Understanding and designing differential filters for communications systems

November 22, 2016 // By Graham Prophet
When it comes to communication systems, differential circuits can always provide better performance over single-ended circuits. They can provide higher linearity, immunity to common mode interference signals, and more. However, there is often a lot of mystery around differential circuits. Some RF engineers feel they’re difficult to design, test, and debug. This seems to be especially true of the differential filter. It’s time to lift the veil off of differential filter design.

To do that, we’ll start with a communication system receive chain IF stage filter. We’ll compare a few types of frequently used filter, and look at how to start with single-ended filter design then transfer that to a differential filter design. We’ll also examine a few points on how to optimize differential circuit PCB design.

 

Differential circuit advantages in RF signal chains

The user can get higher signal amplitude with a differential circuit than with a single-ended circuit. With the same power supply voltage, a differential signal can provide double the amplitude as compared to a single-ended signal. It also provides better linearity and SNR performance.

 

Differential circuits are fairly immune to outside EMI and crosstalk from nearby signals. This is because the received voltage is doubled, and theoretically, the noise affects the tightly coupled traces equally, cancelling each other out. Differential signals also tend to produce less EMI. This is because the changes in signal levels (dV/dt or dI/dt) create opposing magnetic fields, again cancelling each other out.

 

Differential signals can reject even-order harmonics. As an example, consider continuous wave (CW) passing through one gain stage. When using one single-ended amplifier, the output can be expressed as shown in Equation 1, and Equation 2.

 

 

When using one differential amplifier, the input and output are shown in Equations 3, 4, 5, and 6.

 

 

Ideally, the output does not have any even-order harmonics, making a differential circuit a better choice for a communication system.

Filter comparison

 

 

Table 1. Filter comparison

 

The IF-filter in a communications receive chain is basically a low-pass filter or band-pass filter. It is used for rejecting the aliasing signals along with spurs generated by active components. The spurs include harmonics and IMD products, among others. With the filter, the receive chain can provide signals with higher SNR for the ADC to analyze.

 

The Chebyshev Type I