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high frequency solid-state relay

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zoulzubazz

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Hey guys,

I need to switch a 10MHz signal through an array of 8 air-core coils. the maximum peak to peak amplitude of the signal is +-20V and the current i am looking to switch is a maximum of 400mA. The figure attached shows what i am looking to do. I was looking at solid state relays but am not sure if they cant handle signals of 10MHz through them. The switching on/off time required is a few ms. What switch would you recommend for this application? Thanks.

IMG_20150721_231926.jpg
 

betwixt

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I don't think an 8-way multiplexer exists that can handle that voltage and current, even at low frequency. As the switching speed is low, you could try using miniature relays but I think the approach I would be to use the 10MHz to drive 8 individual power driver stages and do the switching by controlling the DC to each driver.

Brian.
 

FvM

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If a solid state relays is suitable for 10 MHz depends on if you can accept the switch capacitance in your application.

Low voltage solid state relays in the 0.5 to 2A range are availaible on the market, if you don't find a suitable type, you make your own one using a MOSFET pair and a photovoltaic coupler.
 

zoulzubazz

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thanks, the SSR with the lowest out capacitance i could find was about 25pF which means about 45mA of current through the switch in off state, which is not acceptable in this case unfortunately. is there a way to circumvent this problem?
 
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betwixt

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Just a thought, given the DC path to ground, I wonder if PIN diodes might be an option. I suspect the losses may be too great. At only 10MHz it might just be possible to use a normal Shottky diode if it could be forward and reverse biased enough and the signal leakage could be tolerated. Some expermentation would be needed but it could be a cost effective solution.

Brian.
 

Dan Mills

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Pin diode switches are the obvious approach, and at 10MHz there are common power rectifier diodes that will work perfectly well for this.

You need to hold off with a DC reverse bias greater then the RF peak voltage (ideally quite a bit greater, 100 - 200V is typical), and for on state you need to forward bias with a reasonably significant current (but the voltage drop is tiny).

Hell, you can do it with 1N4007s at that frequency and power level.

Here is a TR switch for ham radio done with 4007s, aparently good for a KW. https://sites.google.com/site/k7fjdiodeqskswitchbyw5uxh/home/k7fj-original-schematic

Regards, Dan.
 

crutschow

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thanks, the SSR with the lowest out capacitance i could find was about 25pF which means about 45mA of current through the switch in off state, which is not acceptable in this case unfortunately. is there a way to circumvent this problem?
Is the concern about the current itself or that it will be going through the load?
If the latter, then you can use a additional switch at the output to ground, as FvM suggested, to short the leakage current to ground.
 

zoulzubazz

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hey guys, thanks very much for the replies. FvM's ideal to use additional switch is very good but the system would be bleeding too much current unnecessarily.

Pin diodes seem to be a good option, is it possible to use Mosfet drivers to turn the diode on/off quickly?

Keeping the non-linear distortion to the minimum is desired, the signal being switched is AC coupled into the 8 coils.

Thanks very much.
 

betwixt

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Here is a TR switch for ham radio done with 4007s, aparently good for a KW. https://sites.google.com/site/k7fjdi...inal-schematic

There is your example of using MOSFETS. You don't need to replicate all the circuit 8 times, just follow the basic principles of using the diode in series with the RF but ensuring it doesn't work as a rectifier (or conductor) by driving it hard into continuous conduction to 'switch on' and apply a high reverse voltage to 'switch off'.

I think I would have some concerns about using more than one diode in series, I think I would have wired resistors (1M maybe) across each diode so the reverse voltage was shared equally.

Brian.
 

KlausST

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Hi,

you could use an OPAMP for each channel independently. There are OPAMPS with enable pin. So you select just the channels with logic levels.

Like this one from LT: LT1210 (but this doesn´t meet your voltage and current requirements)

Maybe you find a suitable one...

Klaus
 

dick_freebird

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400mA is not huge for a high power RF CMOS switch. But
these all want DC-blocked, ground-symmetric signals.

What's wrong with a plurality of latching relays, if you
need to keep coil power minimal? Or for that matter, a
plain old reed relay? We used to use Coto relays of
DIP-16 or sub-DIP size on ATE load boards at much
higher frequency and at least the same current, a lot.
 

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