Design Note 1030
Design Note 1030: RF Input Impedance Matching Data for the LTC5564 Schottky Detector
Introduction
The LTC5564 is an UltraFast™ RF power detector for applications in the 600MHzto 15GHzfrequency range. It combines a temperature compensated Schottky diode peak detector, a gain selectable operational amplifier and a fast comparator in a 16-lead 3mm × 3mm package.
Performance of the RF detector, specifically detector response and bandwidth, depends on proper impedance matching into the detector RF input. The impedance measurements presented here can be used in conjunction with a CAD simulation tool to simplify the task of RF input impedance matching.
RF Input Impedance Measurement
Input impedance was measured on a LTC5564 demo board DC1646A, modified for minimal RF trace length as shown in Figure 1. For the purpose of this measurement, the detector input DC block was omitted since the test equipment supports DC voltage at its input. In a real world application, a DC block function must be included as part of the matching network design. The measured LTC5564 input impedance is shown in Figure 2 and Table 1
Figure 1. Test Setup Showing Location of the Reference Plane.
Figure 2. Measured S11 for the LTC5564
The phase reference plane for the measurement is 1.1mm away from the edge of the QFN package. This distance was chosen to give a comfortable margin against typical DFM rules regarding the distance between components.
For higher frequencies, the accuracy of the S11 measurement becomes diminished by various factors specific to each individual PCB design, such as ground via inductance, the quality of the microstrip transitions, etc. Nonetheless, the S1P data helps in producing the first cut design, saving development time.
| FILE NAME | CONTENTS |
| LTC5564_-10dBm.S1P | Input Reflection Coefficient, S11, PIN = –10dBm, Touchstone Format. 101 Freq. Points, 0.5GHz to 16.5GHz. |
| LTC5564_0dBm.S1P | Input Reflection Coefficient, S11, PIN = 0dBm, Touchstone Format. 101 Freq. Points, 0.5GHz to 16.5GHz. |
Note that the in put impedance to the Schottky detector is a slight function of RF drive level, as seen in Figure 2. There is no measurable input impedance variation over the specified supply voltage range of 3V to 5.5V.
Refer to the DC1646A demo board manual for several LTC5564 RF input matching circuit examples
Conclusion
The LTC5564 RF input impedance was measured and converted to a 1-port S-parameter file, which can be read by most industry-standard RF CAD programs, expediting the matching process.
| FREQUENCY(GHz) | INPUT IMPEDANCE (SERIES EQUIV.) | VOLT. REFL. COEFF. (ZO = 50Ω) | ||
| RS | jXS | MAG | ANG | |
| 0.5 | 126.2 | –59.8 | 0.521 | –19.4 |
| 1.3 | 65.9 | –58.6 | 0.468 | –48.0 |
| 2.1 | 40.7 | –40.3 | 0.416 | –79.0 |
| 2.9 | 31.0 | –22.5 | 0.350 | –114.7 |
| 3.7 | 28.5 | –11.2 | 0.305 | –144.2 |
| 4.5 | 24.1 | –0.1 | 0.350 | –179.7 |
| 5.3 | 20.9 | 12.8 | 0.442 | 146.0 |
| 6.1 | 21.9 | 27.1 | 0.508 | 115.3 |
| 6.9 | 26.6 | 40.9 | 0.542 | 91.7 |
| 7.7 | 30.3 | 57.7 | 0.617 | 73.1 |
| 8.5 | 38.4 | 76.7 | 0.663 | 57.7 |
| 9.3 | 50.7 | 91.2 | 0.671 | 47.4 |
| 10.1 | 63.7 | 109.7 | 0.700 | 38.9 |
| 10.9 | 92.0 | 132.1 | 0.715 | 29.4 |
| 11.7 | 120.8 | 143.3 | 0.717 | 23.7 |
| 12.5 | 170.8 | 143.6 | 0.712 | 16.9 |
| 13.3 | 211.7 | 78.6 | 0.658 | 9.2 |
| 14.1 | 197.7 | 3.7 | 0.596 | 0.6 |
| 14.9 | 115.7 | –22.0 | 0.415 | –11.0 |
| 15.7 | 95.8 | 46.1 | 0.425 | 27.6 |
| 16.5 | 168.9 | 83.8 | 0.621 | 14.2 |
Note: For a less granular version of this data, use the S1P data file (101 frequency points)
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