mmic switch with dc-transmission

[Vau]

hi everybody!

I´m a student in electrotechniques and working on a thesis.

In this I need to switch up to 100 RF signals (up to 200 Mhz). I found
a lot of different mmic switches, which would be applicable because
off small smd packages and limited space on pcb.

My problem is that all found switches need dc-cut capacitors, but I
need dc transmission because of measuring logic levels.

Does anybody know mmic switches which transfer dc or any other
suggestion to solve this problem?

thanks in advance

Vau

 

[John Larkin]

hi everybody!

I´m a student in electrotechniques and working on a thesis.

In this I need to switch up to 100 RF signals (up to 200 Mhz). I found
a lot of different mmic switches, which would be applicable because
off small smd packages and limited space on pcb.

My problem is that all found switches need dc-cut capacitors, but I
need dc transmission because of measuring logic levels.

Does anybody know mmic switches which transfer dc or any other
suggestion to solve this problem?

thanks in advance

Vau

Most of the integrated RF switches do pass DC, because capacitors are
hard to fabricate in-chip. But they tend to work best at weird DC
offsets; you can get around that be figuring out what their inherent
offset is, and shifting your power supplies to translate that to
ground. Of course, your control logic levels have to move, too.

Depending on your isolation requirements, the little Fairchild cmos
logic switch/mux things might work... they're just analog transmission
gates.

The Hittite HMC253 is a nice 8:1 mux, very fast, and the price just
dropped from $22 to $5. It conducts DC in its transmission path but,
interestingly, has series caps on its terminations!

Don't overdrive any of these things or very strange stuff will happen.

A 100:1 tree will be interesting in any technology.

John

 

[Vau]

Most of the integrated RF switches do pass DC, because capacitors are
hard to fabricate in-chip. But they tend to work best at weird DC
offsets; you can get around that be figuring out what their inherent
offset is, and shifting your power supplies to translate that to
ground. Of course, your control logic levels have to move, too.

Depending on your isolation requirements, the little Fairchild cmos
logic switch/mux things might work... they're just analog transmission
gates.

The Hittite HMC253 is a nice 8:1 mux, very fast, and the price just
dropped from $22 to $5. It conducts DC in its transmission path but,
interestingly, has series caps on its terminations!

Don't overdrive any of these things or very strange stuff will happen.

A 100:1 tree will be interesting in any technology.

John

I made different tests with Hittite HMC253. I removed the DC-Cut off
capacitances on their evaluation board. the switch passes DC now, but
the isolation between the channels doesn´t seem to function properly
any more.

I. e. a sinewave on channel 1 distorted a square wave signal on
channel 8 which should be passed.

Any suggestions?

Vau

 

[John Larkin]

I made different tests with Hittite HMC253. I removed the DC-Cut off
capacitances on their evaluation board. the switch passes DC now, but
the isolation between the channels doesn´t seem to function properly
any more.

I. e. a sinewave on channel 1 distorted a square wave signal on
channel 8 which should be passed.

Any suggestions?

Vau

Too much signal swing will turn on the internal fets; centering the
swing vs the DC supply levels will optimize things. If your signals
are high-level, beyond the specs of the Hittite, you'll have to shop
around for a different technology. Some CMOS analog switches are fast,
but I don't know your exact requirements for crosstalk isolation,
swing, attenuation, etc. No simple answers here.

John

 

[Joerg]

Hi Vau,

Check out the SD5400 series of quad switches. I have done several
multiplexers with these that went from DC to well over 100MHz. You do
need to have some resistive path to ground though so that the FETs don't
start to float.

Regards, Joerg