Pi, T Filters Match RF Impedances

Filter circuits are used to remove unwanted or undesired components from a signal. When coupling more than one signal or band to/from an antenna, networks that filter and match impedances such as Pi filters and T filters are bridges in that last connection from the RF transceiver to the antenna. These circuits not only pass some bands while discriminating against others, they also match impedances and can allow duplexing and circulating circuits to provide full duplex bi-directional communications. When designing RF sections, it is nice to know we have these parts in our arsenal.

This article will discuss how these filters can reject or pass bands complying with specific standards as well as those that allow multi-standard transmission through the same path. All parts, tools, and data sheets mentioned can be found on the Digi-Key website.

Limitations of silicon

Physical constraints do not allow efficient full-range performance from monolithic silicon. Filters, attenuators, transmission lines, isolation barriers, surge suppression, and more just cannot be done (at least not yet) as effectively in silicon as they can when built up from other substrate materials at larger scales. 

For example, we can make an on-chip dielectric insulator between sensitive stages on a single-chip die. But, as voltage spikes increase in amplitude, the properties of the silicon insulator just cannot take the higher voltages. Also, unless we insulate in three dimensions, arcing will occur at some point. This applies to surge suppression, too. There is only so much energy a small area can dissipate, absorb, or shunt.

With regard to on-chip capacitors, dielectric materials are limited in capability when it comes to silicon. Whether charge storage or timing related, capacitors rely on the physical properties of spacing, area, and dielectrics. Without a high dielectric constant and area, the right-sized capacitor just may not be feasible to implement on chip, even with the Miller Effect.

As a result, many optimal solutions for modern RF stages rely on external components to match silicon input/output stage characteristics with an efficient antenna, and provide EMI/RFI protection from undesired sources. Even PCB traces act as components (transmission lines). While simulators are good, the only real measure of success is when you can test and characterize a finished design headed for production.

Choices and characteristics

Single-stage filters can be implemented in one of four ways. These include chokes, R/C filters, L/C filters, and Pi/T filters. These can be combined to wave-shape, discriminate, or pass specific frequencies. 

Choke filters are inductors or ferrite beads that exhibit electromagnetic reluctance (Figure 1). They can also be useful parts of a discriminating RF stage. High-frequency signals cannot pass and will look like resistances at these frequencies. Parts like the Bourns FB20011-3B-RC are rated by resistance at a given frequency, in this case, 415 Ohms at 100 MHz. In this case, a discrete part can be used when coupling a high-power transmitter to an antenna. Current carrying capacity is also an important rating when used in a transmitting stage.

As a result, chokes and common-mode chokes are mostly used in power supplies, but can also be used as filter elements in a high-power RF transceiver. A dual-band antenna, for example, can be shared using a receiver circuit that is protected (in part) by a choke that will not allow the transmitting frequency to pass back to the sensitive receive circuitry. A common-mode choke can also block any high-power induced signal from bleeding back into the circuitry since it will reject common-mode signals.

Figure 1: Simple chokes can remove high-frequency components of a signal and recover the DC or signal part of the modulated signal. They can also protect local receiver circuits in proximity to high-power transmitters.

Capacitive filters block AC and recover a DC level after rectification and are used in power supplies, but can also be combined with resistors, chokes, and inductors to implement multi-pole filters in a purely passive fashion. These R/C and L/C filters can provide single-pole filter characteristics that can be daisy-chained to create passive, multi-pole versions with steeper rejection curves. Note that these passive stages attenuate the signals as well, so amplification will be needed to bring it back to the usable range.

Overall, the flexibility and performance from passive R/L/C filters is combined in a special way with Pi and T filters to optimize performance in RF designs. What’s more, they are integrated into small, single-package devices that can fit easily in the compact spaces found in modern PCBs.

Pi filters and T filters

Pi filters are basically one inductor surrounded by two capacitors and arranged like the Greek letter Pi. The input capacitor is selected to offer low reactance and repel the majority of the nuisance frequencies or bands to block. Its inverse, the T filter uses two shunt inductors and a coupling capacitor. These single-stage filters can act as low pass, high pass, band pass, and band stop.

Figure 2: Low-pass Pi filters (left) can tap into an RF-transmission path allowing just the lower frequencies to pass. Conversely, high-pass T filters (right) block the lower bands and allow the upper frequencies to pass. These parts can be used to both optimize each band’s performance and allow full-duplex operations.

An RF transceiver may use a T filter to block a shared or competing band while using a Pi filter to clean up and pass the desired frequencies. In either case proper selection of component values can match impedances on both sides to maximize power transfer between active stages, switches, and antennas.

Lots of choices

Several quality manufacturers provide well-engineered Pi and T filters. Critical here when trying to find the right part is a good parametric search engine. With so many variables, it is nice to be able to narrow down the playing field.

While many application-specific Pi filters are designed for audio, data line, and power conditioning, several general-purpose parts as well as RF targeting parts are available for use in wireless designs. What’s more, integration of ESD protection lets these monolithic filters multitask in the same physical space.

Consider, for example, the Murata 8 low-pass L/C Pi filter with a center cutoff frequency of 100 MHz. The single, third order polarity insensitive 0805 packaged filter includes a 135 nHenry inductor, with 44 pF capacitors and can pass 200 mA of current.

A family of parts like the Murata EMIFIL NFL21SP Series allows flexibility when choosing the right current rating, inductances, capacitances, and center frequencies to match up your transceiver with the antenna (Table 1).

Table 1: Some suppliers centralize data to help identify the best match for a design. Here, L/C values and cutoff values let designers choose the parts with values that best match their circuits.

In similar fashion, designers who need single 3^rd-order devices that implement 100 mA L/C T filters can take advantage of the TDK MEM2012S25R0T001. This 3-terminal 0805 packaged low-pass T filter has a 25 MHz center/cutoff frequency and attenuates 30 dB from 70 MHz up to 2 GHz. Also, as a member of the company’s MEM Series of 3 terminal filters, cutoffs from 25 to 200 MHz allow 100 and 250 mA power levels at 10 Volt max (Table 2). Other family members feature currents up to 1 A.

Table 2: Within a specific family, selection of frequency parameters will provide solutions with different L/C values to hopefully match up to the stages of the design. Separate tables provide the actual component values used internally.

Ultimately, the best way to know for sure how your design performs is to test it in your configuration. Some nice tools are available that you should investigate, and radio transceiver makers will also often be able to point you to specific components they suggest in their reference designs. A parametric search engine is another invaluable device for helping you find the right filter.

For more information about the parts discussed in this article, use the links provided to access product pages on the Digi-Key website.