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. >> > > -- Light travels faster than the speed of sound, this is why some people tend to appear bright, until they decide to speak. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
