Matt,
-174 dBm is the noise of a resistor at room temperature in a 1 Hz
bandwidth. In a 500 Hz bandwidth (which seems to be the standard for
receiver measurements) you get 10 * log(500) = 27 dB more, or -147 dBm
thermal noise.
So 35 dB excess noise is -147 + 35 = -112 dBm in a 500 Hz bandwidth.
With a 2.7 kHz SSB filter, it should be -104.7 dBm.
The KX3 S meter may not be accurate for noise. I expect the AGC tends
to respond to peak signal level rather than RMS. That would give a
higher than expected S meter reading, which is what you observed.
The way to measure the N-gen output is to turn off the AGC, reduce the
signal level so the audio output is not distorting (using attenuator or
whatever) and measure the audio output with an RMS voltmeter. Then
connect a calibrated signal generator and adjust its output level for
the same meter reading. If you don't have a true RMS voltmeter you'll
be off a dB or two, but it should be in the ballpark.
Alan N1AL
On 12/31/2012 2:33 PM, Matt Zilmer wrote:
Couple of questions on the N-gen:
1. The instruction sheet gives "35 dB" of Excess Noise Output. Is
this referenced to -174 dBm or some other?
I'm seeing S0 on a KX3 with the switch OFF, and S5-6 with the switch
on. Seems usable. I have S5 at -97 dBm, S6 at -91 dBm. So the
reference level is around -132 to -126 dBm, if I read this spec
correctly.
2. Can someone reference a data sheet for the MAR-1 MMIC? I'm going
to search for it, but thought someone might have it around.
The N-gen makes a nice 30 minute build, and is fun. Parents are still
giving me kits even though I'm 55 y/o - because they want to watch me
build them - yes, right on the spot in their living room in Tucson.
I restrained meself until we got back to SoCal and I had my wrist
strap on.
Happy New Year to All.
73,
matt W6NIA
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