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This file type includes high resolution graphics and schematics when applicable. | Designers sometimes need to analyze the spectrum of a modulated carrier. Although they can do this by down-converting and then digitizing and signal processing in the digital domain, the high-speed analog-to-digital converter (ADC) and the processor power required will reduce battery life. Instead, by down-converting in the analog domain and splitting the signals into discrete bands, the design can use a much slower, less power-hungry microcontroller.
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This can be achieved, for example, by using an IC such as Mixed Signal Integration’s Mixed Signal MiXer and Very High frequency Filter (MSMXVHF) to baseband, while the Mixed Signal Low powered Spectrum Analyzer (MSLSA) provides spectrum analyzer outputs at 1/6th-octave spacing.
The MSLSA sweeps with octave step increments using the 74HC4060 oscillator/divider and the 74HC151 one-of- eight selector. By selecting each output from the 74HC4060 with the 74HC151, one can quickly step through the baseband up to 100 kHz.
In the portable spectrum analyzer of Figure 1, the MSMXVHF includes a switching mixer that functions up to 600 MHz. The output of the mixer is limited to 1 MHz by a second-order, continuous-time low-pass filter. The MSMXVHF also features a selectable low-pass/band-pass switched-capacitor filter with operation up to 1 MHz (clock at 12.5 MHz). The output of the mixer is ac-coupled externally to the input of the filter, while the output of the filter is ac-coupled to the input of the MSLSA. The MSLSA has six outputs that can be monitored with a scope or the ADC of a microcontroller.