At any given time, there are a multitude of signals at a variety of frequencies streaming all around us. Each device that relies on receiving the proper RF signals such as televisions, radios, radars, medical devices, and cell phones, requires some level of filtering. While all filters have the same basic job – remove unwanted or out-of-band signals – the specific job requirements of each filter vary depending on the RF architecture used and the needs of the final device.
When designing a filter for an application that depends on receiving and sending RF signals, the first step is to identify the jobs the filter needs to do. These jobs can be very different depending on the RF architecture used in the application. At a high-level, the jobs filters need to perform in the three most common RF architectures include the following:
- Superheterodyne (superhet) – Preselection and removing image and intermediate frequency (IF) spurs as well as LO leakage.
- Direct conversion receiver (DCR) – Preselection, preventing out-of-band signals from saturating the analog frontend, and aliasing with a lowpass filter at baseband.
- Direct sampling – Preselection, preventing out-of-band signals from saturating the analog frontend, and aliasing with a bandpass filter to prevent interference in the alias bands of the ADC.
In these three functional block diagrams, all the necessary components for each of the three main RF architectures are displayed, with A showing a superhet receiver, B showing a DCR, and C showing direct sampling.
Read more about the jobs RF filters need to perform and the trends effecting filter performance considerations today in part 1 of a two-part series we wrote for Microwave & RF.