[TenTec] Receiver Specs - Orion et al

[Bill Tippett <btippett@alum.mit.edu>]

Hi Chuck,

WA4HHG wrote:
>As one who is an R390A enthusiast, what I find remarkable is that a DSP 
based
receiver has a dynamic range of 92db.  The Orion either has an incredibly low
noise floor or, able to handle extremely high strength signals.  Its a little
of each, I expect.

        Orion is more accurately a hybrid analog/DSP receiver.  More of its
excellent RF front-end has to do with narrow roofing filters at 9 MHz, and
less to do with DSP at the 3rd IF of 36 kHz.  Orion's sensitivity (MDS as
measured by ARRL) is about the same as most current radios (-128 dBm with
Preamp Off and -136 dBm with Preamp On), but as noted in the footnote at
the bottom of the page below:

"Pre-Amp On MDS numbers of -130 dBm or more are more than adequate for most 
HF band operating, since band noise is typically above this number. (Lower 
frequencies need less MDS (more +number) due to an increase in atmospheric 
noise.)"

http://www.elecraft.com/K2_perf.htm#5%20kHz%20numbers

Here are some comments by W8JI on the same topic:

"MDS is a measure of sensitivity. -135dBm is 10dB more sensitive than 
-125dBm. This number doesn't matter very much in what you actually hear. 
The real test is if you hear a very noticeable noise increase when you 
connect an antenna to the receiver.  If you hear an obvious noise increase 
when you connect an antenna instead of a dummy load, your receiver is 
sensitive enough! You should check sensitivity at the quietest time with 
the narrowest selectivity you use on every antenna you use."

http://www.w8ji.com/receiver_tests.htm

>The point to illustrate is that this strength is not drwan by the 
amplification
factor of the R390A front end being greater.  Rather, the advantage is 
realized
by its lower system (RF to AF stages) noise floor.  As an FYI, depending on 
the
reciver, I measure R390A noise floors typically between
-135  and -140db with the 4kc filter, AM mode.

        If the R390A had -135 dBm MDS at 4 kHz, that would equate to -144 
dBm at
a standard test bandwidth of 500 Hz for MDS measurements (ARRL, Sherwood, 
etc).
Although this might seem important, it is most likely overkill especially on
the low bands where the noise floor is typically much higher.  W8JI's measured
noise floor on 160 on a quiet winter night was -127 dBm with a 350 Hz 
bandwidth.
Adjusting for noise bandwidth (350 Hz to 500 Hz), this would equate to a MDS of
-125.5 dBm at 500 Hz, so any receiver sensitivity better than this is wasted.

http://www.w8ji.com/receiving.htm

        I'm not 100% positive but believe Sherwood Engineering's 
measurements below
use the standard 500 Hz bandwidth for his MDS measurements.  He shows the
R390A at -137 dBm in the following table:

http://www.sherweng.com/table.html

Bottom line is that sensitivity is not really a critical parameter for most
receivers as long as the RX noise floor is lower than atmospheric noise.  As
W8JI, Sherwood and others have noted, close-spaced IMDDR3 and BDR are much
more important receiver performance metrics.

                                                73,  Bill  W4ZV 

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