Low frequency circuit power divider
In the low-frequency electronic circuit, a signal is divided into multiple channels, or the multiple signals are mixed into one, simple, all signals are connected to a network contact, such as the three-way noninverting adder in the following figure:
Since the input impedance of the eponymous end of the op amp is infinite (the concept of falsehood), the impedance determined by the Ri1/Ri2/Ri3 hybrid structure is the termination parallel matching impedance of the op amp with the same name, so the low-frequency circuit basically does not need to consider the op amp impedance matching, so the Ri1/Ri2/Ri3 value range (degrees of freedom) can be very wide.
Chip resistor power divider for RF PCBs
In the field of RF microwave, impedance matching is considered, so if a signal is divided into several channels, the resistance value is not only unique, but also limits the shunt power.
Look at the one-two-resistor power divider in the figure below:
At least the impedance matching at the input must be satisfied, so looking to the right from pin the impedance must be 50 ohms, resulting in an equation like this:
How did the equation come about?
If it is also required that no secondary reflection occurs at the tap end P1, the impedance of the resistor splitter that P1 looks to the left must also be 50 ohms.
Talk about the principle of absorption secondary reflection: If the P1 impedance is not 50 ohms, a primary reflection reaches the resistor power divider, which is completely absorbed by the network composed of the resistance power divider, the Pin internal resistance, and the P2 internal resistance. Naturally, the equivalent impedance of this network formed by P1 looking to the left must also be 50 ohms.
The same is true of the equation.
Three equations with three unknowns, there is a unique solution.
Such a symmetrical equation, it is obvious that R1 = R2 = R3 = 50/3 = 16.67 ohms.
The intrinsic loss of this resistive power divider is 3dB. The equivalent power to reach either tap is -6dB.
If a different rate of work is required, the first equation is generated:
Obviously solving three unknowns with four equations is unsolvable.
So if a different power ratio is required, then the first three equations must drop one, which means that there must be a port that does not guarantee a perfect match.
The principle of the star-shaped triangle is still applicable in the field of RF microwave:
The conversion formula is not listed, and everyone checks it in Baidu.
If you also want to meet the perfect matching of the three terminals of 50 ohms, you can calculate that the three resistance values of the triangle connection method are 50 ohms.
Theoretically, the resistor power divider has no frequency limit, but because of the size of the resistor package and layout area, it cannot be used in the millimeter wave band.
In the field of RF microwave, when the frequency is not too high (for example, less than 5GHz), this power divider can be made with a chip resistor, mostly using the star connection method:
Pay attention to the package of the chip resistor, the chip resistor itself parasitic parametric effect, may be inductive, may also be capacitive, the optimal package width compared with 50 ohm line width is about 1.2 times.
The chip resistor package width in the left figure above is smaller than the line width, and it will be inductive, so add copper capacitor matching as in the right figure.
It is also important to note that the three resistors must be laid out as close together as possible.
Because the high-frequency characteristics of this smd resistor power divider also depend on the size of the entire power divider compared with the working wavelength, or the smaller the layout area on the PCB, the better, if you want to further increase the operating frequency, you have to use a "no package" thin film resistor power divider to further reduce the layout area:
Thin-film resistive power divider
Thin film resistance is the formation of a nickel chromium nickel film on the alumina ceramic substrate by vacuum deposition, usually only 0.1um thick, only one thousandth of the thick film resistance, and then etched into a certain shape by lithography process. The lithography process is very precise and can form complex shapes, so the performance of thin film capacitors can be well controlled.
The blue area shown in the following figure does not distinguish a single resistor at all:
Do you think this thick film or thin film resistance power divider is a triangle connection method or a star connection method? Please reply in the message.
Specify the appropriate resistivity (or block resistance value) to get an optimal three-terminal match.
It is not difficult to design the graphic with HFSS, and the island owner randomly uses a blue hexagon to get the matching indicator (return loss) as shown below:
Can be used within 20GHz. If you want to continue to increase the operating frequency, it is possible to make some fine adjustments to the graphics and resistivity.
RF PCB multiplexed resistor power divider
The multi-channel resistance power divider on the RF PCB is slightly troublesome in design, mainly because the impedance matching of the port, insertion loss and other indicators cannot be taken into account, and the specific problems should be analyzed specifically, and the circuit should be more important to which index to design.
One-point N-resistor power divider, if N is large, use a 1/4 wavelength impedance converter, so that the wideband resistance power divider becomes a narrowband power divider:
The indicators are as follows:
The black line above is the return loss, which can be seen as a narrowband resistor power divider with a frequency of 2 GHz.
The biggest advantage of this 1-point N-resistor power divider is that it only needs one stage, and the layout area is much smaller than that of the Wilkinson power splitter of one-two and then two-four.
The second advantage is that N can be arbitrarily valued, and the Wilkinson power divider must be 2 to the N power, that is, 2, 4, 8, 16.