Spectrum analysers are one of the most fundamental devices of frequency domain measurement methods. Nowadays the wired and wireless network infrastructures operate on higher and higher frequencies, thus measuring broadband spectrum becomes essential. Modern microwave spectrum analysers do have excellent parameters (frequency range, noise figure, dynamic range, sweep time, class of accuracy), however, many applications do not require an expensive precision measuring instrument but a simpler and less pricy version of that with a high dynamic range.
In this paper we introduce the design of a spectrum analyser operating to around 6 GHz. The core element of the designed hardware is an ISM band radio transceiver which has high receive sensitivity and is able to measure the received signal strength indicator (RSSI). Considering that superheterodyne principles make it possible to extend the receivable frequency band, for the transceiver chip we have designed a microwave interface containing two mixer blocks, a local and a tuneable oscillator. The interface transposes the received broadband signal to a discrete, narrow frequency band. The frequency stages in the signal path are created in such a way that the image frequency is strongly attenuated while the frequency domain requirements against the subunits (filters, amplifiers, mixers, oscillators, etc.) are still met. Most of the components of the system are modern but inexpensive integrated circuits, furthermore two entirely self-designed microwave filters (a low- and a band-pass filter) also take place in the system. The components of the device are controlled by a microcontroller which transmits the measurement data via UART.
In this paper after a short theoretical overview I introduce the system plan which clarifies the architectural structure of the system. Following that, the quite serious requirements against the components and then the circuit diagram and the printed circuit board design process of the whole system are presented. I introduce the building and measuring method of each circuit block. I sketch the structure of the self-developed firmware running on the microcontroller. The implementation of the firmware is reduced only to monitoring the ISM band at 2.4 GHz. In the last section of this paper I evaluate the system properties of the realized device and I also reveal the successful and less effective aspects of the whole design process.