The Log_Out pin creates an envelope proportional to the incoming signal; therefore, the frequency and phase information of the input signal is lost. The frequency detect feature enables the LOG300 to recover this frequency information. The frequency detect pin toggles at the same frequency as the input, and can therefore be used to calculate the input signal frequency or the input signal zero crossing points.

The internal circuit consists of a comparator with one of the inputs connected to the pin Log_In bias voltage and the other input of the comparator connected to the last stage of the Log Detector gain block (see Figure 7-3). Log_In is ac coupled; therefore, the incoming signal is biased to the internal bias voltage.

The comparator compares the gained Log_In (both single-ended and differential) signal biased at the internal bias voltage with the DC internal bias voltage, thereby toggling at every instance of a zero crossing. If the incoming signal is not a symmetric waveform or consists of multiple frequency content, the frequency detect feature compares the zero crossings of the incoming signal.

Figure 7-3 Internal Block Diagram for Frequency Detect