8 Tips for Specifying Filters for RF and Microwaves

In a classic case of what you don't know can hurt you, design engineers with advanced knowledge of digital electronics are discovering an urgent need to brush up on RF basics when it comes to specifying filters for wireless devices. Failure to take into account the fundamental aspects of filter types and minimum specification requirements can result in products that fail "test," thus incurring costly production delays as the product goes back to the drawing board. On the other hand, knowing how to accurately specify filters helps yield products that meet production benchmarks and function correctly in the hands of the customer. In effect, this knowledge helps contain production expenses while upping the product's chance of success in the marketplace.

BACK TO BASICS
The fierce competition for band space in today's wireless world dictates an ever-increasing attention to filter performance. Inaccuracies in specifying the correct filter ultimately translate into frequency conflicts that come back to bite the design team in the form of cross-talk, dropped calls, loss of data and interrupted network connections.

The problem of incomplete or inaccurate specification of filters partly rests on today's emphasis on digital electronics in the electronics marketplace. By some accounts, 80-90% of new electronic design engineers are software and digital oriented. Herein lies the knowledge gap, because no matter that the intelligence being transmitted is in digital form, when it travels through the ether via radio or microwave, the carrier always obeys the laws of electromagnetic physics.

Fortunately, a quick refresher on some of the more essential elements of filter performance specifications can aid engineers in correctly calling out filters that meet the needs of their particular application. Doing it right the first time saves time and money, ensuring more bang for the buck when ordering these indispensable components.

1. KNOW THE BASIC RESPONSE CURVES
Basic response curves for filters include: bandpass, lowpass, highpass, bandstop, diplexer and duplexer, shown in Figures 1A-1F. Each respective profile determines which frequencies get through and which don't.

Far and away, the most common among this group is the bandpass filter. All engineers know that a bandpass filter allows signals between two specific frequencies to pass, but discriminates against signals at other frequencies. Examples include surface acoustic wave (SAW) filters, crystal filters, ceramic and cavity filters. As a point of reference, the cavity bandpass filters manufactured by Anatech Electronics cover a frequency range from 15 MHz to 20 GHz with bandwidths from 1% to 100%. Complete specifications for a lumped component bandpass filter from Anatech Electronics is shown in the Table below. For all manufacturers, the passband of a filter is usually defined at the 0.5 dB, 1 dB, or 3 dB attenuation points on either side of the center frequency.

2. INCLUDE ALL NECESSARY SPECIFICATIONS
Too many times an engineer will send out a short RFQ for "a 100 MHz bandpass filter"--the exact opposite of "too much information." A filter supplier can hardly fill an order in such a vacuum.

Providing all the necessary information begins with detailing all the frequency parameters such as:

• Center frequency (Fo): This is usually defined as the midpoint between the two 3 dB points of a bandpass filter (or bandstop filter), and is normally expressed as the arithmetic mean of the 3 dB points.
• Cut-Off frequency (Fc): This is the transition point from the passband to the start of the stopband in a lowpass or highpass filter. That transition point is normally the 3 dB point.
• Rejection frequency: The specific frequency or frequencies where the signal is attenuated at some specified value or set of values. The region outside the desired passband is sometime defined as the rejection frequency or frequencies, and the attenuation as the rejection.

Filter type determines the specified frequency. For bandpass and band reject filters, the specified frequency is the center frequency. For lowpass and highpass filters, the specified frequency is the cut-off frequency.

To be totally complete, engineers should also specify characteristics such as:

• Stopband: A band of frequencies, between specified limits, that a filter does not transmit.
• Isolation: In diplexers the ability to reject the transmit (Tx) frequency while looking at the receive (Rx) channel and the ability to reject the receive (Rx) frequency while looking at the transmit (Tx) frequency is called Rx/Tx isolation. The more isolation, the better the filter can isolate Rx from the Tx and vice versa. The result translates into cleaner transmitting and receiving signals.
• Insertion loss(IL): This is a measure of power loss in a device, and at any frequency is defined as: IL =10Log(Pl/Pin), where Pl is the load power and Pin is the power from the generator.
• Return loss(RL): This is a measure of filter performance and is an indicator of how close the input and output impedance of the filter is to an ideal impedance value. Return Loss at any frequency is defined as: RL = 10Log(Pr/Pin). Where Pr is the power reflected back to the generator.
• Group delay (GD): The group delay is a measure of the phase linearity of a device. Since a phase delay occurs at the output of a filter, it is important to know if this phase shift is linear with frequency. If the phase shift is nonlinear with frequency, the output waveform will be distorted. The group delay is defined as the derivative of the phase shift versus frequency. A linear phase shift will result in a constant group delay, since the derivative of a linear function is a constant.
• Shape factor (SF): The shape factor of a filter is typically the ratio of the stopband bandwidth(BW) to the 3 dB bandwidth. It is a measure of the steepness of the filter skirt. For example if the 40 dB BW is 40 MHz and the 3 dB BW is 10 MHz, the shape factor will be 40/10=4.
• Impedance: The value specified in Ohms, of the filter source impedance (input) and the terminating impedance (output). Generally the input and output impedance are the same.
• Relative attenuation: The attenuation difference measured from the minimum attenuation point to the desired rejection point. Relative attenuation is usually specified in dBc.
• Ripple(Ar): This is a measure of the flatness of the passband in a filter and is normally expressed in decibels. The amount of ripple in a filter will affect the return loss. The greater the ripple, the worse the return loss, and vice versa.
• Rejection: See above
• Operating temperature: The range of temperatures at which the filter is designed to operate

