In Power Dividers

The reactive power splitter is like Rodney Dangerfield: it gets no respect. Often people will resort to the primitive, high loss resistive power divider simply because a Wilkinson is specified over a limited band, not down to DC. Don’t get me wrong, resistive power dividers have their place. They are much cheaper, anyone can make them, and they can cover very wide bandwidths when made properly.

The truth, however, is that a Wilkinson will work for one application very well outside of the specified bandwidth. The application it will work for is splitting a signal into two well matched loads. While it is true that you can’t have a reciprocal, lossless, and matched three port device in general, you can have such a device if the output ports satisfy one condition: they must be common mode or differential mode. That means that the signals need to be identical to each other (common mode) or opposite of each other (differential mode).

This happens all the time when someone is trying to split an incoming signal into multiple well matched loads. This is why our baluns work so well for so many people. While they don’t have any isolation, and the return loss from a single ended signal put into one output port is terrible (3 dB intrinsically), they are usually used by people to input data to a chip for testing, or combining the differential outputs of a chip. In this case the return loss looks good all around. In contrast, if they were used to combine identical outputs from a chip, then it would be pure reflection. A balun looking into an in phase power divider just looks like an open circuit.

This brings me to the Wilkinson. Outside of the specified band, the Wilkinson will have no isolation and work just like a reactive power splitter. It will have 3 dB nominal splitting loss, but no additional loss (unlike a resistive power divider). If you are using it to split an incoming sine wave, it will work beautifully, as long as the frequency isn’t very, very low (below tens of MHz). Since the group delay and insertion loss are flat, it can also be used to divide data.

Here is the eye diagram from our PD-0020, a resistive power divider:



 This is from a 10 Gigabit per second, 2^31 length psuedo-random bit sequence (PRBS) pattern, so there is significant low frequency content. The input eye is saved in the background. As you can see, the output is very clean, but with significant attenuation (1/2 the voltage). This is typical of a resistive power divider.

Next up is the output from a PD-0140. This has a 40 GHz high frequency cutoff, well above what we need to pass 10 Gb/s data, but the 1 GHz low frequency cutoff is high enough that some data will fall beneath this frequency. Here is the output:


The eye looks pretty open. There is some small overshoot associated with some group delay variation, but nothing too bad. The inside of the eye is wide open. If the low frequency content was compromised, we would see baseline wander. Since the eyebrows are just as narrow as with the PD-0020 case, we conclude that there is no significant low frequency content degradation.

Fair enough, but maybe the data slipped under the 1 GHz limit. Maybe the part has a conservative spec. Next we try the PD-0218, a Wilkinson power divider with a 2 GHz low end cutoff, well above a good amount of frequency content in a 10 Gb/s signal:

Once again, narrow eyebrows, no low frequency content distortion. Once again, there is some wiggle in the eyebrows due to group delay flatness. It is true that the group delay will not be quite as flat in a Wilkinson as a resistive power divider. This is an inevitable result of the fact that a resistive PD is just shorter than a Wilkinson, and the impedance transformation is performed resisitively instead of reactively. The tradeoff is that the voltage out from a Wilkinson is .707 times the input, vs. .5 times the input for a resisitive.

Is there some point at which the low frequency content will eventually catch up to us? Yes there is. At some point the Wilkinson’s reactive impedance match will no longer work, and the input sees a 100 ohm load that it just reflects from, causing significant problems. This frequency is very low compared to the operating frequency of the power divider, though. To illustrate, here is 100 Mb/s data passed through the same PD-0218:

At this point you can see some degradation in the eyebrows due to the Wilkinson power divider. So if you are trying to push data from lower than 100 Mb/s through the same system as 40 Gb/s data, then you’ll have to use the resistive.



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