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There are two primary means of measuring group delay. The most widely used involves frequency domain method converting small-signal S-parameters to group delay. The time-domain method directly measures the time that a pulse takes to move through a network.
Group delay frequency domain measurements
Pay attention, in the following discussion you might learn how to use one of our free downloads to calculate group delay from manufacturers' S-parameters...
At microwave frequencies, we usually measure stuff in the frequency domain. If you have vector S-parameter data over frequency on your DUT (device under test), you already have the means to evaluate group delay as a function of frequency. Quite often your network analyzer will support direct read-out of group delay measurements. But below we will describe how to make the conversion from S-parameters to group delay, you will find this useful when comparing devices from manufacturers data sheets which almost always include S-parameters but seldom include group delay data.
Check out the following simple equation. Group delay is the negative-slope of the transmission phase angle with respect to frequency.
Remember that units must be consistent, if radians/second are used for frequency, you must use radians for phase angles, then the calculated result will be in seconds. We recommend that you use degrees and degrees/second in the formulas, because most data is handled in degrees, so that's one less thing you need to convert. Why bother converting degrees to radians?
To get frequency in degrees per second, multiply Hertz(cycles/second) by 360 degrees/cycle. One other trick to remember: if you start with frequency in GHz, you will end up with group delay in nanoseconds, without having to mess around with moving the decimal place around.
You don't need to do the calculation yourself ever again, because Microwaves101 offers a free download of an Excel file that has equation built in, and plots the data for you! It's called S-Parameter Utilities 101, check it out in our download area!
The plots below are calculated group delay for a TriQuint amplifier based on S-parameter data that we took from their web site while they were sleeping. The plots were all generated using our free download.
One interesting plot is the S21 phase angle, after it is "unwound." The slope of this plot is the group delay. You can generally see were the group delay will not be flat, by any regions of non-linearity on the plot. From the plot you can expect funny things to happen to the group delay of this circuit starting just below 8 GHz...
We included an S-parameter plot in the download. Might as well plot it, the data is there!
Finally we come to the group delay plot. It does suck at eight GHz by golly!
Group delay time domain measurements
Coming soon! Maybe!
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