[BC] IBOC question

[Craig Healy]

One thing I have never seen addressed in the IBOC discussion is 3rd order
intermod interference.  Most transmitters I've seen located near each other
have some 3rd order products.  Sometimes this can be a significant level,
though often masked by nearby signals.  One example here in Providence is
the product of a co-located 94.1 and 95.5 site.  That creates a spur on 92.7
that is quite noticeable in the vicinity of that site.

What about the increase in products with the two new adjacent frequency
carriers?  Has anyone measured this?  It would appear the cumulative noise
when most stations run IBOC may be quite high and on many more channels than
at present.

To use the above mentioned pair with IBOC added:

Lower IBOC on 94.1
(93.9 * 2) - 95.3 = 92.5
(93.9 * 2) - 95.5 = 92.3 (local WPRO)
(93.9 * 2) - 95.7 = 92.7

Analog on 94.1
(94.1 * 2) - 95.3 = 92.9
(94.1 * 2) - 95.5 = 92.7
(94.1 * 2) - 95.7 = 92.5

Upper IBOC on 94.1
(94.3 * 2) - 95.3 = 93.3 (Local WSNE)
(94.3 * 2) - 95.5 = 93.1
(94.3 * 2) - 95.7 = 92.9

Then we could reverse the frequencies to get the other set, but you get the
picture.  Some frequencies even get a double chance at intermod, like 92.7
and 92.9.  I should also point out that the two stations mentioned have a
pretty new ERI filter/combiner, yet there is still some interference
generated and radiated.

Is this even been looked at by anyone?  Are these newer transmitters linear
enough so spurs are below 80db down?

Craig Healy
Providence, RI