Thread: Franklin VFO ?
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Old 22nd May 2019, 10:29 pm   #58
G0HZU_JMR
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Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Franklin VFO ?

Yes, at the time I bought a batch three of them because the prices were so low. I sold on two of them and kept the best one. It only had 120 hours' use on the internal usage timer and looked new even though it was quite old.

At work I managed to get hold of one of the E5052A analysers to do some testing of the Franklin oscillator and the MMIC oscillator. Sadly, this E5052A doesn't function on its baseband input port and I think this is because it is a discontinued/unsupported option on the E5052 'A' version. So on the first day I could only test the 10.7MHz MMIC oscillator as this E5052A only works down to 10MHz.

The most disappointing thing was the awful amount of RFI in our work labs. My work area is surrounded by wall wart chargers and power supplies and various power and networking systems. Plus there is a lot of high power equipment being worked on. So there is lots of spurious pickup across 10Hz through 100kHz on the E5052A plots. Sorry about this!

I hope it's OK to add these plots as some people may find them interesting from a noise theory point of view. The MMIC works best (in terms of low phase noise) with 2dB attenuation in the feedback and this means the gain margin is about 4dB. So it is fairly compressed when oscillating and it produces about +12dBm at the PCB output port. The loaded Q is 41, the system noise figure is about 9dB and this accounts for resonator loss and a slightly degraded noise figure for the MMIC as it is in compression. So this 9dB figure is a fudge factor and it is quite normal for this part of leeson's equation to be 'fudged to fit'. I think the fudge amounts to about 1dB of adjustment so nothing major in this case. The MMIC has a typical NF of 6dB on the datasheet so not a great example of a low noise MMIC but it is fine for this type of demo.

I set the flicker corner at 2kHz but this number varies with device current etc. In the plot the slope of the phase noise is still 20dB/decade between 1kHz and 10kHz so the flicker corner might be a bit lower than I thought with this MMIC at this current setting. The oscillator output is about 12dBm so there is 10dBm at the resonator after the feedback attenuation. Putting these numbers into Leeson's equation produces the graph below and I've also included the plot from the E5052A. The agreement is very good I think although the awful noise pickup in the lab spoils things a bit.

I chose a MMIC for the Leeson demonstration because it is a well defined amplifier in terms of gain and noise figure and port impedance. It also runs in class A so the results really should be very close to those predicted by Leeson. I've added blue dots on the E5052A plot and these correspond to the Leeson graph on the right. I think this demonstration deserves a better test setup than this and I have to apologise for all the noise pickup in the lab. I'm not sure how to improve on this, we used to have a big metal/screened 'quiet' room for this stuff but that got sold off years ago. There are a few R&S TS7124 shielded boxes available at work but I don't think these will help down at these frequencies. They are very heavy so I'm not to keen to try one. I might try and find a mu metal enclosure but I have no idea how big/thick it would need to be. The alternative is to move the E5052A to a quieter location.
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Regards, Jeremy G0HZU
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