On 9/21/2016 8:01 AM, David C. Partridge wrote:
Looks like all I've managed to do is measure the baseline noise of the 
analyser.  Ho-hum looks like I need something a *lot* quieter to do these 

Indeed Dave, yes. I made the same mistake a few years ago when I tried to compare the results of three different DDS clocks on the output of my homebrew transceiver DDS LO using a 3585A spectrum analyser and John's program. It took me a while to figure out why they were identical and then the penny dropped that I was just measuring the 3585A...

The plot of the three thunderbolts I sent before using my -hp- phase noise system is interesting in that the white plot was from the Tbolt I keep on the bench to run all the stuff there, and it uses the switch mode power supply that came with the TAPR ones, built into an Extron distribution amp case. All the spurs that are shown with dotted lines aren't all spurious, most are probably real. The other two are from units with linear supplies and are significantly improved.

Most older phase noise systems use some kind of low noise oscillator for reference, mixing the signal under test down to AF and going from there with low noise amps and then the actual measurement. Wenzel has a very good application note describing a system that can be copied in the home lab.

I attach a plot of my -hp- 3048A system noise floor showing what levels can be achieved with twenty year old technology. The spurs the system shows dotted may not be real. The modern system John Miles designed can no doubt do better and in about one hundredth of the volume, but is a little more expensive :^)


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