Chuck-I agree that the R390A is a great receiver, but the noise floor
performance you quoted in a 4 kHz BW make me suspect that you may have a
faulty attenuator and/or leakage in your signal generator and associated
cables.
For example, the thermal noise floor (kTB) is defined as -174 dBm/Hz at room
temperature. Assuming that the R390A's 4 kHz filter is actually close to
this BW, 10log(4000)=36 dB, so the thermal noise alone is -174 + 36 = -138
dBm in a 4 kHz BW. After factoring in an estimated receiver noise figure of
at least 5 dB, the actual R390A noise floor is probably no better than -133
dBm in a 4 kHz BW (probably closer to -130 dBm).
When making measurements on an exceptionally sensitive receiver, I like to
use the "kTB+noise figure test" to confirm that the numbers I'm coming up
indicate that leakage is not a potential source of measurement error.
Scott
N7NB
-----Original Message-----
From: tentec-bounces@contesting.com
[mailto:tentec-bounces@contesting.com]On Behalf Of Chuck Rippel
Sent: Friday, 09 July, 2004 12:21
To: tentec@contesting.com
Subject: [TenTec] Receiver Specs - Orion et al
> 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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