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LTC5532 20dB coupling circuit for dual-band WiFi 2.4GHz - 6.0GHz

We are using LTC5532 in our design (prototype stage) and would like somebody from Analog to advice/explain on 20dB coupling circuit for 2.4GHz - 6.0GHz frequency range (dual-bend WiFi).

Also, we would like to receive an advice on "safe" zone gain settings - would x6 or x8 gain be ok?

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  • Hello,

    An RF coupling circuit can be a 1) resistive/capacitive voltage-divider, or 2) based on coupled transmission lines. Each is explained below:

    1) LTC Design Note 335 and App Note 91 discuss the concept of using a voltage divider for measuring a Tx power. Be aware that stray + parasitic capacitance of the resistor touching the RF mainline can introduce some tilt and ripple onto the detector response. Another problem with the voltage divider method is no directivity, which means that antenna mis-tuning or dielectric loading can (and will) affect the reading at the detector. Nonetheless, many customers find that the voltage divider method is 'good enough'. 

    2) For more accurate measurements, a directional coupler can be utilized. Such devices have a directivity spec, which characterizes the ability of the coupler to separate the measurement of incident wave from reflected wave. There are some small, low-cost ceramic components which perform this function, for example the Johanson 2450CP05A5400 dual-band coupler, also available under different part number from ACX. When using these couplers, be sure to follow all manufacturer's recommendations regarding PCB layout. 

    Regarding LTC5532 gain: Higher gain is OK, but understand that the amplifier's closed-loop bandwidth is inversely-proportional to gain. And for higher gain settings, it becomes increasingly likely that the output voltage could hit the positive rail. Gain= 2 or Gain= 4 is recommended for correlation with datasheet. Gain= 6 maybe OK, but Gain= 8 seems excessive, somewhat limiting the detector's useful dynamic range.

    For Wi-Fi signals, best power measurement accuracy will be obtained with an rms-responding detector, for example the ADL5501 (linear in Volts per Volt), or ADL5903 (linear in dB's per Volt). RMS detectors are designed for high accuracy with high crest factor waveforms such as Wi-Fi. Schottky detectors such as LTC5532 are more responsive to the peak of a complex signal, thus introducing an error which may or may not be of interest, depending on the application. 

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  • Hello,

    An RF coupling circuit can be a 1) resistive/capacitive voltage-divider, or 2) based on coupled transmission lines. Each is explained below:

    1) LTC Design Note 335 and App Note 91 discuss the concept of using a voltage divider for measuring a Tx power. Be aware that stray + parasitic capacitance of the resistor touching the RF mainline can introduce some tilt and ripple onto the detector response. Another problem with the voltage divider method is no directivity, which means that antenna mis-tuning or dielectric loading can (and will) affect the reading at the detector. Nonetheless, many customers find that the voltage divider method is 'good enough'. 

    2) For more accurate measurements, a directional coupler can be utilized. Such devices have a directivity spec, which characterizes the ability of the coupler to separate the measurement of incident wave from reflected wave. There are some small, low-cost ceramic components which perform this function, for example the Johanson 2450CP05A5400 dual-band coupler, also available under different part number from ACX. When using these couplers, be sure to follow all manufacturer's recommendations regarding PCB layout. 

    Regarding LTC5532 gain: Higher gain is OK, but understand that the amplifier's closed-loop bandwidth is inversely-proportional to gain. And for higher gain settings, it becomes increasingly likely that the output voltage could hit the positive rail. Gain= 2 or Gain= 4 is recommended for correlation with datasheet. Gain= 6 maybe OK, but Gain= 8 seems excessive, somewhat limiting the detector's useful dynamic range.

    For Wi-Fi signals, best power measurement accuracy will be obtained with an rms-responding detector, for example the ADL5501 (linear in Volts per Volt), or ADL5903 (linear in dB's per Volt). RMS detectors are designed for high accuracy with high crest factor waveforms such as Wi-Fi. Schottky detectors such as LTC5532 are more responsive to the peak of a complex signal, thus introducing an error which may or may not be of interest, depending on the application. 

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