DC Blocking capacitor's effects

Hi all,

I am designing a multiplier (biased one).

The input has a dc blocking capacitor as shown in Diagram A. I am working frequency 5.3 GHz. As per theory (that is Xc should be small for RF frequency, or Capcitor high) I have selected a capacitor of 100pF (Xc = 0.3 Ohm approximately at 5.3GHz)

Now if I do not have the capacitor at input the current across the diode is 0.11 mA and voltage at its input 215 mV.

When now I attach the capacitor current and voltage change to 0.43 mA and 255mV respectively. This also decreases my conversion gain.

My question is why is that capacitor affecting my results, should it? or is it that the capcitor has some resistance which forms network with the resistance R and changes current for the applied voltage?(as shown in diagram B)


Thanks for your replies in advance...

Could it be that the DC resistance of the RF input generator, is placing a 50 ohm resistor across the diode in your first test. Putting the cap in causes the diode to rectify the input RF, this causes a voltage to build up across the cap, altering the bias point. Try reducing the RF drive and see if the current/voltage changes.
Frank

I agree, the capacitor allows dc current only to flow thru the inductor/resistor. With the capacitor gone, the DC can also flow out the input port--changing the bias point.

In general, you want a lowpass filter structure at the Fin to the diode, though. Unless you have more circuitry between the input and the diode than shown...you are losing a lot of output power out the input port.