R: Topband: Receiver Dynamic Range

[Maurizio Panicara]

The bandwith of pulsed signals exists but actually most of the problem lay
in the phase noise of the PLLs used in the transmiters that spreads some
energy in the spectrum around the nominal frequency. Also the nowadays
receivers have PLLs in the heterodyne oscillators and create a local problem
that adds to the transmitted one.
Actually, unless a signal strenght doesn't exceed a certain level, nothing
is practically noticed, but when the received signal is huge it's well
possible to receive the transmitted noise as a larger occupied band or to
"feel" the own receiver limits in the matter of phase noise and not only
about its IMD distortion.
I believe that a receiver using an high level balanced mixer (better if
passive) and without a preamplifier has a decent noise figure for 160m (even
using a beverage) and shouldn't suffer an overloading unless the antenna
picks up much more than milliwatts by the interfering signal.
When the problem is really an incontrollable receiver overloading, the best
way to fix it is to use a null steering device. Unfortunately the commercial
products I know, in order to equalize the signal level between direct and
sense ports, are mostly using symple wide band low level preamps very prone
to suffer IMD problems in case of huge signals, often worsely than receivers
do with their own preamps. With such kind of steerers the huge signal is
dramatically reduced, but unless direct and sensed signals are both
dramatically attenuated before beeing applied, an horrible band spectrum is
delivered to the receiver input. This happens because the interfering signal
is only cancelled by the phase reversal in the output but is still present
in the input of the amplified port/s of the steerer at a huge level, so the
overloading continues to take place there and the IMD products are sent to
the receiver input. When such situation exist, weak DX signals are not
discernable in spite the original QRM has apparently gone.

73,
Mauri I4JMY



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