Tag Archives: BiasTees
Recently a customer asked what the settling time of the DC port on our surface mount bias tees was. Settling time is a tricky spec. When you apply a DC voltage to the bias tee, you want it to show up at your device immediately so you can do whatever it is that you need to do. Conversely, you don’t want any high frequency noise from the DC line contaminating your bias, so we try to make the DC to common port isolation high down to low frequencies.
When you master phase, you become like a God, capable of performing wonders that mere mortals can only dream of. Wonders like making laser beams (using phase engineered quarter-wave reflectors), communicate tremendous information great distances through thin air (all modern communication formats use both amplitude and phase), and create amazing products (balanced amplifiers, balanced mixers, phased array antennas, Mach Zender modulators, the list never ends).
BUT…phase is the hardest thing to understand in microwaves, RF, and photonics. It is hard to measure, hard to visualize, and makes some very confusing homework problems that kept me in the late night coffeeshops of Champaign-Urbana well past my bedtime.
In this post we will make a dent in the universe of phase understanding by clarifying the difference between phase and group delay, and in the process explain why you can’t match phase with variable line lengths. When you buy a phase shifter, it is sometimes what I would call a real phase shifter, and sometimes what I would refer to as a ‘group delay shifter’. The trombone type variable delay lines (we like the ones from sage) are actually variable time delay elements, and not phase shifters.
A group delay (or time) shift is easy to understand: it is how long the pulse (or wave) takes to arrive at your measuring receiver. Differential delay is therefore the difference in how long it takes for two pulses or waves to arrive. In passive components it is just the distance divided by the speed of light (or whatever your wave is) at your frequency in your material.
Phase is much more difficult. It is the integral of group delay over frequency (plus an offset), or differently the group delay is the derivative of the phase vs. frequency. This is why filters can be used as time delays; the edges of the filter have significant phase variation that leads to significant group delay variations over a narrow bandwidth (this is called Kramers-Kronig relation).
A variable length delay line, therefore, can only change the phase by changing the group delay. But by changing the group delay, you are changing the integral (slope) of the phase vs. frequency. This means that the phase change will be different at different frequencies. This is very different than what you get from a quadrature hybrid coupler, or a balun, where the phase shift is constant across frequencies. The difference is shown below. First is a plot of the phase difference between the two outputs of a BAL-0520 Balun (180°), a QH-0226 quad hybrid (90°), a coupler plus two 37.5° Schiffman phase shifters we developed as a custom (165°), a PD-0220 wilkinson power divider (0°), and a PD-0220 with an extra .570″ adapter on one side (variable).
As you can see, the phase is flat across the bandwidth of the device for everything except the PD-0220 with the extra delay line (adapter). This has a rapidly changing phase across frequencies. If we take the derivative of this we should get the group delay, but instead I measured the differential group delay with the PNA-X.
Here you can see that the differential group delay between outputs for each of the devices is 0, except for the power divider with the adapter, which has a flat constant group delay (ignore the big hump, I think that is from the calculation the PNA is doing with the phase flip).
So what is the lesson? You can phase match two outputs using a variable delay line, but only at a single frequency. Otherwise you have to do it with a coupler, a balun, a Schiffman, or some other true variable phase circuit.
We get this question a lot: how much power can part XYZ handle?
Power handling is a difficult topic, because the ways in which a device can fail depend so much on the operating conditions that it is subjected to.
We specify the max power on (for example) the PD-0165 as 1 watt only to be extremely conservative. Here are some use scenarios for the PD-0165 and the power handling I would estimate:
– Ideal use case: 50 ohm matched at all ports, using the device as a power divider. In this case the device is only dissipating the excess insertion loss. Depending on the heat sinking it has attached, it should be able to handle 10s of watts of CW power or more at 43 GHz. At a high enough power the connectors will fail.
– Worst case CW performance: Out of phase reflections at both output ports, or use as a power combiner with two signals that are 180° out of phase. In this case all the power will be dissipated in the isolation resistors, which means that the power is limited to what the resistors can dissipate. This is where the power handling will be limited to about 1 W before the resistors pop.
– Pulsed case: In this situation the power is limited by the voltage breakdown in the device. If the peak power is high enough the voltage will break down the dielectric either in the connectors or the substrate, this isn’t clear. The amount of power it can take depends on the pulse width and hence the peak power.
So the amount of power that you can put through the device depends on how you are using it and how much heat sinking you provide to it.