Page 19 - EngineerIT July 2022

ADVANCED FILTER TECHNOLOGY

How digitally tunable filters enable

wideband receiver applications

By Brad Hall, Systems Applications Engineering Manager, and David Mailloux, Product Applications Engineer

n today’s multichannel, wideband multi-octave tuning RF RF signal chain overview
receivers, it is often necessary to eliminate unwanted blockers A typical wideband signal chain covering 2 GHz to 18 GHz is
Ito preserve the fidelity of signals of interest. Filters have shown in Figure 1. The basic theory of operation of this signal
played an essential role in reducing these unwanted signals, chain is the following: the antenna receives a broad spectrum
particularly in the receiver RF front end and local oscillator (LO) of frequencies. There is a series of amplification, filtering and
portions of these systems. This article will explore filters within attenuation control (the RF front end) before the frequencies are
RF signal chains, discuss the concept of blocker signals, review converted to an IF signal that the ADC can digitise.
traditional filtering technologies, and conclude with the latest
product solutions for optimising signal chain performance. The filtering functions in this block diagram can be divided into
four main categories:
Introduction • Pre-selector sub octave filtering
With the goal to continuously reduce size, weight, power and • Image/IF signal rejection
cost, while increasing or maintaining performance, it has become • LO harmonics
necessary for RF system designers to evaluate each component • Antialiasing
in the signal chain and look for opportunities to innovate. As
filters have traditionally consumed considerable space, they are The pre-selector sub-octave filtering needs to be near the
an obvious area to explore size reduction. beginning of the signal chain and is used to address second-order
At the same time, receiver architectures are evolving with intermodulation distortion (IMD2) spurs that can show up in the
the ability for analog-to-digital converters (ADCs) to sample at presence of interferer signals (also known as blockers). This occurs
higher input frequencies. With a higher ADC input frequency, the when two out-of-band (OOB) spurs add or subtract and create a
constraints placed on filters in the signal chain have changed. In spur that falls in band, potentially masking a desired signal. A sub-
general, this trend means a relaxation of rejection requirements octave filter removes these interfering signals before they can hit
for filters, which opens them up to further size and tunability a non-linear component in the signal chain (such as an amplifier
optimisation. or mixer). Often, the absolute bandwidth requirement for the sub-
To start this exploration, a general overview of RF signal chains octave filter becomes narrower as the center frequency reduces.
and definitions can assist in explaining where and why filters are For example, the first band in a 2 GHz to 18 GHz signal chain may
needed. Further, a review of traditional technologies can give only cover 2 GHz to 3 GHz and would need good rejection at
insight into the status quo. Then, by comparing these traditional 1.5 GHz on the low side (F high/2) and at 4 GHz on the high side
technologies vs. the latest product solutions, it becomes clear how (F_low × 2), whereas the highest band in the signal chain may
system designers can easily achieve their goals. cover 12 GHz to 18 GHz, with good rejection at 9 GHz on the low

Figure 1: 2 GHz to 18 GHz receiver block diagram.

EngineerIT | July 2022 | 17