Signal Chain Discussion Series: Difference Between IF & RF

In many cases in RF/Microwave electronics it is advantageous to generate, modulate, receive, and demodulate communication and sensing signals at frequencies lower than the RF transmission/reception carrier frequency. The frequency difference between the modulation/demodulation (baseband) signals and the RF signals may be so vast, that multiple frequency conversion stages may be required. In these instances, an intermediate frequency (IF) is created during the upconversion/downconversion stages between the RF and baseband signals.

The reason for using an IF stage is that there is a limitation in frequency conversion electronics as to how much conversion can be done while maintaining adequate signal quality. On the other hand, there are also limitations to direct digital synthesis (DDS) and direct digital conversion (DDC)/direct RF sampling (DRFS) using analog-to-digital converters (ADC) and digital-to-analog converters (ADC) for zero-IF or homodyne architectures. These limitations consist of how high in frequency the DDS/DDC technology can operate, as well as their bandwidths.

Currently, there are RF ADCs and RF DACs that can operate to millimeter-wave frequencies, though these devices are not widely available and may not be at viable price points for many applications outside of aerospace and defense. RF ADCs and RF DACs that operate into the gigahertz range are more common and accessible for mainstream applications and have replaced IF stages in many applications. These applications include consumer wireless internet, cellular, and other applications that push for low cost and highly integrated solutions.

For sensing and communications applications that operate at much higher frequencies, DDS and DDC technologies may still be inaccessible. Moreover, for many test and measurement applications, the performance of DDS and DDC technologies don’t meet some metrology level requirements for precision testing and measurements. In these cases, it is still common to see IF stages, or even several. Other applications that may see even several IF stages are radiotelescopes and other precision sensing applications.

Heterodyne and Superheterodyne transmitter/receiver circuits rely on IF stages made up of a mixer, a local oscillator (LO), and possibly several filters to upconvert or downconvert signals between the RF and baseband frequencies. As mixers are imperfect active devices, they tend to generate images, spurs, and harmonics that are undesirable in the transmit or receive signal chain. Hence, image reject filters and other filters are often used to clean the transmission/reception signals for improved signal quality. A high performance LO is also needed to avoid signal degradation phenomenon, and often high Q filters are needed to ensure adequate signal quality to the receive circuitry without introducing excessive losses.

Another factor to consider is that each component and device within a heterodyne or superheterodyne signal chain intrinsically presents different impedances at their respective ports. In order to achieve optimal signal chain performance, mismatches need to be eliminated to reduce reflections. Hence, it is common for these signal chains to use a standard impedance for all of the interconnect between the components and their ports. 50 Ohm and 75 Ohm are common standard impedances used for this purpose. Though using a standard impedance solves the mismatch problem, it also requires additional engineering design and electronics, in some cases, to convert the impedances of the signal chain devices and components to 50 ohms. This naturally requires some performance tradeoffs, such as when matching an LNA output to an image reject (IR) filter 50 ohm input.