Posted by
alorona on
Aug 26, 2017; 7:36pm
URL: http://elecraft.85.s1.nabble.com/A-very-desirable-capability-tp7633678p7633779.html
Once again, a beautiful post by Bob. His description suggests a feature that could be put into any new transceiver.
Imagine a graphical depiction of Bob's scenario below on the color screen of the radio which would help you optimize the placement of the dynamic range of the radio with respect to the band noise. A simple vertical graph, showing measured band noise floor at the bottom and a vertical axis calibrated in dBm, with a movable rectangular "window" shaded with a color that moves up and down according to the setting of the RF Gain, Atten, and Preamp of the radio.
Maybe this already exists in one of the fancy SDR radios and I don't know about it? But it seems to me that this would be a great aid to visualizing where the dynamic range of the radio is set. If too much of the colored window I'm talking about is below the band noise, it would be obvious that reducing gain or adding attenuation would optimize the dynamic range without sacrificing sensitivity -- a concept that many hams have difficulty understanding.
In the K3, for example, the shaded rectangle would have a fixed height of about 104 dB, the dynamic range of the receiver. As you decreased the RF Gain, that window would move up, and as you increased RF Gain it would move down. Turn on the ATT and the window jumps up by 10 dB. Turn on the preamp and the window jumps down by whatever the gain of the preamp is. According to Bob's excellent description you want the bottom of the window to be about 10 dB below the noise floor, but no lower. Think of how easy it would be to adjust your controls if you could *see* what's happening on the screen of the P3. Those of us with "preampitis" would finally realize that the preamp is unnecessary in many situations.
The key to this would be to give the receiver the ability to measure the current noise floor in dBm so it could be plotted on the screen, showing the operator the noise floor in real time, and for the receiver to know at all times what its maximum signal level handling point is. The P3, of course, already measures noise floor; maybe each receiver would need a pre-measured dynamic range burned into a ROM at the factory. But I don't even think that's necessary... some nominal window height as I mentioned earlier would probably be good enough, plus or minus a couple of dB.
R,
Al W6LX
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One thing that also helps is to use minimum necessary RF gain and
correct utilization of Attenuation. Thus on 160M and 80M and to some
extent 40M, I use 15 dB Attenuation and reduce the RF gain a wee bit.
In doing so, the signals pop out of the noise.
A point to consider is optimum receiver performance occurs when the band
noise floor is 10 dB to 15 dB above the receiver noise floor. Since we
can't really change the receiver noise floor, we must change what is
arriving at the front end. This means some degree of attenuation and RF
Gain reduction is required. For the math.......if no signal band noise
is -97 dBm or S-5 and the receiver noise floor is - 125 dBm, a
difference of 28 dB, the receiver should have 15 dB of attenuation
activated. You can do the same with the RF Gain by reducing it about 3
or 4 S units. Too much RF gain being utilized in an attempt to hear weak
signals is not necessarily a good thing. As one goes higher in
frequency, usually band noise decreases and finally to a point where the
PRE should be used. In many cases I've heard hams describe the use of
ATTN is to prevent receiver overload. While true under some cases, most
residential QTH locations and antennas won't encounter enough signal to
overload the receiver. Thus the ATTN function is more beneficial when
used in correct receiving gain structure.
73
Bob, K4TAX
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