Frequency Range (MHz)	Number of Sections	3dB BW %	VSWR (max)	Impedance In/Out	Response	Avg. Input Power	Temp (deg. C)	Shock	Vibration
1 to 1000	2 to 10	5 to 100	1.5:1	50 or 75 Ohms	Chebyshev	2 to 20 watts	-25 to +70	30G 11msec	10G 5 to 200 Hz

3. DO NOT SEEK UNREALISTIC FILTER CHARACTERISTICS
Cases exist when an engineer has made a request such as, "I want a passband extending from 1,490 to 1,510 MHz, and I want 70 dB of rejection at 1,511 MHz." This cannot be done. In reality, the rejection is gradual, not an abrupt 90° drop off, a more realistic specification would be approximately 10% off of the center frequency.

Another instance involves requests for a filter that "rejects everything above 1,960 MHz," for example. In this case, the engineer must be reminded of the impossibility of attenuating every frequency from that rejection frequency out to infinity. Some boundaries must be set. A more realistic approach might involve attenuating two to three times the specified rejection frequency close to the passband.

4. SHOOT FOR A REASONABLE VSWR
Often used as a measure of the filter efficiency, the voltage standing wave ratio (VSWR) is a ratio ranging from 1 to infinity that expresses the amount of reflected energy. A value of 1 indicates that all of the energy passes. Any greater value indicates that a portion of the energy is deflected, i.e. wasted.

However, in the real world of electronic circuits, a 1:1 VSWR is almost impossible to attain. Typically, a ratio of 1:5 represents a more nominal goal. Requesting anything lower than that corresponds to a decreasing benefit-to-cost ratio.

5. CONSIDER POWER HANDLING
Power handling is the rated average power in watts beyond which the performance of the filter may degrade or fail. Also note that filter size is driven somewhat by the power handling requirements. In general, the greater the power, the larger the footprint of the filter on the circuit board. While manufacturers like Anatech stay constantly at work creating new algorithms to accommodate these competing interests, up-front planning here can save costs.

6. FACTOR IN ISOLATION FOR SIMULTANEOUS, TWO-WAY COMMUNICATIONS
An especially important aspect in diplexers, isolation represents the filter's ability to reject the transmit frequency while simultaneously looking at the receive channel, and vice versa. The more isolation, the better the two can be separated. This separation translates into cleaner transmitting and receiving signals.

7. BE AWARE OF TRADEOFFS
Higher performance usually incurs higher costs. All the more reason why accurate specifying— which curtails unneeded extremes—helps avoid unnecessary expenses.

Beyond that, other factors deserve weighing against each other. For example, the closer the rejection frequencies are to the center frequency, the more complex the filter. In some cases this may result in greater insertion loss.

Additionally, higher performance usually necessitates a filter with a larger footprint. For example, a very sharp transition from passband to rejection requires a more complex filter with a greater number of cavities and sections. But when real estate on a circuit board is at a premium, performance may have to be scaled back.

8. FIND A MANUFACTURER WHO CAN BALANCE COMPETING DEMANDS
While not an inherent characteristic of filter performance, as much care should go into identifying a filter vendor as specifying the component itself. A quality, on-shore manufacturer who specializes only in filters can oftentimes create a custom part to accommodate a shortcoming in the product design.

At Anatech, for instance, parameters such as insertion loss, selectivity, and power handling capacity can be enhanced by using special design techniques. In-house design and manufacturing capabilities allow center frequencies, for example, to be shifted without scrapping the original circuit design of a product. By the same token, last-minute packaging changes can be handled by changing an input/output connector from one type to another. Such flexibility can save a project from cost-overruns.