[opendtv] Re: 5th generation Test

Tom Barry wrote:

> It sounds like "IM3 products" (whatever they are) and
> good RF frontends are supposed to be known technology.

IM3 is third order intermodulation distortion products.
Created in receivers when multiple incoming carriers
and the local oscillator beat together. Ideally, any
such IM products would fall way outside the IF passband
and be attenuated nicely. But with single conversion
tuners, they are not all outside the passband.

It appears that the low cost of single conversion
tuners is enough of an incentive that manufacturers
seem to jump at the chance of going that route. As we
were informed a couple of weeks ago, even COFDM did
this. As Al Limbert and others explained, if your
RF amp up front is tuned to the channel you want and
can reduce its gain in the presence of strong signals,
the IM distortion can be greatly reduced, so single
conversion tuners can work okay.

With co-located transmitters, the problem might not be
so acute. Because you will never see unwanted signals
that are louder than the desired signal. But if the
unwanted signal is louder than the wanted signal, then
this can be a problem. The numbers we saw showed that
a 3 dB increased level to the receiver, from unwanted
signals, can cause up to 9 dB increase in this in-band
IM3 distortion (IIRC). So, big deal. And a 3 dB
increase into a receiver is not so hard to imagine if
there are loud transmitters close by, even if your RF
amp is tuned. The shoulders of the RF amp's passband
won't be vertical. Dual conversion, i.e. where the
receiver goes through two IF conversions, takes care
of this nicely.

> But if that's the case then maybe we (collectively,
> not me) also know what it would cost to add them to a
> box with an LG5 chip in it.
> How much extra are we talking about here?

Well, since everyone was marveling at the low cost of
the first gen Freeview boxes, and they were dual
conversion designs, the cost cannot be prohibitive.
However, it won't follow Moore's law, so the cost
will not erode over time quite so fast.

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