[G0HZU_JMR]

This may be of interest... many years ago I designed a basic 'trainer' 10.7MHz oscillator using a 50Ω MMIC gain stage and a (low) tapped resonator. It had a transformer splitter in the circuit that created an auxiliary 50Ω output port. This allowed the signal to be fed to a spectrum analyser. There was also a pair of 50Ω coaxial connection points that allowed the feedback path to be broken and the loop analysed on a network analyser. So direct measurements of gain margin, loaded Q, phase response and noise figure could be made using regular 50Ω lab gear . The option of fitting an attenuator and/or a delay line was possible at this point as well.

This allowed Leeson's equation to be demonstrated. I'm pretty sure I've posted about this old oscillator before on here and probably on Eevblog. Somewhere, I have some old phase noise plots taken of it with the company E5052A analyser. I dug it out yesterday and rebuilt it as I had pinched some parts from it a while ago. The plots below are for the group delay of the model of the resonator/oscillator and also of the real circuit. Both are measured in open loop at the 50R breakout point.

They agree very well and the loaded Q is about 41 with this resonator. By playing with the attenuator it is possible to control the loop gain. A loop delay can be added here as well (using coax cable) and the effect of spoiling the phase response can be explored. Because the MMIC is a well behaved 50R gain block with well controlled gain it is also possible to explore the region between regenerative gain and the onset of oscillation with this setup. I think I demonstrated it once as a regenerative detector but I had to tweak it down to the 40m band for this. It was possible to show the improvement in gain and selectivity as the loop gain was adjusted in tiny amounts.

It normally runs at 10.7MHz and this is twice the frequency of the Franklin oscillator. So that normally brings a 6dB penalty in phase noise. But because the MMIC has low flicker noise and because it runs at a higher power level and because I have the phase response optimised it is probably about 6-8dB better than the Franklin oscillator (in it current state) at offsets between 100Hz and 1kHz. It was only really designed to demonstrate Leeson's equation and it achieved this quite well I think. It wasn't designed to have ultra low phase noise or low drift.

This circuit is a bit like the Franklin oscillator in that it has two 180degree phase shift components in the loop. However, in this case the second one is a passive transformer and all the gain happens in the MMIC. I tap into the resonator at low impedance points in the resonator rather than tap into the top of the resonator. This seems to work really well and this suits the 50R MMIC. This setup makes it really easy to analyse and tinker with.