Diode impedance depends on the circuit where it is used.
If used as a detector, it depends on input RF power. If used in a mixer, then it depends on the LO input power, and also if the mixer is balanced, etc.
Check textbooks on detectors and mixers for details.
The "impedance" of a diode is the sum of bulk resistance and differential resistance rd = dV/dI which is current dependent according to the exponential diode characteristic.
You can derive both numbers from the datasheet forward characteristic. OA 90 bulk resistance range is 25 - 100 ohm according to datasheet, typically 50 ohm.
So it is roughly already matched or I have to add the Rd to this value as well?
The circuit I consider is this. It uses a aproximately 50R input attenuator to the diodes. The diodes are used as charge pumps, so I think impedance matching would not be too critical on HF anyway.
In this app, Rd is varied so I think I cannot consider constant impedance...
But isnt't that the case even with the capacitor in place?You need a capacitor between the 500 ohms and the two diodes, else the diode connected to earth will short out the RF.
Never mind, you are right, it is a voltage doubler configuration, it needs a capacitor at the input.But isnt't that the case even with the capacitor in place?
During the negative half of the sinewave, the shunt diode conducts rf to the GND.