[Elecraft] XV-144 with high NF?

[Bob DeVarney W1ICW]

I guess I'm not going to worry too much about it.

I just did a MDS measurement with my HP Service Monitor. MDS was around 
-157 dBm in 400 HZ bandwidth. Preamp on, AGC off. My HP 8921 only goes 
down to -137 dBm do I added a 20 dB pad in line to get down to -157 dBm. 
Still barely audible in the headphones at that level.
I set the rf preamp bias to 40 mA=188 mV at TP2, and tweaked C1 for best 
S/N at -157 dBm and I think I'll call it good.

Thanks for the sanity check, Don.

I've been trying to find a culprit for missing a couple juicy EME 
dxpeditions lately, but I guess I can't blame it on the receiver just yet.

73,
Bob W1ICW

On 12/5/2010 7:50 PM, Don Wilhelm wrote:
>  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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