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Frequency range for 1N4148

I am testing the switching function of 1N4148 that I thought would work under VHF or at least HF. I input a 200KHz square wave of 100mv-800mv to a 1N4148, which has a 100k ohm between the output end and the ground. There is also a 50 ohm resistor between the input end and the ground. I am expecting to see also a square wave with 0v-300mv at the output end. But what I got has a DC component in it. Where does this DC come from? Does it mean the diode didn't cut off even when the input voltage is below its threshold? Or What's the frequency range for

1N4148 to work well? Thanks a lot.
Reply to
tester
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Remember that the diode has a junction capacitance (about 4 pf for low bias voltages). Think of the time constant of that capacitance with a

100k resistor. 100k*4pf=0.4us

At any frequencies higher than 1/(2*pi*.4us)= 400kHz, the diode will look mostly like a capacitor, charged to some offset by the average diode current. At 200kHz, the capacitance should be showing a measurable, but not totally dominant effect.

In other words, the frequency capability has a lot to do with the rest of the circuit it takes part in.

Reply to
John Popelish

Diode switches are either AC coupled or use two diodes against each other. I'd suggest two diodes anyway for compensation. As to the frequency range, I'd expect a few MHz.

Rene

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Reply to
Rene Tschaggelar

Such as the capacitance of the probe being used to see the waveform. This can have a significant effect on the displayed waveform in a high impedance circuit.

Cheers!!!!!! Dave M MasonDG44 at comcast dot net (Just subsitute the appropriate characters in the address)

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Reply to
DaveM

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