Fixed and Programmable Filters for RF Designs

RF applications are exploding. The encapsulation of complex RF functional blocks into silicon is transforming the types of devices and equipment we use.

Most RF designs are standards-driven, using specifications formalized by IEEE or a consortium group encompassing several manufacturers that have agreed to develop and adhere to a set of guidelines. However, not all designs are public domain and open to anyone’s receiver. Indeed, secure, nonstandard encryption, or hidden transmissions may be needed to assure privacy, stealth, or security, as well as to allow customized control functions.

Filters play a role here. Mixing signals and manipulating bandwidth, modulation and data can add information that will not be detected unless you know exactly what to look for. Much like data that is hidden in a picture file, data can be hidden in an audio stream as short bursts of power at specific frequencies and at key moments that will be undetectable to the listener. This procedure may allow even a standards-oriented design to boast unique features and functionalities, since a control channel is now available to the user.

For example, if you are in a car having a phone conversation using a Bluetooth earpiece and another call comes in, a piggy-backed and detected signal in your earpiece can transmit a very-low-power near-field personal area network (PAN) signal to a heads-up display on your windshield, letting you know that you have a call coming in and providing the phone number and identity of the caller – all without interrupting the audio stream. You can now decide if you want to interrupt your call or not without becoming distracted.

This article discusses standard and nonstandard filter parts and techniques that can be used to help customize an RF link. All parts, datasheets, and training information referenced here can be found on the DigiKey website.

Mixed and modulated data

Radios are basically mixers, filters, amplifiers, and modulators. At a specific radio frequency, the bandwidth of the recovered signal is proportional to the Intermediate Frequency (IF) mixed with the carrier. The mixed signals have each constituent signal as well as sum and difference frequencies (Figure 1). As such, the recoverable signal bandwidth, whatever the modulations scheme, is set by Nyquist and is directly proportional to the IF frequency.

Figure 1: Filters are essential when recovering and separating mixed and modulated data in RF stages.

Not every radio design is a complex digital wrapper strapped onto an RF transceiver. Basic RF stages are still useful and take advantage of well-refined and well-understood AM, FM, FSK, and phase-shift modulation schemes. These designs also are typically very low cost.

As a result, and without great expense, you can theoretically extend the bandwidths of your transmitted signals to allow higher-frequency components to get through, while standard receivers that do not have expanded bandwidth will not recover these signals. Engineers can play with IF stages here using standard RF specific filters or create their own filters with discrete components.

However, a word of caution: the wider the bandwidth, the higher the resulting noise level in the signal-to-noise ratio. Narrower bands have less out-of-band noise since effective notch filters remove virtually all out-of-band noise.

Filters for given standards

Let’s now consider radios designed for interoperable compliance to standards. Examples include specific functions such as voice links, networking, audio/video, gaming control, etc. These have defined bandwidths and signal characteristics, so you really cannot play with parameters on this level. But, making our lives easier, canned filters tailored to these standards are available from many suppliers at competitive cost.

For example, Bluetooth designs – including the new Low Energy flavor – can take advantage of the 2.441 GHz center, with 2.45 GHz center frequency, 78 MHz, and 100 MHz bandwidth-tuned monolithic parts that save PCB space and reduce cost. Similarly, Wi-Fi designers can choose from 2.45 and 5.5 GHz filters with varying bandwidths from 100 MHz to 1.02 GHz. Specific configurations for WiMAX are available off the shelf as well. Often, these filters are actually multiple inductors, capacitors and sometimes resistors packaged as a single part.

Very little modification will be made to filters for these and other well-defined standards, but you can always create your own tailored low-pass, bandpass, notch, and high-pass filters using the many available discrete RF specific filter components at your disposal. Taking this design route could give your product better receive sensitivity or transmit range, resulting in a distinct competitive edge.

For example, Johanson Technology offers the 2593BP44B186E 2.593 GHz bandpass filter for WiMAX designs with a 186 MHz bandpass range. The company also provides a Training Module on the use of its MLCsoft software, which allows you to iteratively vary parameters and fine-tune the performance (Figure 2).

Figure 2: Helping to improve the accuracy of filter designs, the MLCsoft program from Johanson Technology helps you customize filter response in an iterative way.

While targeted parts for standardized designs certainly simplify matters, custom radios can live in these bands as well and can operate outside the fixed limitations that a specific standard may impose. In this regard, many free use and “experimenter” bands allow a lot of wiggle room as to how you design your custom secure or enhanced radio (as long as you adhere to FCC style rules). One technique is to play with modulation schemes (see the TechZone article “”) or even combine them to allow hidden transmission and reception of data.

Hidden signals

While you can tinker with bandwidths and enhance recovered signal ranges, you can also embed information within the standard data stream that only you know how to recover. This is a lot easier than designing a custom radio since you can take advantage of standard parts at your disposal.

While encryption may be available, and may be quite effective in protecting the actual data stream, an alternative approach is to have a hidden signal that is undetectable without specific information.

A perfect example of this can be found in amateur and professional band radios, which have been using sub-audible tones mixed into the transmitted audio signals to allow remote-control functions. Residing below the typical human audio hearing threshold make them basically “hidden” from detection, but you can design circuitry to capture it.

Helpful here are active filters that can tuned to very specific tones and bands, or can be dynamically programmed to look for power in different bands at specific times. This is not unlike time-division-multiplexed frequency sequencing.

Take, for example, Linear Technology’s LTC1060CN#PBF universal switched-cap filter. Part ranges and performance can be influenced by the clock rate and resistance values, and the two filters inside can create low-pass, bandpass, high-pass, and notch filtering with good response (Figure 3).

Figure 3: Switch Cap filters offer good performance with few external components. Clocking can also affect filter characteristics.

Some parts, such as the Quickfilter QF4A512A-LQ-B, contain multiple programmable filters with anti aliasing, programmable gain, precision voltage references, and SPI control (Figure 4). The single or differential inputs can be processed under control of a local microcontroller to set the sampling rates and finite impulse response (FIR) filter coefficient parameters.

Figure 4: Four individual filters can be programmed and controlled dynamically through an SPI port. On chip EEPROM stores filter coefficients for all channels.

The company also provides a software design tool as well as a development system to help prototype and verify a filter design. The Windows-based Quickfilter Pro includes all the hardware and software necessary to design, implement, and test a complete filter design. On-the-fly system control is available through USB-based SPI interfacing (Figure 5).

Figure 5: The Quickfilter Development Kit allows rapid prototyping and testing of filter designs. The software allows you to configure the gain stages, filter coefficients, sample rates, and types of filters.

A continuing need for filters

More and more, so-called software-defined radios are making inroads into the way in which radio links and even protocols are implemented. As such, the radio designs and components we use today will be the older legacy protocols of tomorrow.

It is pretty much a given that, in the future, DSP functions inside of high-end communications chips will take over all the modulation, demodulation, signal recovery, protocol implementation, and handshaking functions. Even so, engineers will still be able to hide data, or piggyback signals in the content to develop custom or differentiating features, if they know how and when to filter.