High Isolation Circuit - RF Cafe Forums
Post subject: High isolation circuit Posted: Mon Feb
16, 2009 5:47 am
Joined: Mon Jul 18,
2005 12:55 pm
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
Thanks and regards
Post subject: Re: High isolation circuitPosted: Mon Feb 16,
2009 10:52 am
Joined: Mon Feb 09, 2009 12:26
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.
Post subject: Re:
High isolation circuitPosted: Mon Feb 16, 2009 11:18 am
Joined: Thu Sep 25, 2003 1:19 am
What is your frequency range?
Why not use 2 switches in series.
Check out Peregrine Semi.
Post subject: Re: High
isolation circuitPosted: Mon Feb 16, 2009 12:11 pm
Joined: Mon Jul 18, 2005 12:55 pm
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!
Post subject: Re: High isolation circuitPosted: Mon
Feb 16, 2009 12:35 pm
Joined: Fri Feb
17, 2006 12:07 pm
Location: London UK
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
in and blowing out.
Re: High isolation circuitPosted: Sat May 02, 2009 9:27 am
Joined: Wed Apr 08, 2009 11:07 am
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.
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.
Maguffin Microwave Consulting