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How to use MOSFET as a switch to introduce a capacitor into a LC tank in VCO?

Alex Liao

Hi guys,

In my VCO design, if I introduce a fixed capacitance, Cap_fix into the C tank, it works fine and give me the target frequency I want. If I disconnect this path (in parallel with the total C) to disable the introduction of this Cap_fix, it gives me higher frequency and it is reasonable as it follows：
w = 1/sqrt(C*L).

But if I want to implement this on/off feature using a MOSFET it does not work.
It always generates strange frequency. I was observing the target frequency through Cadence DFT function of the output in the ADE panel.

Working as a switch, I treated the D and S ends as the switch's two ends. I biased the MOSEFT in triode (ohmic) region, which means,
give me a small Ron (1/gds) when it is on and a infinite large Ron when it is off. For MOSFET size, I tried several combinations, still not working. Either the harmonic signal's strength is high or sometimes output some unreasonable DFT waveform.

Is it such tricky on just using a triode region MOSFET as a simple on/off switch in RF circuit? Or was I implementing the switch using MOSFET in a wrong way? or any tips on bias or sizing this MOSFET? Shouldn't be the reason of my core design as it works fine by simply connecting/disconnect a regular capacitor into the LC tank.

Any reply is appreciated!
Thanks,
Alex

Parents
  • I have looked at different things that mentioned by your reply. The most important of all is the how the MOSFET works as a switch in large signal case. As it is known that if it is properly biased, the working state will not vary in the small signal analysis like applications in amplifier. But in the VCO, the D and S ends are in the large signal situation as you mentioned in (b). It cannot work steadily in one region. (This knowledge is gained by the point that looking at the AC output of different nets/ends in the circuit by you.) If you can refer to the schematic that would be very helpful to understand. http://www.edaboard.com/attachments/117951d1432864293-vco.png The main parameters as described as follows: My target Frequency is 5GHz. L = 1.33nH. The total C vary from 30pF +/- 600fF depending on the introduction of different paths which has parallel connection with Capacitance. The MOSFETs as switch work at this part and thus the D and S ends are connected one at the Vout_negative and the other is connected with the capacitance (or the capacitor device). After passing through the capacitor the Vout_positive is connected to it. Several path can be introduced in parallel to increase or make a combination of the C_equivalent. The total voltage is supplied by a Vpulse, 0V @ 0ns, 1.2V after 1ns. The total I_bias is 2.2mA.

    I have noticed that the AC current drop on the ideal switch is almost 0, but on the MOSFET device, Vds_AC drops a lot. This makes fewer AC voltage drop on desired capacitance I wanted to introduce on the specific path. The AC voltage between the Vout_N and Vout_P is around 3V to 550mV in the ideal case for the 5G desired target frequency. But in the MOSFET_switch case, if this voltage is applied into the path (one MOSFET (under 1.2 V supply or bias) + one capacitor), the MOSEFT cannot only working in a stable region.

    This probably the biggest problem not appearing in the low frequency domain or small signal application.
    Therefore I would think of on how to use MOSFET as a switch in the large signal case providing not over supplying 1.2V for 90nm technology.
    Or any direction on which knowledge is suitable for desiring such a large signal MOSFET switch which is also Resistance and Capacitance sensitive one?

    Switch capacitor might be a good topic but I do not think the circuit need extra knowledge on how to clock it. Isn't it?
Reply
  • I have looked at different things that mentioned by your reply. The most important of all is the how the MOSFET works as a switch in large signal case. As it is known that if it is properly biased, the working state will not vary in the small signal analysis like applications in amplifier. But in the VCO, the D and S ends are in the large signal situation as you mentioned in (b). It cannot work steadily in one region. (This knowledge is gained by the point that looking at the AC output of different nets/ends in the circuit by you.) If you can refer to the schematic that would be very helpful to understand. http://www.edaboard.com/attachments/117951d1432864293-vco.png The main parameters as described as follows: My target Frequency is 5GHz. L = 1.33nH. The total C vary from 30pF +/- 600fF depending on the introduction of different paths which has parallel connection with Capacitance. The MOSFETs as switch work at this part and thus the D and S ends are connected one at the Vout_negative and the other is connected with the capacitance (or the capacitor device). After passing through the capacitor the Vout_positive is connected to it. Several path can be introduced in parallel to increase or make a combination of the C_equivalent. The total voltage is supplied by a Vpulse, 0V @ 0ns, 1.2V after 1ns. The total I_bias is 2.2mA.

    I have noticed that the AC current drop on the ideal switch is almost 0, but on the MOSFET device, Vds_AC drops a lot. This makes fewer AC voltage drop on desired capacitance I wanted to introduce on the specific path. The AC voltage between the Vout_N and Vout_P is around 3V to 550mV in the ideal case for the 5G desired target frequency. But in the MOSFET_switch case, if this voltage is applied into the path (one MOSFET (under 1.2 V supply or bias) + one capacitor), the MOSEFT cannot only working in a stable region.

    This probably the biggest problem not appearing in the low frequency domain or small signal application.
    Therefore I would think of on how to use MOSFET as a switch in the large signal case providing not over supplying 1.2V for 90nm technology.
    Or any direction on which knowledge is suitable for desiring such a large signal MOSFET switch which is also Resistance and Capacitance sensitive one?

    Switch capacitor might be a good topic but I do not think the circuit need extra knowledge on how to clock it. Isn't it?
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