A colleauge of mine is building an automated test jig for measuring rise times under different capacitive loads ranging from 5 to 70pF. Switching small capacitances with relays is difficult/impossible and I proposed he use electronic switching. Here are the schemes we've considered:
.-- 1V step from 100-ohm source impedance | 10ns rise time (unloaded) | -------o-----------------o----------------o----> To 'scope | | | | --' --- --- --- C1 --- C2 --- C3 --- R3 10k | | | ___ .-----o | o----|___|-> +10V | | | | L1 C| | PIN |/ |/ 1uH C| V Diode .----| Q1 .----| Q2 C| - D1 | |> | |>
| | | | | | .-. | .-. | .-. | R1 | | === R2 | | === R3 | | === 120 | | GND 1k | | GND 1k | | GND '-' '-' '-' | | | | | | 3.3V Level CMOS Logic Control Lines
C1/C2/C3 in 15 - 50pF range Q1/Q2 = 2SC4774 or BFS20W
Our first thought was to use PIN diode switching; but we see problems with the above circuit in simulation when the diode is supposed to be off. Large negative DC bias is required to stop the fast rise time of the signal turning on the diode; and, unless an impractically large inductance is specified, choke L1 passes a sizeable AC current, causing severe waveform distortion.
The bipolar transistor circuit seems much better. There is no waveform distortion except for the expected RC low-pass filtering effect of the selected load capacitance. The output capacitances of the 2SC4774 and BFS20W are both specified as about 1pF at Vcb = 10V; however, it seems to work equally well in simulation with or without DC collector bias.
Any comments or advice?