Linear RF Power Meter

Posted on Feb 5, 2014

The National Semiconductor LMV225 is a linear RF power meter IC in an SMD package. It can be used over the frequency range of 450 MHz to 2000 MHz and requires only four external components. The input coupling capacitor isolates the DC voltage of the IC from the input signal. The 10-k resistor enables or disables the IC according to the DC voltage

Linear RF Power Meter
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present at the input pin. If it is higher than 1. 8 V, the detector is enabled and draws a current of around 5 8 mA. If the voltage on pin A1 is less than 0. 8 V, the IC enters the shutdown mode and draws a current of only a few microampG¨res. The LMV225 can be switched between the active and shutdown states using a logic-level signal if the signal is connected to the signal via the 10-kR resistor. The supply voltage, which can lie between +2. 7 V und +5. 5 V, is filtered by a 100nF capacitor that diverts residual RF signals to ground. Finally, there is an output capacitor that forms a low-pass filter in combination with the internal circuitry of the LMV225. If this capacitor has a value of 1 nF, the corner frequency of this low-pass filter is approximately 8 kHz. The corner frequency can be calculated using the formula fc = 1 G· (2 p COUT Ro) where Ro is the internal output impedance (19. 8 k ). The output low-pass filter determines which AM modulation components are passed by the detector. The output, which has a relatively high impedance, provides an output voltage that is proportional to the signal power, with a slope of 40 mV/dB. The output is 2. 0 V at 9 dBm and 0. 4 V at 40 dBm. A level of 0 dBm corresponds to a power of 1 mW in 50 R. For a sinusoidal wave-form, this is equivalent to an effective voltage of 224 mV. For modulated signals, the relationship between power and voltage is generally different. The table...

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