[TenTec] Receiver Specs - Orion et al

Chuck Rippel WA4HHG at R390A.com
Fri Jul 9 15:20:43 EDT 2004


> The thing I find impressive in the Sherwood report is that 50 year old
> technology (AKA R-390A) does better then most of the current rigs...
>
> 73, Steve
>

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.

I asked a FT1000/Icom V/S Orion question a few days ago.  This thread sheds 
some light on one of the arrows aimed at the Orion, a supposed lack of 
sensitivity.  What is being experienced is most likely a very quiet receiver 
which most are never exposed to.  No "YaWoodCom" I have ever come across has a 
has a receiver section with particularly low noise floor.  Even on a quiet 
band, the "S" meter is always bouncing between S-1 and S-3.

Conversely, a properly restored and aligned R390A exibits what some have called 
the "R390A" effect.  Tuning across the same quiet band as above, the Carrier 
meter will sit on zero and there is little band noise.  When nearly the exact 
frequency of a strong (but clean) signal of an international broadcaster is 
tuned, the station will nearly "pop" in and the Carrier meter jump to 60-90 (0-
100db scale).  The lack of band noise is only an illusion which might cause 
some to conclude the receiver is not sensitive.  

One test I run here on the East Coast is tune 4915.0 from about 1930UTC.  The 
internal service of Radio Ghana is on this frequency and is one of the first 
sub-Saharen trans-atlantics to be detected.  When the YaWoodComs can only 
detect the beat of a carrier when in SSB mode with a 2.4kc filter, the R390A is 
actually pulling audio off the signal in AM mode with a 4kc filter.

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.  Because those levels are so 
very low, the tests are not made in an RF proof room and the absolute low limit 
of the HP signal generator is -140db, I cannot stand on those numbers are being 
completely scientific.  However, whatever the exact value, the noise floor is 
incredibly low.

Going back on topic with the Orion.  Perhaps it is this effect that is causing 
some to conclude the receiver is "not sensitive."   Thus, a real world test is 
to see which rig is best at recovering audio from low strength signals.

Chuck
WA4HHG


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