The implementation of a spectrum analyzer with a tracking generator, which as a result of the measurement gives an accurate representation of the conditions at its input, has remained mostly unchanged for the past 50 years. Advances in the manufacture of semiconductors and integrated circuits allow for more compact installation, complex processing, and a lower price of the final device. The latest technology paves the way for completely digital signal processing, but the most prestigious instruments from reputable manufacturers still resort to a much more expensive and developmentally more complex analog version, conceptually identical to those from half a century ago.
We can trust the result of completely analog signal processing with certainty, since errors occur mostly due to our improper use (excessive input power, inadequate power level of the analog circuit). The processing is performed by many dedicated circuits, which appropriately transform and process the measured signal. The problem arises while making very narrow frequency filters to achieve better selectivity, as these are much easier to implement in the digital domain, so much so that it tempts even more serious manufacturers. Unfortunately, digital implementations are often the source of insidious errors that produce an unreliable measurement result, which are very difficult to detect and even more difficult to correct, since the instrument software is protected by heavy encryption mechanisms.
Most modern spectrum analyzers with a tracking generator are still beyond the reach of the majority of engineers, as even the finest research institutions can hardly afford them. Manufacturers approached the reduction of the price of measuring devices by replacing the most expensive dedicated parts for cheaper widely used building blocks, and above all by switching to the digital domain very early in the analog signal path (implementing all band-pass filters in dedicated DSP routines). This significant cheapening of the instrument brings its limitations. An analog-to-digital converter is limited by its dynamic range and its sampling frequency defines the widest filter range that dictates the highest rate of frequency sweep. Measuring the transfer function of high-quality filters or observing fast avionics signals is almost impossible with such devices.
This work presents an innovative approach to the design and construction of a spectrum analyzer with tracking generator, working between 100 kHz and 4 GHz, and fully analog signal processing, without the use of expensive dedicated building blocks, and with manufacturing processes that can be replicated in the home workshop. Due to the way of design, the final price of the product is much lower than the professional versions, and the key components, which can be destroyed by improper handling, are easily replaceable. The spectrum analyzer acts as a receiver by repeatedly mixing the input signal to different intermediate frequencies, whereby the local oscillator of the first mixer performs a frequency sweep, while all other oscillators are set to constant frequencies.
The dissertation presents a novel method of performing wide-band frequency sweep between 4.3 GHz and 8.3 GHz using several narrow-band oscillators and switching between them. It thereby provides a high-quality signal for the first local oscillator with low phase noise and without unwanted products of fractional phase-locked loop operation. The drop in output power with increasing frequency is eliminated by the innovative use of a photovoltaic module to bias the gate of a dedicated low-noise amplifier.
All filters are made analog. Cavity filter at the first intermediate frequency of 4.3 GHz is constructed with wide-access aluminium profiles, and LC filter at lower frequencies by changing the quality of the resonant circuit and thus electronically selecting the bandwidth between 3 MHz and 30 kHz. An even narrower filter is designed using piezoelectric resonators, which enables the implementation of the narrowest bandwidth down to 1 kHz.
In addition to the desired high-frequency output signal, the tracking generator also includes a duplicate structure of mixers and local oscillators that provide a reference signal to the phase detector, thereby expanding the range of various measurements of the end device. This processing is also completely analog, which allows for independent changing of the frequencies of all local oscillators and preservation of calibration data in case of power loss. Signal path can be independently controlled via simple character commands understood by the microcontroller software.
All building blocks are thoroughly described, together with all of the requirements and constraints. Detailed electrical diagrams of all circuits and key parameter measurements that shape the instrument's end capabilities are shown. Special emphasis is given to the phase noise of the first local oscillator, which limits the usefulness of the narrowest bandpass filters. The generation of the frequency sweep is performed by sequentially connecting two phase-locked loops with the corresponding low-noise narrowband oscillators, whereby the first oscillator of the chain operates in fractional mode, and its output is used to clock the phase detector of the second oscillator in integer mode. By choosing the STuW81300 integrated circuit, 128 narrowband oscillators, managed by a microcontroller, are available.
The choice of the first mixer in the analog chain will have a significant impact on the measured signal's distortion. The use of a commercially available building block in place of the mixer proves to be sensible due to its low cost and easy installation. Because it is a sensitive component that limits the input power of the instrument, accurate measurements of the third-order intermodulation distortion intersection and saturation points are given for the entire frequency range.
Furthermore, a new version of the bandpass filter with electronic selection of its width is presented. Controlling the quality of the LC resonator, and thus the width of the passband, is performed via PIN diodes, whereby the change in diode capacitance during switching is small enough to not cause a shift in the central frequency of the oscillating circuit. Tuning is much simpler than in case of the industrial equivalents, which simplifies the handling of the instrument and does not require additional calibration during the operation of the device itself.
Digital sampling of the analog signal after final processing is performed by the STM32G441 microcontroller, which simultaneously performs the required steps in setting the phase-locked loops of all local oscillators. It sends aggregated data of the detected signal's amplitude and phase to the user interface, which are shown on an interactive touch screen.
The presented instrument stays true to the proven analog design, employs modern, reasonably priced building blocks for the generation of local oscillator signals, and increases the scope of tracking generator measurements by adding a vector measurement mode.
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