[Elecraft] KX3 "Full" SDR

[Bruce Manning]

So I have been setting here with the ARRL handbook and trying to understand
this stuff. This is what I have come up with using the formula in the
handbook;
Aqrm = NL + 10 x log (BW)

Aqrm = interfering signal in db
NL = noise level on the receive frequency in dBc
BW = receiver IF band width in hZ (I am using a 500 hZ filter)

Aqrm = -77 + 10 x Log (500)
           = -77 + 10 x 2.69897
           = -77 + 26.9897
           = -50.01 dB
           If the tx signal is S9 +20 dB then you add +20dB to the -50.01dB
and you get -30.01dB which  is a S4 signal. So a S4 signal is a lot of QRM.
    If the band width is 1000 hZ the signal would be S5 plus 3 dB. And all
of this is at 350kHz away from the Tx signal.

K3
Doing the same math but using the 120 dBc at 360kHz we get a interfering
signal of -73 dB, which is 75 dB below an S1.

Now no one has ever accused me of being the brightest bulb in the box, but 
this is what I have come up with.

73 and HNY looking forward to SKN in a few hours!
Bruce NJ3K

-----Original Message----- 
From: Bruce Beford
Sent: Tuesday, December 31, 2013 4:38 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] KX3 "Full" SDR

> For those, the noise from a Flex at 100w, at -77 dBc (which is -27
> dBm), can be quite substantial. Recall that S9 is approx -72 dBm. A
> Flex is transmitting wideband S9+45 noise.

OK, I'm not a Flex user. But, I don't agree with your math. I am not trying
to defend the transmitted phase noise performance of the Flex products,
but... Your math is flawed.

Assume 100w transmitter is delivering +50 dBm to an antenna. (100W is 50 dB
greater than 1 milliwatt) Also assume you are at a receiving station, and
hearing this station at S9+30 dB, on a receiver calibrated to 6dB/S unit,
S9=50uV at 50 Ohms, or -73dBm for S9. Your receiving the fundamental signal
at -43dBm (S9+30dB). With the transmitted phase noise 77dBbelow the carrier,
it would be received at -120dBm, or about S2.

Granted, it would be 10 dB stronger (abt S3.5), if you were receiving the
fundamental transmission at S9+40dB, etc. But... the Flex is not "
transmitting wideband S9+45 noise."

Let's not blow this out of proportion.

Bruce N1RX


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