I. Interpretation of Power Filter Parameters
1. Passband Rippy: The variation of insertion loss within the passband with frequency. The passband ripple within a 1dB bandwidth is 1dB.
2. In-band VSWR: An important indicator for measuring whether the signal within the passband of a filter is well matched for transmission. Ideally, the VSWR is 1:1; in case of mismatch, the VSWR is greater than 1. For a practical filter, the bandwidth satisfying VSWR < 1.5:1 is generally less than 3dB of bandwidth (BW), and its proportion of BW3dB is related to the filter order and insertion loss.
3. Return Loss: The ratio in decibels (dB) between the input signal power and the reflected power at the port, also equal to |20Log10ρ|, where ρ is the voltage reflection coefficient. Return loss is infinite when all input power is absorbed by the port.
4. Stopband Rejection: A crucial indicator of filter performance. A higher stopband rejection ratio indicates better suppression of out-of-band interference signals. There are generally two approaches: one is to specify the required suppression level (in dB) at a given out-of-band frequency fs, calculated as the attenuation at fs, As - IL; the other is to define the degree to which the filter's amplitude-frequency response closely approximates the ideal rectangular shape—the rectangularity coefficient (KxdB > 1), where KxdB = BWxdB/BW3dB (where X can be 40dB, 30dB, 20dB, etc.). Higher filter orders generally result in higher rectangularity—meaning K is closer to the ideal value of 1—but also increase the manufacturing complexity.
5. Delay (Td): refers to the time required for a signal to pass through the filter. Numerically, it is the derivative of the transmission phase function with respect to the angular frequency, i.e., Td = df/dv.
6. In-band phase linearity: This parameter characterizes the magnitude of phase distortion introduced by the filter into the transmitted signal within the passband. Filters designed according to a linear phase response function have good phase linearity.
II. Power Filter Design Principles
1. Filter type selection
Power supply filters are mainly classified into high-pass, low-pass, and band-pass filters. The appropriate filter type should be selected based on the signal frequency and impedance matching. For example, a high-pass filter should be selected for power lines that need to suppress high-frequency noise; a low-pass filter should be selected for power lines that need to suppress low-frequency noise; and a band-pass filter should be selected for power lines that need to suppress noise within a specific frequency range.
2. Filter parameter design
Filter parameter design is the core of power supply filter design. Based on the desired filtering effect and actual circuit parameters, the filter should be designed and optimized to determine parameters such as passband width, cutoff frequency, capacitance, and inductance. Furthermore, other factors such as filter size, weight, and heat dissipation performance should also be considered to meet the requirements of practical applications.
3. Filter housing and connections
The choice of filter housing and connection method also has a significant impact on its performance. Considering factors such as housing material and connection method, it is essential to ensure a good connection between the filter and the circuit, reduce contact resistance and parasitic effects, and improve overall performance.
