Variable capacitance component/circuit for significant AC voltages - Page 1
Forum section: electronics / rf-microwave

author: crosswick
author_rank: Newbie
timestamp: October 02, 2020, 10:08:44 pm
content: Hi all,I'm looking into recreating/miniaturizing (SMT) a system that a friend designed using a mechanical variable capacitor, used to tune an LC circuit. In my recreation of this, the AC sine voltage is about 20 volts peak-to-peak; if I understand correctly, this prevents me from utilizing something like an Infineon BB640 as a varicap, because in those typical circuits, the AC signal through it must be in the millivolts range.So now I'm looking at theMuratavoltage-controlled variable capacitors, specifically the LXRW19V201-058, through which 30Vpp can be run. Unfortunately though, this part is not (yet) in the JLCPCB SMT catalog, which I was aiming to utilize.Can anyone here perhaps suggest an alternative approach that can be built with JLC components? I would also like to have a larger capacitance range than the 100pF to 200pF that the Murata '058' offers. Thanks in advance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2020, 06:16:14 am
content: What ESR / loss tangent? What frequency?Power diodes often have steep C(V) curves, and drop off at higher voltages. Maybe one would work. You'll still have to deal with IMD or parametric wonkiness due to the large swing. Also I'm guessing the swing varies with C.An underappreciated variable element is the VDMOS transistor. Basically any power transistor will do. Substrate depletion depends almost exclusively on drain voltage: this makes Ciss vary, so long as you don't drive the gate into conduction (Vgs > Vgs(th)), which would short out your control bias. With a typical 20, sometimes 30V limit, this gets pretty close to your 20Vpp requirement, or a transformer can be used to extend it a bit further. Downside: typically Cgs is constant while Cgd varies over a huge range, but Cgd is only comparable to Cgs so the total variable range is maybe 2:1. I will note you didn't give a tuning range so maybe this is good enough.An old school method is simply switching regular capacitors. Multisync CRTs simply used capacitors in series with MOSFET drains, to provide an adjustable capacitance (S-correction). When off, the MOSFET body diode ensures the capacitor charges to the peak signal voltage, and the MOSFET remains reverse-biased most of the time. When on, the capacitor is held to ground of course. Downside: turning on and off injects a slug of charge. Can be reduced by using analog switches instead; integrated cap DACs do this. (It's digital (a power DAC of sorts), so you need enough values to cover the resolution required, or enough values plus a continuously variable element to fill in the gaps.)And also, the non-solution: you can't simply cook up the function with an ADC, DAC and power amplifier, for example?Tim

author: crosswick
author_rank: Newbie
timestamp: October 03, 2020, 11:14:33 am
content: Thanks for that comprehensive reply Tim - I'll probably have to do some more research into the things you mentioned first in order to answer all the questions you raised; what I can say is that the intended frequency range is between 200 kHz and 1 MHz, with a primary focus on around 400 kHz. The inductor I've been testing with is around 3 mH with an impedance of 190 Ohms, but that can change - it's a flat PCB Tesla coil that I found a design tool for called Spiki.The general idea is to get the LC circuit in resonance to get a strong emission out of the inductor; I'm intruiged by the idea that this could also be achieved using a power amp stage of sorts. Perhaps this would be less desirable in terms of SNR though, I'm not sure.