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High isolation circuit - RF Cafe Forums

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Corvus
Post subject: High isolation circuit Posted: Mon Feb 16, 2009 5:47 am

Captain


Joined: Mon Jul 18, 2005 12:55 pm
Posts: 8
Hi everybody!

I am designing a RF high isolation switch of 6 samples. I am thinking into use HMC252QS24 from HITTITE. The problem is that the isolation I obtain between signals is around -50dBm but I need more. I am thinking into interconnect a monolitic amplifier into each signal and switch ON/OFF the amplifier in order to increase the isolation...

Any more ideas?Better low cost solution?Another interesting solution¿

Thanks and regards


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RF Head
Post subject: Re: High isolation circuitPosted: Mon Feb 16, 2009 10:52 am

Captain

Joined: Mon Feb 09, 2009 12:26 pm
Posts: 11
Looking at the data sheet the best you can do from the 'common' line to any other is 38 dB (DC to 1GHz) and this drops to 26 dB at 3 GHz. The data sheet does not show the isolation between channels that are both 'off' If this is what you are referring to then you can measure this and if this is the 50 dB figure you mention then thats not bad. Note that much of the isolation will be down to your board layout. If this is done well you may be able to improve on your 50 dB figure. A simple way to work out what is down to the layout is to remove the IC, terminate the unused pads with 0402 size 51 ohm resistors and measure the isolation and see what you get. This removes the 'leakage' that goes on inside the IC from the problem and tells you what the layout is like. I suggest you then ask the IC manufacturer what they think is possible.

Ultimately, if you want a lot more isolation I suggest you use separate IC's and get some physical separation between your channels. You will then be able to go a log higher in isolation.

Ensure you use a microstrip layout or better still a coplanar.

Enjoy


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yendori
Post subject: Re: High isolation circuitPosted: Mon Feb 16, 2009 11:18 am

General


Joined: Thu Sep 25, 2003 1:19 am
Posts: 50
Location: texarcana
Hi,

What is your frequency range?
Why not use 2 switches in series.

Check out Peregrine Semi.

Rod


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Corvus
Post subject: Re: High isolation circuitPosted: Mon Feb 16, 2009 12:11 pm

Captain


Joined: Mon Jul 18, 2005 12:55 pm
Posts: 8
I found a good solution by using x6 units of MINICIRCUITS with code M3SWA-2-50DR+ for each sample and then connect the outputs of each switch to the input of switch of HITTITE HMC252QS24. By simulating in ADS I obtain very good isolation between inputs around 90dB. The problem is that solution is expensive around 25 dollars only in IC's. I am using six signals of 10MHz, 36MHz, 1950MHZ, 1056MHz 1052MHz 1550MHz.

This first solution sounds good but on the other hand is expensive and fills a lot of space in the PCB. Any cheap idea to do the same??

Thanks and best regards to everyone!


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nubbage
Post subject: Re: High isolation circuitPosted: Mon Feb 16, 2009 12:35 pm

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
I had a similar problem with a 2 pole circuit, but my switching speed was low, about 250 microsecs. So I was able to use reed relay elements in a quasi-50 ohm stripline electro-mech swicth at 1300MHz. The isolation was around 85dB and power handling a couple of watts.

_________________
At bottom, life is all about
Sucking in and blowing out.


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biff44
Post subject: Re: High isolation circuitPosted: Sat May 02, 2009 9:27 am

Colonel

Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
You are having troubles because the laws of physics are against you. Switches have isolation because either:
1: they use a device that provides a dead short circuit shunt to ground.
2) they use a device that provides a series open circuit

In the first case, the "dead short circuit" involves connecting the bottom of the chip to the ground plane by via hole(s). If the via hole is a real-world part, it has inductance and resistance. In other words, it is not a "dead short circuit", and as such has limited ability to totally reflect a signal.

In the second case, the "open circuit" is not pure either. There is often some capacitance in the diode or fet that lets some signal blead thru, instead of being totally reflected. Also, there can be crosstalk between two arms of the switch, especially where the arms have to converge onto a small space to connect to the semiconductor itself.

So, you have practical limitations as to how much isolation you can get out of one semiconductor. To solve this, you want to cascade additional devices, physically separated from one another. To do so successfully, you need to know something about the internals of the device. For a FET series shunt sp2t switch, you could add an additional shunt short circuit element (like a pin diode) quarterwave away from each of the inputs. Alternatively, you could add an additional shunt element at any distance away from the IC, but only IF the IC has a nonreflective port when in isolation mode. You do not want a resonant length between the IC and additional elements, causing a "bandpass" response instead of the desired isolation.

_________________
Rich
Maguffin Microwave Consulting
www.MaguffinMicrowave.com







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