>If I am running two tones at say 30 and 30.1 MHz and am generating
>harmonics at 60 and 60.2 MHz, will those harmonics beat together to form
>intermod products as well? Or in order for intermod to form does the
>signal have to appear at the input of the amplifier? Can intermod be
>generated by signals appearing only at the output?
Yes, intermod can be generated in a nonlinear device coming through the
"back door". Broadcast sites with many XMTR's co-located have to install
notch and BP filters, to prevent the neighboring RF from getting back into
their final. It's not only a safety and performance issue, but IMD
generation. The most susceptable are broadband finals, like solid state
amplifiers. Tube amplifiers, by nature of the tuned plate circuit, reject a
lot external RF off frequency, and have better IMD from the output. Class C
and other high efficiency amplifiers would be more prone to generating IMD
as they operate the amplifying device in saturation, where linearity is not
good.
Television transmitters, with the video portion (not ATV but plain old
analog NTSC or PAL) coming from linear amplifiers, are also prone to IMD
in the final. For two nearby channels at the same antenna site, Fspur = 2 A
- B. If this comes within the BW of the cavity amplifier, where the "turn
around loss" is only a few dB, then they create spurs in the final of one
(or both!). Turn around loss is the term for the rejection of an amplifier
to off frequency energy.
Geoff Mendenhall, (W9NEZ?) now of Harris Broadcast, wrote a good summary of
the IMD process in PA's for FM broadcasting in the National Assoc of
Broadcasting (NAB) handbook, 7th edition 1985. I'm not sure if the info is
still in later editions, but it is certainly not obsolete. We made the
measurements in a Chicago FM station atop the Sears or Hancock tower as
well as in the lab, with lower power and various couplers and filters.
Page 3.3-102 states: "The output of the first transmitter F1 is coupled
into the non-linear output stage of the second transmitter F2 because there
is not complete isolation between the two output stages. F1 will mix with
the second harmonic of F2 producing an in-band 3rd order term with a
frequency of 2F2-F1. In a similar fashion the other 3rd order term will be
produced at a frequency of 2F1-F2. This implies that the second harmonic
content within each transmitters output stage along with the specific
non-linear characteristchs of the output stage will have an effect on the
value of the mixing loss (turn around loss). "
.....
"The transmitter output circuit loading control directly affects the source
impedance and therefore affects the efficiency of coupling the interferring
signals into the output circuits where it mixes with the other frequencies
present to produce IM3 products. Light loading reduces the amount of
interference...."
.....
"Second harmonic traps or low pass filters in the transmission line of
either transmitter have little effect on the generation of IMD....."
Another thing I remember is that a repeater site I worked on had a lot of
probs with IMD, some coming in the back door, and a lot from rusty metal on
the tower, connectors, and dirty transmitters. It was and still is a
constant threat to the commercial and amateur operator on a heavily loaded
tower site.
John
K5PRO
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