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Re: Analog switch methods

How much isolation do you want? =A0At what bandwidth?

-40dB would be good. DC-20MHz. 50 ohms switching would be good, so I can just run everything low-assed-impedance straight through (with the understanding that some signal will be lost across the switch, which can't be a nonlinear response!). Peak amplitude about a volt, so it should handle up to 20mA.

=A0If you need more > than a single SPDT switch can provide, have you thought of using two > layers of switches to defeat blowby?

I have, take for instance these examples;

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I don't think I can find JFETs small enough (2N3819 grade capacitance) and conductive enough (J105 grade conductivity) to do 50 ohms in one pair, so they'll have to do it in two -- a tee switch. And it's two tees for the A/B action. This would have to be followed by a follower (possibly with another JFET for high impedance input, though right into a diff pair probably wouldn't be too bad anyway) to get back down to 50 ohms.

MOSFETs, like the SD214, are pretty close, but I'd have to hunt for them too. 1N914s I have in my parts bin...

Sure, I like discrete switches - I designed a few into some Tek > 'scopes, > both junction devices and SD21x's. =A0But if your bandwidth is as modest > as you say, there's no reason these days to do that unless you want to > learn more about the old method.

Old methods? Ha! Just be glad I'm not using tubes in the output stage! ;-) The low capacitance of a 6DJ8 is tempting, though.....

Tim

Reply to
Tim Williams
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You need to be clearer on exactly what you are trying to do. 50 ohms? Why? What sort of voltage/power range? You cannot expect to get sensible answers if we don't have your context.

Reply to
cassiope

Oh, real specs? Sure...

A/B switch for ALT/CHOP function on (mostly) discrete oscilloscope design (I think I alluded to this earlier). Source impedance 50 ohms (or something else, like open collector or emitter follower (Zo < 10 ohms), if need be). Load impedance whatever, 50 ohms (50 ohms straight through) preferred, but if I need for example a FET follower, then I need a FET follower. Isolation as mentioned. Switch time under 20ns, without much glitching if possible. What else?

Tim

Reply to
Tim Williams

?

I still say try the FST3125, though you will need the awkward +4V/-1V supplies. I have used these switches to handle +/-1V or so signals in

50 Ohm systems up to a few MHz (though they would ba OK for higher frequencies) and found them very satisfactory, especially for the price. If you want to have one with proper analogue specifications, then you can buy something similar (but maybe a bit slower switching time) from Analog Devices, but the price will be higher since it is not marketed or specified as a logic device.
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Chris

Reply to
chrisgj198

Oh, real specs? Sure...

A/B switch for ALT/CHOP function on (mostly) discrete oscilloscope design (I think I alluded to this earlier). Source impedance 50 ohms (or something else, like open collector or emitter follower (Zo < 10 ohms), if need be). Load impedance whatever, 50 ohms (50 ohms straight through) preferred, but if I need for example a FET follower, then I need a FET follower. Isolation as mentioned. Switch time under 20ns, without much glitching if possible. What else?

Tim

I was wondering what you really wanted because channel switching is almost never voltage switching but rather current switching. Think current steering... Have a look at any reasonnably modern Tek scope service manual.

-- Thanks, Fred.

Reply to
Fred_Bartoli

0 ohms?

ect to get

Needed to switch a 50ohm, 50MHz, 2V sine via two LC quadrature networks (DPDT), feeding a NE602 mixer. Hugely optimistically I ignored the datasheet and wired in a '4053 with predictable results. Desperate for some numbers, started putting together a discrete version and 6 hours later had a breadboard stuffed full of bits, massive signal loss and virtually no isolation. 3 hours further on and a second breadboard dissed me with mediocre signal level and 20dB isolation. I know when to call a halt, so the next day bought a cheap Fairchild FSA450 video switch from Farnells. Sorted in one!. I'm a fanatical believer in cheap discretes but occasionally packaged Silicon offers the best option.

Reply to
john

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