Posted by
Don Wilhelm-4 on
Dec 06, 2010; 12:50am
URL: http://elecraft.85.s1.nabble.com/XV-144-with-high-NF-tp5806265p5806389.html
Bob,
I do not have calibrated Noise Figure measurement tools, so I do a quick
assessment of the XV transverters using the calibrated attenuator output
of my HP8640B signal generator, which will go down to -140 dBm. I view
the K2 output on Spectrogram and use a 400 Hz bandwidth. With a
properly functioning XV144, I normally can see the signal at 10 dB or
more above the receiver noise floor.
MDS = -174 +10(log Bandwidth) + NF + SNR
If I assume the SNR is zero (yes that will give the worst case NF)
The 10(log Bandwidth) value is 26 dB for a 400 Hz bandwidth
The implied MDS of a signal seen 10 dB above the noise floor is -147 dBm
That measurement further implies a NF of less than 1 dB, which is within
specification.
I would suggest you do a similar MDS measurement as a quick sanity check
on your NF measurements.
If it is as bad as you indicate, you may have a bad preamp in that XV144.
Your RF Amplifier Bias adjustment is not unusual - 175 mV is the
minimum, but it usually maxes out in the 210 to 230 mV range - I have
not yet seen one that would go as high as 275 mV.
73,
Don W3FPR
On 12/5/2010 6:36 PM, Bob DeVarney W1ICW wrote:
> I an wondering if anyone has encountered this before.
> I have a recently built ( this spring ) XV-144 external transverter
> teamed up with my elderly K2. The K2 was updated to all the most recent
> mods, builders alerts, and firmware upgrades as of mid April.
> The transverter has a high noise figure in the neighborhood of 10 dB
> when measured with a known ENR noise source and either the VK1OD of
> G8KBB noise figure software.
> I have also measured the transverter using my Kenwood TS-2000 as an IF
> rig and have similar results.
>
> Before I yank the covers off and get out the magnifying glass, can
> anyone point me in a good direction?
>
> I set C1 for a noise peak at 145 MHz per the manual, but the RF amp bias
> would not go above 220 mV at TP 2 so I suspect something in that area.
>
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