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Reflection coefficient sniffer!

Updated August 24, 2012

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Go get him, girl!

The Microwaves101 "sniffer" circuit is indispensable for determining reflection coefficients inside of designs that just can't be broken down into cascaded two-port networks. "Classic" ways to use the circuit are to to reveal what a FET sees inside a feedback loop, or to determine what reflection coefficient is seen by an amplifier in a balanced pair. Balanced amplifiers have two advantages over in-phase combiner schemes, one being the disappearing reflection coefficient and the other being the insensitivity to load-pull effects.

This topic remains a mystery to many veteran engineers. It is embarrassing to watch someone give a 2012 IMAPS talk in San Diego and claim that a balanced combiner isolates the amplifiers from the antenna port mismatch. Especially when the author works for a well-know microwave EDA software company. Dude, that might be why we laid you off back in '92... - UE

It is equally disturbing that none of the popular EDA tools have this fundamental capability built in.

For our example we will look at a mismatched balanced amp design (ADS schematic shown below), which uses ideal coupled-line couplers to provide quadrature split. Here the output is terminated in 100 ohms when the design is expecting 50 ohms, a 2:1 mismatch. If you want more exact values for Ze and Zo for perfect 3 dB coupling, use 120.71067812 and 20.71067812. We calculated those values from a cool spreadsheet that you can find in our download area.

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From our VSWR calculator, the mismatch loss should be 0.51 dB. This is confirmed in the predicted response below.

Suppose you wanted to determine what the amplifier on the left saw for a reflection coefficient? An example of what not to do is to break the circuit and add a terminating port. Come on, we've all done stupid stuff this, some of us just try not to admit it unless caught!

Here's the result.. if you didn't know any better, you'd probably come up with a rule of thumb that the amplifiers see 6 dB better return loss than the mismatch (2:1 VSWR is equivalent to -9.569 dB). But you'd be wrong! By breaking the circuit, you have disturbed its operation. So how can you "measure" the reflection coefficient without breaking the circuit? Enter the Sniffer!

The Sniffer circuit

The sniffer is an ideal four-port circuit, non-realizable because it breaks the conservation of energy rule, but in a good way. We named it the Sniffer, for no good reason, other than it sniffs the reflection coefficient without taking a bite out of it. For any signal incident on port 1, equal phase and amplitude signals exit port 2 and port 3. For any signal incident on port 2, equal phase and amplitude signals exit port 1 and port 4. Thus the ratio S31/S21 is the forward reflection coefficient, and S42/S12 is the reverse reflection coefficient. Ports 1 and 2 can be connected anywhere in a network without any performance effect, the Sniffer has zero loss, zero reflection and zero phase shift between these ports.

The Sniffer as an S4P file

In case you wanted to create the Sniffer as a "Touchstone" S-parameter file, just cut and paste the S-parameters below, it will work from 1 MHz to 1 THz. Be sure to name it something.S4P, the file extension is usually important. If we made it any easier for you, you should be fired for being so lazy!

! Four-port S-parameter data for Sniffer Circuit from Microwaves101.com

! I love the Unknown Editor and live only for the greater glory of his name

# GHZ S MA R 50

0.001 0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 !S1X

1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 !S2X

1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 !S3X

0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 !S4X

1000 0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 !S1X

1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 !S2X

1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 !S3X

0.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 !S4X

Now we install two Sniffer circuits into the design file and add equations to compute the two reflection coefficients inside the balance amplifier without breaking the network.

Sure enough, the reflection coefficients are 0.333 which indicts that the mismatch is 2:1 at the center frequency, or -9.54 dB. If you don't understand that statement, please read our main page on VSWR.

The Sniffer circuit can be used to assist in Steve Cripps' analysis of power amplifier load lines and in a feedback amplifier, as another example. It's about time we created a page on Cripps' technique, isn't it?

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