I am new to circuit and has a very basic question about how to set biasing circuit of varactor. I need to use varactor to tune the resonance behavior of Split ring resonator (SRR) structure when illuminated by plane wave. (If you are not familiar with SRR, you can think that SRR illuminated by plane wave is just a RLC circuit driven by AC voltage), See Fig.1.enter image description here

I tried to find some guidance online. Some said that I need to use RF choke (an inductor) to isolate DC voltage from AC circuit, see Ref.1. Somme said either inductor or resistor could be used, see Ref.2. Some uses both inductor and capacitor, see Ref.3. (Fig7, Page12). This make me confused, so, what should I do?

My own understanding is that, RF choke is to make the DC biasing circuit 'unseen' by AC signal, so, at working frequency of the AC circuit (f0), the impedance of inductor should be much larger than that of the varactor, so that the current flowing in the DC branch is negligible comparing to the AC branch,See Fig.2. Am I correct? enter image description here

If there is anything wrong, please correct me. Besides, external resources could be helpful, too. I looked up several textbooks on circuit, but no detailed information about how to bias varactor was found.

Thank you very much for your kind help! (By Jingwei)




3. RF Diode design guide


Sure you can do a RF choke .Remember that the varicap diode is running on reverse bias voltage to change the effective Capacitance.This means very low DC current.So you could use a resister instead of the harder to find choke .The resister value should be high enough to not load the tuned circuit .I have used 100Kohm .Your application is different .If in doubt start with 100K. Practical RF chokes have a finite impedance .Getting above 10K is not easy so the resister should work better.

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  • \$\begingroup\$ In discrete VCOs, humans often use 47,000 ohms ...... just because. The noise floor will be set by the capacitance of the varactor, in any case. \$\endgroup\$ – analogsystemsrf Jan 22 '19 at 2:29
  • \$\begingroup\$ Hi Autistic, thank you very much for your reply. So is my circuit diagram correct? If so, is it just to replace RF choke with resistor according to what you say? \$\endgroup\$ – Jingwei Xu Jan 22 '19 at 14:33
  • \$\begingroup\$ Your diagram is set up for a Negative control voltage .If this is not convenient then swap the varicap polarity .Consider a cap in paralell with the varicap if you do not need a big tuning range .Back to back varicaps make fewer harmonics and are described in old ARRL handbooks. \$\endgroup\$ – Autistic Jan 23 '19 at 9:27

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