Some better news, actual a lot better news, after reviewing my test setup, I discovered that my SDR was overloaded, and the products at 20 kHz either side on SSB were IMD in the SDR, they vanished by using less gain in the SDR. The other spikes which contain no modulation are about 76 dB down on the wanted signal, an awful lot better than I had measured earlier. They are at about 12 kHz spacing and must be some DC - DC converter noise I guess.
On transmit on CW with no key down there are no sign of those spikes. They only appear when the key is down or in SSB irrespective of mic audio. So things are looking perfectly OK as regards purity at least for a low power rig. I still wouldn't put this into an amplifier however. The heat sinking issue with the PA is another matter, fortunately I don't use FM or AM, and FSK may not work anyway as the stability may not be sufficient, though it looks not too bad. Let me be the first to apologise for my hasty review based on hurried and unscientific testing on my part. I clearly was in a panic having looked forward to getting this transverter and then sing what appeared to be major problems with its purity. The acid test will be on air testing with a close neighbour to see if they notice any problems. I don't have my yagi installed yet, so that will have to wait. 73 from David GM4JJJ > On 1 Jun 2016, at 19:32, David Anderson via Elecraft > <[email protected]> wrote: > > I have just fitted a 4m internal transverter into my KX3 today. I wanted to > check the signal stability and quality on a load before putting it on the > air, and used my SDR as a spectrum analyser to examine the output. I have > used this SDR in the past for examine other transmitters so know it's > limitations and strengths. > > Here are my observations so far: > > 1) There is on frequency carrier leakage from the receiver. This isn't a big > deal, and is explained in the manual. > > 2) When in transmit on SSB or CW with no keying or audio input, there are a > forest of spurs close to the operating frequency, they are independent of the > power setting. Perhaps 50 dB or less down on PEP. This is concerning. I will > do more accurate tests. > > 3) When modulated there are products either side of the main signal that do > vary with modulation. They are again not very many dB down. This is very > concerning. > > 4) I tried a transmission at 3W for a couple of minutes, and the PA > temperature quickly rose to the cut off point. Yes, I have tightened the > screw on the PA to the side wall. This will be a problem if using the rig on > high duty cycle modes like JT65. The PA of the transverter is a FET bolted to > the side wall of the KX3 and does not directly make use of any third party > heatsink that may be bolted to the top. > > 5) Mysteriously a couple of times I have seen a High SWR warning briefly at > start of transmission. > > On the positive side the frequency stability is acceptable for SSB and CW. It > may even be for JT65 or FSK441. > > I have not found any specification for the spectral purity of the 4m > transverter, so don't know if this is what Elecraft expect or not. It > certainly isn't up to my standards for a commercial amateur transmitter and I > wouldn't subject my friends to such a signal on the air. > > The only thing in the installation manual that was unclear is the exact > routing of the LO cable from the RF board to the transverter by the heatsink > and the filter board and battery holder. I suppose there is a small > possibility that the routing of that cable may affect the levels of Spurs on > the signal. I may try some small adjustments and see. > > I was going to build a medium (30 W) or high power (150 W) amplifier for the > transverter, but clearly I would not like to amplify a signal as impure as > this. My plans for 4m have now changed to an IC7300, and I may keep the KX3 > 4m transverter as a backup or for portable work with QRP. > > If anybody else has tested their 4m or 2m internal transverter for spectral > quality I would be interested in their comments. > > 73 from David GM4JJJ > ______________________________________________________________ > 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 > Message delivered to [email protected] ______________________________________________________________ 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 Message delivered to [email protected]
