[R-390] Noise Floor question

[DJED1 at aol.com]

In a message dated 3/29/2006 1:12:58 P.M. Eastern Standard Time,  
sandeenpa at yahoo.com writes:

In ads  for the R390A the term of "noise floor close the theoretical limit"  
yada  yada yada. is
often mentioned.  A search on google didn’t yield  anything comprehensible.

If we follow Roger Ruszkowski’s method for  achieving a 30db ratio, then by 
logical definition we
are lowering the  noise floor by a ratio-metric method.

I know the receiver has a very  low "noise floor" but how would us mere 
mortals measure it?  Short
the  antenna input, terminate the audio out put with a 600 ohm resistor turn 
up the  RF gain and
audio controls and then measure the AC voltage on the  termination resistor 
with a micro-voltmeter?

If one did Chuck Ripple’s  audio improvements are we lowering the noise floor 
or just making a
better  sounding audio output or both?

Inquiring minds ever seek knowledge for  improving our radios.  Regards,  
Perrier





I took a while to do some research on Perrier's questions.  Here's my  
answers, although I'm willing to be corrected-
First, people have thrown around "a noise floor of -143 dBm, close to the  
galactic limit".  It sounded pretty ambitious to me, and I calculate that  the 
noise of a resistor in a 0.1 Kc bandwidth is about -154 dBm.  So that's  the 
ultimate limitation on sensitivity, although the R-390A doesn't nearly reach  
that.  I'll use my radio for further discussions, although it has not  been 
tweaked for optimum sensitivity per Roger's procedure.  One  measurement on my 
radio indicated that the noise figure was about 10 dB, thus  putting the radio's 
noise floor at -144 dBm.  So that may be where the  claims come from, although 
most of us measure AM sensitivity at a 4 Kc  bandwidth. But the radio was 
originally used extensively for CW intercept work,  and so sensitivity on a 0.1 
Kc bandwidth is relevent.
 
Measurement of sensitivity can be done in one of two ways- using  a 
calibrated noise source and adjusting it for a 3 dB rise  compared to receiver noise 
(this measurement is independent of bandwidth), or  using a calibrated signal 
generator and adjusting the output for a given rise,  usually 10 dB, in receiver 
output.  This can be measured using the VU meter  on the receiver or a 
separate AC voltmeter on the line output. Techniques are  well covered in W Li's 
Pearls of Wisdom.  The consensus is that the  measurement should be using AM in a 
4 Kc bandwidth, and turning  the modulation on and off to measure the 10 dB 
difference.  A CW  measurement using the BFO will give significantly better 
sensitivity.  My  radio measures 0.6 microvolt AM sensitivity, and 0.16 microvolt 
CW  sensitivity. 
Finally, Chuck's audio improvements will not change the sensitivity of the  
radio.  However, his recommendation on setting the IF gain will help the  
sensitivity.
Hope this helps- and the Pearls of Wisdom will tell you plenty about this  
topic
Ed