10MHz spectrum analyzer monitors 550MHz signal

The circuit for monitoring 550MHz signals with a 10MHz spectrum analyzer is shown in Figure 1. The inverter gate acts as an oscillator, generating the necessary harmonics. The 74HC04 inverter constitutes an oscillator, and its frequency is determined by the power supply voltage. When the power supply changes from 5 to 5.5V, its frequency changes between 80MHz and 250MH. Table 1 gives the corresponding relationship between power supply voltage and frequency. The third harmonic is 750Mz, and the fundamental wave is filtered out at 16dBm.

The mixer is an LMX2216 (its low noise amplifier is not used in circuit 1). The mixer output is passed through a simple bandpass filter to reduce spectrum analyzer aliasing. Harmonics and the sum and difference frequencies from the oscillator in the measurement are important to observe the correct signal and must be treated with great care.

The mixer and oscillator should be mounted on a copper-clad board, and all connections should be soldered to the copper board. Traces should be kept as few and short as possible (1 inch of wire represents 25nH). The power supply pins on active devices should be decoupled. The small capacitor is of sm COG type. Rf should be connected via 50Ω.

Table 1