There are two commonly utilized methods for measuring the SAW device response [27
]. One is frequency detection, as shown in Figure 2
a. This basic testing system consists of an RF amplifier for a feedback loop and a frequency counter. The electronic circuit should satisfy the Barkhausen stability criterion: (1) The loop gain is equal to unity in absolute magnitude, that is, |βA
| = 1; and (2) the phase shift around the loop is zero or an integer multiple of 2π, that is, entire phase shift = 2πn (n = 0, ±1, ±2, ····), upon which, the entire circuit would oscillate as a resonator around the center frequency of the SAW device [28
]. Though this method only requires very simple circuit design, the system is not stable and is easily affected by the environment, which usually causes frequency hopping. As there could be many frequency points that satisfy the oscillation starting conditions, once the SAW is affected by a big perturbation, the circuit will self-oscillate at a different frequency. In addition, this system also requires a high-speed frequency counter that is not easily miniaturized for portable application. The other method is phase detection using a phase detector, as shown in Figure 2
b, comparing the input/output difference of the SAW device [21
]. The system gets an RF source with a certain frequency and splits the signal two ways, one passes through the SAW device and the output signal is compared with the other signal. The phase difference as a voltage output can be easily measured by a voltmeter. Since it is not based on feedback, this detection system is not susceptible to frequency hopping and would be very stable. However, to achieve simultaneous sensing and NSB removal, there will be a problem of supplying a RF signal source for both the sensing and removal inputs. Direct digital synthesis (DDS) [29
] would be a good method of producing an analog waveform by utilizing a time-varying signal in a digital form and converting using a digital-to-analog converter (DAC). It can offer fast switching of frequency, a wide output bandwidth, and very high-frequency resolution. Thus, we designed a DDS based system with time division multiplexing for this dual function sensor system.
3.1. Overall System Design
The electronic circuit system consists of three boards and is assembled as a portable instrument, as shown in Figure 3
. The bottom layer is based on the Arduino development board with Intel®
Curie™ Module. This layer provides the microcontroller for the main program operating the system. It provides serial communication with the local computer and the control with other IC chips. The middle layer, which is the main circuit board, is designed for the power supply, DDS signal generation, signal processing, and data acquisition/converting, etc. The top layer is a specially designed PCB board as a chip holder for the SAW device with connecting clips and buffer circuit.
To realize a real portable system, some essential peripheral designs are added to this system. The power supply of this system can range from 7–12 V which is easily obtained from a commercial power adapter. Communication between the portable prototype and compute is via USB cable. The data communication, storage, processing and displaying are conducted with the self-developed SAW sensor data monitoring software. The SAW device is loaded on a specially designed board with eight clips touching with each electro pad of the SAW sensor. This board is connected with the main circuit board via SubMiniature version A (SMA) connectors of 50 Ω impedance. In addition, these electronics are housed in a 3D printed shell package. The dimensions of the portable device are 110 × 110 × 80 mm.
3.2. Main Circuit Design
shows the designed electronic system to measure phase, transmission loss, and frequency for the SAW sensor device. The RF signal is generated from the 32-bit DDS chip (AD9858, ADI, Norwood, MA, USA) and the working frequency is calculated and set by the microcontroller. This synthesizer is able to offer high resolution of 0.233 Hz and a wide bandwidth sine wave up to 400 MHz with special designed temperature compensated crystal oscillator (TCXO). The TCXO provides a stable reference with frequency stability of 0.5 ppm at 1 GHz, which is eligible for biotesting at room temperature. The sine wave is amplified by a digital controlled variable gain amplifier (DVGA), which can provide a +19 dB gain, with final output power about +16.4 dBm. The internal-integrated digital controlled attenuator of this amplifier provides a power attenuation coefficient ranging from 0 dB to −31.5 dB (the final gain is from −12.5 dB to +19 dB). After amplification and low-pass filtering, the RF signal is delivered into two channels controlled by the RF switcher, according to the purpose of use. One channel is separated via the 2-way 0° power splitter, and utilized to load the two sides of the removal IDTs. The other channel is separated into two paths via the power splitter as well, with one signal sent to the SAW sensing device as the input signal for sensing, and the other signal as one input source of the gain/phase detector as a reference signal. The output signal of the SAW device, as another input source, is compared with the reference signal via the gain/phase detector. The output voltage of the gain/phase detector, as a function of the two input signals’ amplitude/phase differences, represents the insertion loss and phase shift of the SAW device. The voltage value is obtained by the microcontroller via the 12-bit analog-to-digital converter (ADC), which offers a phase angle shift resolution of 0.044° and insertion loss shift of 0.0147 dB, respectively. Eventually, after the processing and calculations, the data are received by the local computer device.