Topband: ft1000 mp vs omni 6+
w8ji at contesting.com
Fri Jul 18 10:57:55 EDT 2003
> During the 160 cw contests we hear phantom cw signals on the ft1000 mp,
> While the omni 6+ is clean. The difference is the omni can hear the
> weak EU
> Stations while the ft1000mp is garbled by phantom spurious signal that
> To be cw but make no sense.
This is the effect of odd-order IMD. It sounds like the old crowded novice
bands with a BFO off, or like random undecipherable CW. Many people dismiss
it as "actual sloppy CW signals".
The FT1000MP, like the other Yaesu rigs, has a problem with design of the
noise blanker. If you have the NB gain control UP in the 1000MP, even if
the NB is OFF, this effect is very pronounced. In some other Yaesu's, like
the 1000D or 1000MKV's, you can NOT easily cure the problem by reducing the
front panel NB gain control.
If you fix the MP, or set the front panel NB gain off, it cures this
The INRAD IF modification, because it adds gain before the narrow CW and SSB
filters, makes this problem worse by about the additional gain of the
amplifier inserted into the 70MHz IF. It would be better to just filter the
audio output to get rid of hiss when using narrow filters, rather than mess
up the gain distribution of the receiver by starving the detector and adding
gain ahead of filters.
The actual root of the hiss problem is in the lack of filtering in the last
low frequency IF stage right before the detector. This allows broadband
noise from the high gain last IF stages to enter the product dector
unfiltered, and making things worse the detector sees both USB and LSB
noise. The actual correct design cure would be a cheap filter just before
the product detector, but a $100 or less audio filter is just as good in
practice and will not hurt close-spaced IM3 (like adding 70MHz IF gain and
starving the product detector does).
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