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Is there something wrong with my math?
I purchased and built an Elecraft XG-2 tri-band signal generator to
go with my K2 and my TS-850SAT. The manual for the XG-2 gives (roughly) this procedure for MDS testing: 1) Turn off your AGC 2) Select CW mode with a narrow filter, if you have one. 3) Crank the AF/RF gain up to full 4) Setup a VRMS DVM hooked to your ext. speaker jack Measure the background noise on the DVM then hook up the XG2, set to the proper band and set to 1 microvolt (-107dbm) and re measure the voltage on the DVM. The first reading is N and the 2nd reading is S+N. Do the following math: Divide Signal (Vrms) "XG2 on" by rec noise floor (XG2 "off") = R (Log(R))(20)+107=MDS With this procedure on 40m my K2 scored -136.3dbm but my TS-850SAT scored -137.2dbm. Puzzled by the results, I Googled and found those were typical numbers, although the Kenwood seemed a bit on the hot side. OK - Here's where it get's interesting - today I bought a Telonic Berkeley rotary step attenuator, model 8107s. Now this device can add 0-59db in 1 db steps. The manufacturer claims 0.2db accuracy and 0.4db insertion loss up to 2ghz (worst case). To hook up to the XG-2 I needed a BNC male/male adapter and a BNC female/SMA male adapter. To hook up to the radios I used a SMA male/SO-239 adapter, 7' of RG-8x coax feedine into a MFJ 815c swr bridge then through 2 coax jumpers (2' ea) with a SO-239 barrel connecting them. Yes, I know - not an ideal low loss connection for MDS testing. I'll try that later.... Anyways, I figure I've got 3db of losses (40m) through all those connections/meter/coaxes. The XG-2 is supposed to be VERY accurate as far as output levels go. Never the less I'm fudging the numbers based on W3AB's recent tests of a XG-2 on an HP 8562A spectrum analyzer which showed a real world output of -106dbm +/- 0.5db. Let's add it all up: -106dbm - 0.4db - 3db = -110dbm. If that is accurate, I figured I'd be able to crank in 26db of attenuation before I reached -136dbm and lost the signals (I'm not measuring now - I'm listening with headphones for the tone). I went to 26db and the tone was EASY to hear! I eventually found that I had to crank in **46 db** of attenuation before I lost the signal! That's -156dbm for MDS, guys..... So what did I do wrong? 73's de Ken KG0WX - Kadiddlehopper #11808, Flying Pigs #-1055, Grid EM17io, TS-850SAT, Elecraft XG2, 4SQRP Tenna Dipper, Heath GD-1B |
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