Bandpass Filters

Transversal SAW filters are extremely general: any linear bandpass filter may be synthesized, with arbitrary amplitude and phase, limited only by line width and substrate size. Resonator SAW filters are minimum phase designs with low loss. Both devices are small, rugged, very stable, and capable of high volume low cost production.

Delay lines are simply transversal bandpass filters with additional separation between the transducers to achieve the desired insertion delay.

Applications

Applications of bandpass filters include anti-aliasing, digital communications, MSK modulation, channel selection, matched filtering, TV broadcast, cable headend, channel processors, RADAR matched filtering, EW matched multi-channel receivers, and filter banks.

Design

Adjustment of electrode length and position allows the SAW designer to synthesize any finite impulse response. For filters specified in the frequency domain, FIR digital filter design techniques are used to find the initial optimum, i.e. shortest, time response. Filters may also be specified directly in the time domain. Filters may be specified by defining templates or mathematical functions in the frequency or time domains.

Performance Typical Limit Typical Limit
Type              Transversal                   Resonator
Center frequency fo MHz 20 – 1000 10 – 2500 100 – 1000 50 – 2500
PassBandWidth B*100/fo for Quartz .5 – 5 .1 – 10 .05-.1 .025-.2
                           for Lithium tantalate 5 – 10 1 – 20 .5-5
                             for Lithium niobate 10 – 30 10 – 150 1-15
Amplitude ripple in B dB pp >1 .2 >1 .5
Phase ripple in B degrees pp >10 2
StopBandwidth SB/B >1.5 1.1 >5 3
StopBand rejection dB in SB<2|f-fo|<fo -40 -60 -40 -60
Min insertion loss IL in B dB >25 10 >5 2
Return loss in B dB none 14 none 14
Time spurious dB 2*IL -60
Max response length or Delay us <10 100