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Agilent Balanced Measurement Example: SAW Filters Application Note 1373-5 Introduction SAW filters are commonly used in wireless communi- cation products because of their very sharp response characteristics, relatively low insertion loss, and low cost. In addition, their physical structure is such that they can be designed with single-ended RF ports, dif- ferential RF ports, or a combination of the two. This allows circuit designers to take full advantage of the benefits of either unbalanced or differential circuit topologies. Typically, a SAW filter is most conveniently designed with relatively high port impedances. This high impedance works to the benefit of designers of hand- Port 1 Port 2 held wireless subscriber products who are also con- cerned about minimizing power consumption. Port 3 Port 4 The combination of balanced RF ports and non-50 reference impedances tend to make characterizing SAW filters, and designing them into products, a challenging undertaking. Agilent's balanced-measure- ment solutions address this challenge, and make test- ing and designing with SAW filters as straightforward as using any 50 single-ended component. Mixed-mode S-parameters Single-ended S-parameters do not adequately The 350 data now shows the reflections closer to describe the performance of balanced devices the center of the Smith chart. The significant capaci- because the circuit is electrically different depending tive component to the impedance is clearly visible. on whether it has a single-ended stimulus or a differ- ential stimulus. Therefore, using conventional single- The transmission parameters now give the user a ended S-parameters to design differential circuits can better idea of the filter characteristics since much of lead to misleading and erroneous conclusions. the return loss has been removed. In addition to multiport single-ended S-parameters, The change in terminating impedance has also affect- Agilent balanced-measurement systems give the user ed the passband ripple and the rejection characteris- the ability to examine the mixed-mode S-parameters. tics, as shown in Figure 4. The mixed-mode S-parameters allow the two modes of propagation of a balanced device (differential and common) to be examined independently in both stim- ulus and response. This is best illustrated by examin- ing measured data on actual devices.