Posted by
Bob McGraw - K4TAX on
May 22, 2019; 11:09pm
URL: http://elecraft.85.s1.nabble.com/Level-of-K3S-audio-out-to-digital-apps-on-Mac-tp7652106p7652126.html
Regarding receiver set-up implementation, it is usually best when the no
signal band noise is some 10 dB above the noise floor of the receiver.
Per Sherwood Engineering data, the noise floor for the K3S is -135
dBm. Thus 10 dB above that would be -125 dBm. Now since one can't
change noise floor one must use the correct amount of attenuation and RF
Gain reduction to attain the -125 dBm target for the no signal band noise.
Here's the way I handle that. This example, the no signal band noise
is S-4 or -103 dBm. That is determined with the Attn off and RF Gain at
max. Then the difference between my target of -125 dBm and -103 dBm is
22 dB. Thus I need 10 dB of attenuation and 2 S units of RF Gain
reduction. Lets take this a bit further. The no signal band noise,
no attenuation and RF Gain at max, is S-8 or -79 dBm. To attain our
target of -125 dBm we need a total of 46 dB of attenuation and RF Gain
reduction. If we choose 15 dB of attenuation then we need 5 S units of
RF Gain reduction. To easily accomplish this, I switch the radio from
the antenna to a dummy load. Then adjust the RF Gain until the S meter
indicates S-5 where 1 S unit is = 6 dB. Thus with 30 dB of RF Gain
reduction the switch in 15 dB of Attenuation I have the required 46 dB
or so.
Then to the audio. Here I adjust the Recording Gain of the Windows
mixer. Normally this level is about -10 dB or 24% which ever you choose
to display the value. If the signal level on the WSJT-X is above 30
dB then you should reduce the Windows mixer level a bit more to bring it
into ideal range.
There is a caution in the K3S manual regarding advancing the K3S Line
Output level which could cause distortion. The default value is 10.
As to using AGC, on or off, from the WSJT-X help file: "/Use the
receiver gain controls and/or the computer’s audio mixer controls to set
the background noise level (scale at lower left of main window) to
around 30 dB when no signals are present. It is usually best to turn AGC
off or reduce the RF gain control to minimize AGC action./" I find using
my method for setting the receiver gain based on band noise, and the use
of slow AGC will prevent any likely overload of the K3S audio path, the
computer audio path, or the WSJT-X software processing.
The receiver setting of gain values works quite well for other modes as
well. But today I hear hams say; "gotta have more gain to hear those
weak signals". Yep, 50 years ago this was correct, but not with
today's receivers.
In looking at the Sherwood Engineering Test data 05/10/2019, the K3S has
the lowest noise floor of any others at -135 dBm, it has the best
sensitivity of any others at 0.27 uV, and it has the best 100 kHz
blocking range of any others at 150 dB. The point being.......you have
a darn great receiver in front of you. Now tweak its user parameters to
obtain optimum performance which it is capable of providing.
Now for those that say "well I heard........" or that "someone told
me......." I say go research for yourself and learn the real facts.
73
Bob, K4TAX
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