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Switching small capacitive loads

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?

TIA

Reply to
Andrew Holme
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The third circuit looks OK. The bias will reduce Co.

PIN diodes act very differently for short (below carrier lifetime) and long pulses, so aren't general-purpose wideband switches.

An NE3509 has about 6 ohms of on resistance at Vg=0, and about 0.35 pF of capacitance when off, -1 volt or so on the gate.

John

Reply to
John Larkin

Pickering do a low capacitance SIL relay (

Reply to
David L. Jones

Q1 seems to be more practical if you add a discharge circuit at the source when it's goes low. You may already have that in mind? I can't tell if your source signal is actually pulling to common when off. This is keeping in mind, that Q1 remains selected so that discharge can take place.

Q2, will introduce voltage on the source line in the deselected transition. You may want to think about that.

Those are my observations at first glance.

Reply to
Jamie

It may have 1pF to GND!

I have the measurements somewhere, but I'd have to dig them out.

Jeroen Belleman

Reply to
Jeroen Belleman

The "Guard screen only" (103G) version is speced at 0.1pF contact to coil capacitance and

Reply to
David L. Jones

They *do* meet their specs. With the (grounded) guard screen in between, the coil-to-contact capacitance is indeed very small. It's the contact-to-guard capacitances I was thinking of. I dug up my measurements: It's about 2.8pF for each contact.

Anyway, in view of the intended application, the measurement of rise time with varying capacitive loads, it may be useful to keep in mind that the open contacts of the relay with guard screen look like a short piece of open-ended transmission line. It's up to the OP to judge if that's relevant.

Jeroen Belleman

Reply to
Jeroen Belleman

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