2.1

The general construction

A microwave frequency discriminator in a quadrature version based on microwave ring couplers (rat-race couplers) was developed for the works on methods and devices for monitoring electromagnetic space.

The production of this type couplers in the microstripline technique does not require complex technology. Such elements can be made by photolithography or milling. The visual structure of the developed discriminator was shown in Figure 1. A microwave signal of unknown frequency f_inp is fed to the RF Input port made as an SMA connector. At next stages, the input signal is divided into several paths by PD1, PD2, PD3 power dividers. These are single-step Wilkinson-type power dividers with resistor R. Then, the rat-race couplers RRC1 and RRC2 perform vector summation of the particular signal pairs. Results of summations from output ports of directional couplers are passed to microwave amplitude detectors D1, D2, D3, D4 which have square transient amplitude characteristics. The voltages U1, U2 U3, U4 from the detectors are proportional to power of directional couplers output microwave signals’ and carry an information about frequency f_inp of signal feeding RF Input port.

Figure 1.

Structural diagram of the integrated quadrature microwave frequency discriminator with rat-race couplers.

The temporary value of input signal’s frequency can be estimated and expressed by the symbol f_m according to formula (1).

where: f₀ – center frequency of MFD measurement range, W_m, k₁, k₂, k₃, k₄ – coefficients distinctive for given discriminator.

Transmission lines connecting the ports of individual elements are symbolically represented by straight lines in Figure 1. The bands of discriminator’s unambiguous frequency measurement is determined by the relation of physical lengths of lines L1 and L2. The greater the difference in the length of these lines, the better the measurement resolution is, but at the same time the width of the discriminator’s unambiguous measurement range becomes narrower. The phase shifter PF inserted in front of one of the input ports of rat-race coupler RRC2 enables to achieve a quadrature dependence of voltage differences U1 – U2 and U3 – U4 in reference to frequency f_inp of the input signal.

The entire discriminator’s correlator, i.e. transmission lines, power dividers and ring couplers, was made in an integrated version on one printed circuit board (Figure 2a). Microwave detectors were connected using SMA connectors, which were mounted on the bottom side of PCB (Figure 2b). Thanks to such configuration, crossings of correlator’s internal lines were avoided. Method of signal flow graph was used at the initial stage of device development. The final version of PCB was designed using the Genesys RF simulator.

Figure 2.

Integrated correlator of microwave frequency discriminator with rat-race couplers, a) view from side of paths, b) view from side of SMA connectors.

2.2

Selected results of measurements

The measurement results of the developed and fabricated quadrature microwave frequency discriminator with rat-race couplers are presented in Figures 3 – 6. In the Figure 3 there are presented plots of output voltages U1, U2, U3, U4 versus frequency f_inp of the input microwave signal. They are close to sine-type waveforms around the designed frequency f₀ ≅ 2.45 GHz. Their shapes below a frequency of around 1.675 GHz and above of 3.145 GHz are unacceptable for instantaneous frequency measurement, but between these frequencies they are appropriate for such purposes. Discriminator’s transient characteristic i.e. estimated frequency f_m versus frequency f_inp of input signal was shown in Figure 4. As it can be seen, between frequency of 1.675 GHz and of 3.145 GHz, its shape is very close to a straight line. These frequency values are the boundaries of the discriminator’s operating frequency band. Over this band measurement of frequency is unambiguous. According to the assumptions, the discriminator can be used to observe and measure the instantaneous frequency of signals used, among others, by 2.45 GHz WiFi systems. The operational bandwidth is close to one octave. The ratio of the upper (f_up=3.145 GHz) and lower (f_lo=1.675 GHz) frequency of the band (Figure 4) is about of f_up / f_lo = 1.88. In the frequency range from about of 1.95 GHz to about of 3.145 GHz, the measurement deviation (Figure 5), determined in conditions that the estimation of the f_m value will be made directly without the use of any correction algorithms, is not greater than ±20 MHz. However, around of 1.8 GHz, the frequency measurement deviation increases to around of 60 MHz.

Figure 3.

Integrated microwave frequency discriminator’s output voltages versus frequency of input microwave signal.

Figure 4.

Transient characteristic of the developed quadrature microwave frequency discriminator with rat-race couplers made in the integrated version.

Figure 5.

Frequency measurement deviation of the developed integrated quadrature microwave frequency discriminator with rat-race couplers (results of measurement).

Figure 6.

Polar version of transient characteristic of the developed integrated quadrature microwave frequency discriminator with rat-race couplers (results of measurement).

Microwave frequency discriminator’s properties can be very accurately illustrated by means of polar graph, which was presented in Figure 6.

Axis X in this graph was fed by difference U4-U3 but axis Y was fed by difference U2-U1. This kind of visualization of information from MFD provides estimation both of temporary frequency value and of temporary amplitude (strength) value of input signal as well.