[TenTec] Carl, Pmni, 756 etc

[W8JI@contesting.com (Tom Rauch)]

Hi Carl,

> Let me describe the main receive characteristics of the rx340.  I do
> not consider the 10 khz dynamic range specs to be meaningless.  If you
> have ever run in field day with lots of transmitters in the near
> field, one of the most challenging things for a receiver to do is
> operate on SSB with another transmitter on CW on the same band or
> viceversa.  Most receivers fail miserably at this.

That actually is one of the least challenging applications, and has 
little to do with how the receiver works with close signal spacing. 

Only the stages in front of the roofing filter are involved in wide-split 
overload. If we look at manufacturer and old ARRL tests and 
reviews, we see they all like to measure outside the roofing filter 
passband with 10-20kHz spacing or wider. Almost anything looks 
good at wide spacing.
 
> That said, if your are inside the roofing filter, you are obviously
> limited to the dynamic range of the DSP chip.  This is 40 db.  So for
> signals that are closer than 10 khz to the desired signal, 40 db is
> all the dynamic range you have.  When an undesired signal is within 10
> khz of the desired signal and stronger than 40 db above the desired
> signal, the analog AGC will come up some to keep the DSP processor
> from overloading.  This is done as best as it can be by the rx340
> hardware and I find I rarely get into this situation.

I don't know what the selectivity ahead of the DSP is.

If the blocking or IM DR of the DSP system is 40 dB and if the 
receiver **can't** use a 500Hz or narrower roofing filter on CW or a 
2.5kHz roofing filter on SSB, the receiver would offer poor 
performance in many real-world situations.

It might be fine for casual SSB use (wide filters) in urban or some 
suburban locations, but stay away from weak-signal CW on the low 
bands, especially if there are multiple signals in the passband of 
the roofing filter!

As a matter of fact, that DR is about the performance of a typical 
PSK system. You should hear the PSK folks whine and bellyache 
about overload when a high power (100 watt transmitter) signal 
comes on a kHz from them!
73, Tom W8JI
W8JI@contesting.com