[R-390] Noise Floor question

[Roy Morgan]

At 01:11 PM 3/29/2006, Perry Sandeen wrote:
>In ads for the R390A the term of "noise floor close the theoretical limit" 
>... how would us mere mortals measure it?

Others will have more modern and authoritative answers, but here is mine, 
briefly:


Basic ideas:
1) The front end of the radio, that is the "antenna", the rf coils, and 
first RF amplifier tube contribute most of the noise to the radio.

2) The stuff in the front end, without any external signals, makes noise 
just because it has warmth compared to absolute zero. Thus, there is an 
equation that tells the theoretical noise floor contributed to these 
parts.  And thus, folks in radio astronomy at least used to put liquid 
nitrogen onto their first amplifier tubes to reduce the noise they generated.

3) If we assume no or little noise is contributed by the later stages in 
the radio, and the amplifier is perfectly noise free, then this calculated 
amount of noise would show up at the speaker terminals and we can measure 
it.   This would be the theoretical minimum noise or noise floor. In 
addition, in normal real radios, the first RF amplifier contributes some 
noise, thus we get the real minimum  noise or noise floor.

NOW:
If we add a carefully controlled amount of additional noise at the antenna 
terminals, by magic or by careful engineering, and the noise measured at 
the audio output DOUBLES, then we have added noise equal to the actual 
noise generated by the front end.  There you have it.  This is not 
impossible for us average folks to make an approximation of. Just make an 
input network (attenuator at the antenna input) that presents the needed 
impedance to the radio, and have a way of injecting a calculate-able amount 
of noise extra.  A noise generator or maybe a URM-25 and a 100:1 attenuator 
would let you at least get started on this.

If you use a switched noise generator (pulsed on and off) and look at the 
receiver output noise with a scope, you can see a sort of square wave and 
easily get an approximate setting for the added noise to guess the self 
noise of the radio.  Make adjustments to reduce this self noise, and you 
have made an improvement in the radio. This is more or less what Roger's 
method does, though with different noise levels being compared.  And this 
is exactly what is done with converters, either with a manual 
knob-controlled noise generator, or with a pulsed one and a scope to see 
the effect of adjustments.

>  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?

Yes. but you won't need a MICRO voltmeter - it will be plenty loud enough 
to measure with more normal voltmeters.  Don't short the antenna terminals, 
use the expected antenna impedance.

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

If the plain noise of the audio section adds a LOT to the receiver's noise, 
the audio section needs fixing (such as replacement of leaky caps, bad 
tubes, and bad resistors.)

  If the audio section makes *distortion* on normal sounds at normal levels 
(Which it most certainly does) then this distortion is noise to you 
listening, but is not from the front end of the receiver.  Most audio 
section modifications are likely to reduce the distortion you get from the 
radio.  This is why the diode load audio pick-off method sounds so good.

Chuck's method of setting the IF gain has the basic effect of making sure 
the IF gain is not set so high that the IF section contributes (lots of) 
noise itself. Most of the noise in a normal radio should be coming from the 
RF front end amplifier.  Some modern and very expensive radios have no RF 
front end amplifier, partly in order to eliminate the noise from the active 
part (tube, transistor, traveling wave tube, or whatever).

>Inquiring minds ever seek knowledge for improving our radios.

Improving our understanding of what's going on is a very good start. I hope 
this helps, and expect that other comments/posts will help, too.

Roy



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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