Introduction
The Sanderling project is an integrated six-port sub-circuit designed using 65 nm CMOS technology and optimized to operate at 15 GHz microwave frequencies. This innovative circuit aims to facilitate non-invasive measurement of blood glucose levels by analyzing the dielectric properties of dual-layer materials, such as human skin and underlying tissues.
Circuit Design
The Sanderling circuit features six ports, each serving a specific function:
- Port 1 (CM): Common Mode input
- Port 2 (DM): Differential Mode input
- Port 3 (MEAS1): Measurement port for Device Under Test 1 (DUT1)
- Port 4 (MEAS2): Measurement port for Device Under Test 2 (DUT2)
- Port 5 (DUT1): Connection to Device Under Test 1
- Port 6 (DUT2): Connection to Device Under Test 2
Ports 1 and 2 are inputs to a -3 dB, 180-degree hybrid coupler, optimized for operation at 15 GHz, which splits and phases the signals appropriately. The outputs of this coupler are connected to two directional couplers, also designed for microwave frequencies, whose forward paths lead to DUT1 and DUT2. Reflected signals from the DUTs are directed to MEAS1 and MEAS2, enabling precise measurement of impedance mismatches at high frequencies.
Operating Modes
- Common Mode (CM): When a signal at 15 GHz is applied to Port 1, the hybrid coupler outputs signals that are in phase, exciting DUT1 and DUT2 simultaneously. This mode generates longer-range electric fields between the conductors and ground, allowing deeper penetration to measure subcutaneous layers, including blood.
- Differential Mode (DM): Applying a 15 GHz signal to Port 2 results in outputs that are 180 degrees out of phase, exciting the DUTs in opposite phases. This mode produces shorter-range electric fields between the two conductors, ideal for measuring the skin layer due to shallow penetration.
Dual-Mode Open-Ended Coaxial Probe
Connected to DUT1 and DUT2 is a dual-mode open-ended coaxial probe (OECP) designed for measuring the dielectric properties of layered materials at microwave frequencies. The probe consists of two conductors placed at a specific distance (x) apart, with a ground plane positioned at a distance of n × x (where n > 1) away from the conductors. This configuration allows for:
- Differential Mode Measurements: Short-range electric fields between the conductors for skin layer analysis.
- Common Mode Measurements: Long-range electric fields between conductors and ground for deeper tissue analysis, including blood.
By comparing data from both modes at 15 GHz, it’s possible to isolate the dielectric properties of blood and calculate blood glucose levels accurately.
Varactor Integration and Phase Tuning
To enhance the system’s adaptability and measurement accuracy at microwave frequencies, the design incorporates six varactors within the hybrid and directional couplers. These voltage-controlled capacitors enable dynamic tuning of phase and impedance characteristics by adjusting the varactor voltages. This tunability allows for:
- Phase Correction: Ensuring signals reaching DUT1 and DUT2 are properly phased for both CM and DM excitations at 15 GHz.
- Impedance Matching: Minimizing reflections and optimizing excitation of the DUTs.
- Dynamic Adaptability: Adjusting to variations in the DUTs or environmental conditions for robust measurements.
Testing and Future Work
Initial testing of the S-parameters at 15 GHz indicates that the circuit performs as expected, with the varactors effectively correcting phase discrepancies and improving impedance matching. The next steps involve integrating the OECP and conducting comprehensive tests to measure the dielectric properties of dual-layer materials at microwave frequencies. These efforts aim to refine the model for isolating blood dielectric values and enhance the accuracy of blood glucose calculations.
Conclusion
The Sanderling project represents a significant advancement in non-invasive biomedical sensing technology. By combining sophisticated circuit design optimized for 15 GHz microwave frequencies with innovative probing techniques and tunable components, it holds promise for accurate, real-time monitoring of blood glucose levels without the need for invasive procedures.