RF bandpass filter: How to Verify Compatibility Before Selection

Both filters are labeled with an operating frequency close to 500 MHz and put into the same RF chain, but the results may be different. One can cover the complete signal, and the other may weaken the band edge; their suppression of adjacent interference cannot be judged by “the same center frequency. When selecting the type, first write out the frequency band actually occupied by the signal and the interference frequency point that needs to be lowered, and then look at the device curve.

First draw the frequency band to be retained

The bandpass filter allows the target frequency band to pass while suppressing unwanted signals on both sides. Only one center frequency is reported, which cannot indicate whether the passband covers the entire signal. List the lowest and highest operating frequencies, frequency tolerances, and possible offsets at the operating temperature, and compare the passband boundaries of the candidate devices. When the edge happens to be stuck on the nominal pass band, also look at the actual loss of the band edge, not the start and end frequencies in the read-only table.

In Nandesai’s MMIC bandpass filter selection table, the NDMF040001 frequency range is 0.35~0.55 GHz and the NDMF040002 is 0.46~0.54 GHz. Both cover 0.5 GHz, but they are not directly interchangeable options. The former gives a wider nominal passband, the latter a narrower range; which 1 is only appropriate, depending on the full range that the signal needs to retain.

Locate the interfering frequency to a specific position

“High inhibition” is still not enough. Write down whether the interference is below or above the passband, how far it is from the edge of the passband, and how much it must be attenuated at least at that frequency. The transition band is especially easy to ignore: when the target signal is very close to the interference, looking at the stop band value far from the passband alone will overestimate the device’s help to the current problem.

The suppression indicators of the above two models are marked at different test frequencies. NDMF040001 gives at least 20 dBc at 0.27 GHz and 0.65 GHz; NDMF040002 gives at least 40 dBc at 0.3 GHz and at least 27 dBc at 0.68 GHz. These numbers cannot be sorted directly from the corresponding frequency points. Identify where your interference falls, and then ask the supplier for a complete frequency response covering that frequency and nearby range.

Don’t let the passband index swallow the link margin

When filtering bandpass filter rf, the insertion loss should be placed in the link where it is located. In the same selection table, the NDMF040001 insertion loss is listed as 2.3 dB and the NDMF040002 is 3.2 dB. If the filter is located before the low-noise amplifier, the front-end loss will occupy the noise budget; if used in the transmit chain, check the average and peak power, waveform, mismatch and thermal conditions. A loss value in the table is not a substitute for checking the full pass band, operating temperature and actual assembly condition.

Port matching also needs to be checked. The device table may be represented by VSWR, and the system evaluation often looks at the input and output return losses. Impedance at both ends, wiring on the board, pad and ground will all affect the results after installation. If the signal is sensitive to amplitude flatness or delay, you should also look at the in-band ripple and group delay, not just the insertion loss at the center point.

Perform the final check using the actual reference plane

After obtaining the candidate device, first confirm the port impedance, test temperature and reference surface corresponding to the data. The two-port filter can use the calibrated vector network analyzer to check the passband, insertion loss and stopband response of S21, and use S11 and S22 to check the matching at both ends. The test reference surface should clearly fall on the connector, fixture or package pad; If the template has more switching and routing losses, all the measured differences cannot be counted on the filter.

Finally, the candidate curve is superimposed on its own “reserved frequency band-interference frequency point-allowable loss” requirements. As long as there is no evidence to support an item under actual temperature, power or assembly conditions, continue to check the item, and do not rely on the center frequency to determine the item number.