Application notes

• Selectivity
• Ripple 
• Insertion loss 
• Delay 
• Intermodulation distortion 
• Shock and vibration 
• Phase matching 
• Packages 

Selectivity

Rockwell Collins builds filters from two poles to 12 poles; selectivity is a function of the number of poles and the type of design. The adjacent figure shows the selectivity for a .01 dB Chebyshev design with 3, 5, 7, 9 and 11 poles. The 3 dB bandwidth of all the filters is 3 kHz.

3, 5, 7, 9 and 11 Pole Designs

The closer a design is to a Butterworth function the greater its stability but the lower the selectivity. For optimum stability, Rockwell Collins designs all filters of more than three poles with designs ranging from Butterworth to .05 dB Chebyshev.

Ripple

The filters are designed using theoretical models with ripple less than .05 dB. Actual ripple, due to process variations, will be higher. A typical Low-cost filter has room temperature ripple of less than 1 dB. Ripple over temperature is conservatively specified as less than 3 dB.

Ripple is defined in several ways. The definition we use most often is the difference between the minimum loss and the lowest dip within the passband. Using this method a monotonic round top filter would have 0 dB ripple. A second method to define ripple is from the highest to lowest points within a specified frequency range. A third method, which is never used by Rockwell Collins Filter Products, is to use the largest adjacent peak to dip value. This method understates the true ripple.

Insertion loss

Insertion loss, for our torsional series, is typically two dB or less for filters with a bandwidth of greater than 1000 Hertz. Filters of lesser bandwidth can have insertion losses that are several dB higher. Loss will increase with temperature, nearly doubling at 85°C.

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Delay

3, 5, 7, 9 and 11 Pole Designs

Delay is also a function of the number of poles and type of design. A Chebychev filter will have more delay than a Butterworth filter. The delay for a 3 dB bandwidth .01 dB Chebychev is shown above for 3, 5, 7, 9 and 11 poles.

Delay compensation can be achieved by bridging across two resonators within a filter. This is shown in the plot below.

Intermodulation distortion

Intermodulation distortion (IMD) in torsional filters is quite low. The figure below shows the inband 3rd-order IMD of a typical SSB filter. The reference voltage is the source voltage Vo in the filter test circuit. The wider the bandwidth, the higher the intercept point.  The slope of the third-order product is 2.6 and not 3, as would be expected of a LC bandpass filter. Crystal, ceramic and all electromechanical filters tend to have slopes that are below 3. The usefulness of knowing the 3rd order intercept point is reduced because the value is not 3. When doing system calculations, the intermod slope and 3rd order intercept point need to be considered. A typical 3rd order intercept point for our filters is +30dB.

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Shock and vibration

Rockwell Collins torsional mechanical filters pass the following shock and vibration standards as a minimum:

• Vibration: MIL-STD-202, Method 201,10 G’s, 10 Hz to 55 Hz.
• Shock: MIL-STD-202, Method 213B, 50 G’s, 11 ms, half-sine.

Rugged construction of our torsional mechanical filters results in low sensitivity to vibration and shock as shown in the measured data below.

Phase matching

Rockwell Collins Filter Products builds phase-matched sets of filters. These are used in range finders and sonar systems. A typical match would be ± 3 degrees over 70 percent of the passband. Typical groups are from 2 to 8 filters. If the system has the capability of subtracting out the phase slope difference then lower cost and improved specification tolerances can be achieved.

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Packages

Most popular case styles for our Torsional Mechanical Filters

PS and PL

FP and LP