The design and manufacture of the device comprise three phases that are explained below:
2.1.1. Data Acquisition System
The proposed system consists of an array of fractal antennas (FRACTUSANTENNAS, Barcelona, Spain) to cover the frequency range between 78 MHz and 6 GHz. These antennas are suitable for this design because they can cover large bandwidths and have small dimensions compared to other types of commercial antennas. This makes it possible to optimize space on the printed circuit board (PCB).
Table 3 shows the models and bandwidth of the fractal antennas used for the proposed exposimeter. The plots of radiation pattern of each antenna are shown in
Appendix B.
Figure 1 shows the general block diagram of the system operation, where the fractal antennas from
Table 3 are multiplexed by an Analog Devices HMC321ALP4E RF mux (Analog Devices, DE, USA), which has eight inputs and one output that enables 3:8 binary TTL decoding control; the switching time (
TS) is 125 nsec; and the RF mux switch to the next antenna when the device has measured all its bandwidth. The signals from the antenna are amplified by an Analog Devices ADL5542 low noise amplifier (LNA), which has a maximum gain of 20 dB. The Analog Devices ADF4355-3 fractional-N phase-locked Loop (PLL) works as an RF signal generator, covering the range from 54 to 6600 MHz; the total lock time of the PLL to generate a signal (
TPLL) is 45 µs. The signal generated by the PLL and the signal received from the antennas are multiplied in the mixer in order to lower the frequency of the antenna signal to a frequency that matches the 315 MHz center frequency and 300 kHz bandwidth of the SAW bandpass filter; the Analog devices ADL 5802 mixer and the AFS315E-T filter from Abracon LLC were chosen. The filtered signal is measured by an Analog devices AD8309 logarithmic RF detector that provides a voltage output signal proportional to the power of the input signal in dBm; the output settling time of the logarithmic detector (
TLD) is 220 ns. The output voltage of the logarithmic detector is sampled using an Analog-to-Digital Converter (ADC) and recorded in the microcontroller; the ADC rate (
TADC) is 1 Msamples/s (1 µs for each sample). The time to measure one sample (
T′) is described as follows:
The proposed system measures the received power in the spectrum from 78 MHz to 6 GHz with 300 kHz of resolution bandwidth, measuring approximately 19,500 narrow frequency bands. Therefore, the total time to measure all the bands is described as follow:
where
TT = 901.915 µs is the total time to measure all the bands;
nS is the number of samples (19,500);
nA is the number of antennas; finally,
TS is the switching time of the RF mux.
Finally, the measured signals are stored with the GPS geolocation data in a non-volatile SD memory that can store up to 2 Gbytes of data. The system has a USB communication port to communicate with the computer and download the set of records made. In
Figure 2, the flow chart for the measurement process is shown.
The PLL works as a tuner of the signals recorded by the antennas; therefore, consider that the PLL generates a signal
S1, and the antenna array receives a signal
S2. These can be described as follows:
where
k is the amplification value due to the low noise amplifier,
A1 and
A2 are the amplitudes corresponding to each of the signals, and likewise, their frequencies are
f1,
f2 and phases are
θ1 and
θ2, respectively. These are multiplied by the mixer, and the following equation is obtained with the signal mixer theory [
36].
The signal generated by the PLL signal synthesizer is 315 MHz different from the received signal; this allows it to match with the center frequency of the bandpass filter; therefore,
f1 −
f2 is equal to 315 MHz, and Equation (7) would be as follows:
Then, through the bandpass filter, the signals with a frequency different than 315 MHz are filtered. Giving the next equation:
Then, by the signal theory [
37], the power of the
PS1S2 signal is expressed with the following equation:
According to Equation (11), the power of the signal depends only on its amplitude; therefore, the logarithmic detector detects the power of the PS1S2 signal expressed in dBm. This value is registered and stored in non-volatile memory using communication with the microprocessor, which also has the function of controlling the RF mux, the PLL, and the GPS to obtain the geolocation of each measurement performed.
As shown in
Figure 3, the proposed system consists of two PCBs fitting one on top of the other, incorporating all the components mentioned above. The first PCB (
Figure 3a) integrates the radio frequency components; the second PCB (
Figure 3b) includes the microcontroller, GPS, SD memory, the components for battery charging, and voltage regulators. The antennas share space between the two PCBs and are connected using coaxial cables. The PCBs were designed with enough free space in the PCB in order to avoid electromagnetic interference; this is specified in the datasheets from FRACTUS ANTENNAS. The system uses 2-cell lithium-ion (Li-ion) batteries of 3500 mAh in series, with low noise linear regulators 3.3 and 5.0 V for power supply. The average current consumption is estimated at 450 mA, in continuous operation mode, allowing the system to run for several hours without the need to charge the batteries.
The proposed system has a sampling rate of 20,000 samples/s, taking less than one second to measure the entire frequency spectrum. The memory provides up to one week of non-volatile memory space. The system has a dynamic power measurement range of 90 dB with RF input power from −70 to +20 dBm and a 0.04 dBm resolution.
Table 4 shows the specifications of the proposed system.
2.1.2. System Calibration
The calibration of the device was performed in the anechoic chamber of the Universidad Politécnica de Madrid. The measurements were taken at a distance of 2 m between the transmitting antenna and the proposed exposimeter, evaluating only the far-field from 698 MHz to 6 GHz. The PNA network analyzer (Agilent Technologies E8362B, Santa Clara, CA, USA) was used to generate the signals, and 10 dBm of power was configured.
Figure 4 shows a block diagram of the exposimeter calibration system.
As shown in the block diagram in
Figure 4, a computer controls the PNA to generate the signals to be measured and communicates with the exposimeter to perform the corresponding measurement, to obtain: first, the data of return losses delivered by the PNA and, second, the measurements of power received by the exposimeter. According to the antenna theory, have the following equations [
38]:
where
RL is the return loss expressed in dB generated by the PNA, and
Γ is the reflection coefficient, which indicates the percentage of the reflected signal. Solving Equation (12), the reflection coefficient is obtained:
To find the percentage of transmitted power, the following equation was used:
In Equation (14), T is the percentage of transmitted power and allows the output power of the transmitting antenna to be calculated. In order to obtain the power received by the antennas, the free space attenuation equation was used [
39]:
where
Lbf is in dB,
f is the frequency in MHz, and
d is the distance in meters. Therefore, the power received by the antennas
Pr as a function of the transmitted power
PT, is expressed as follows:
The signals obtained by the PNA are processed using Equation (16) to obtain the received power value and relate it with the measurements obtained by the exposimeter to create the calibration curves.
2.1.3. Software Development: Data Processing and Visualization System
The stored data by the exposimeter is transmitted to the PC using LabVIEW software (National Instruments Company, Austin, TX, USA) and, the data is processed using Matlab software (MathWorks Company, Natick, MA, USA), where each power measurement is corrected through the calibration wave. To obtain the data in power density (W/m
2), the following equations according to the theory of radiation parameters were used [
40].
where
S is the power density,
Pr is the received power in dBm,
λ is the wavelength expressed in meters, and
G is the maximum orientation of the antenna that is considered to be 1.5 so that the effective area
A is a function of the frequency. Therefore, Equation (17) can be written as:
Equation (20), for power density, is expressed in mW/m2 and is a function of the frequency, which is applied to each measure in order to obtain the final result.