 1350*3=4050, 1350*4=5400, etc...
Given the poor state of maintenance of many small AM stations, it's surprising this doesn't happen very often... but I chose 1350 for a reason, if you have a shortwave receiver handy you might check these multiples.....
* in most cases. Lower attenuations may be permissible at lower-powered stations.
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Spurious emissions:
This is a generic term for when a transmitter transmits on frequencies other than the one it's designed to transmit on. It includes harmonics -- and any other situation where you hear a transmitter on a frequency it isn't trying to use.
For example... last night on my way home from work, one of the local FM stations was coming in not only on its "real" frequency, but 0.4MHz either side of that frequency.
It can be very difficult to tell the difference between a spurious emission (transmitter fault) and spurious response. (receiver fault) I'm pretty sure last night's issue was a transmitter fault, as I was
not hearing the same fault on another station of the same power and at the same site. (and I travel that route every day & had never heard the "faulty" station off-frequency before)
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OK, I think I'm out of letters now
