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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 30th Jan 2022, 2:06 pm   #41
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 2,787
Default Re: s2p file from VNA

I had some free time this weekend so I had a go at measuring the MMBTH10 again in common base on my VNA. I got similar results again but one thing to bear in mind is that a typical 56nH inductor won't look like 56nH at 800MHz due to its distributed capacitance. Unless it is wound using a very small diameter it will look like much more than 56nH at 800MHz. This will make it oscillate at a lower frequency than the simulation predicts.

I also had a go at making a 2.3GHz oscillator using the MMBTH10 in common base and you might find the images below to be interesting.

The simulation uses my VNA model of the MMBTH10 when at 10Vce and 5mA Ic. You can see that there is a resonance at about 2.3GHz with a series -4 ohm resistance and the equivalent circuit is drawn below this using -4R in series with 1pF in series with 5nH.

The next picture shows the oscillator as built on bare copper. You can see that the base pin of the MMBTH10 is grounded directly to the copper PCB and the 5nH inductor is a short piece of wire. This circuit does oscillate at 2.3GHz as predicted by the simulation. At first glance this doesn't look like an oscillator but the feedback path is within the transistor itself. There is also about 0.25pF stray capacitance in the large 3.3k resistor and this affects the negative resistance as well.

By increasing the length of the wire the 5nH can be increased and the oscillation frequency will go down.
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Regards, Jeremy G0HZU
G0HZU_JMR is offline   Reply With Quote

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