Wireless Circuits and Systems Laboratory:Single-Ended Mixer

Abstract

The aim of this experiment is to become familiar with single-ended mixer, its fabrication, tests, and learn about DC return path, harmonic tuning and how its absence and presence affects the mixer performance. A mixer is a device which combines the RF and LO signals to produce IF signal using a non-linear device like diode. The experiment involves simulation analysis, RF choke characterization, and diode behavior as a video frequency detector.

1. Introduction

The primary objective of this experiment is to study and analyze the performance of a single-ended mixer.

The experiment consists of four parts:

1. Simulation of mixer performance with and without harmonic tuning using Advanced Design System (ADS).
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2. Characterization of RF choke inductors to block RF and LO signals while allowing DC current to pass.
3. Measurement of RF and LO signals using a mini-circuits signal generator and VCO.
4. Investigation of diode behavior as a video frequency detector.

2. Mixer Simulation Using ADS

In this section, we conducted simulations of two mixer configurations, Test A (with a DC return path but no harmonic tuning) and Test B (with a DC return path and harmonic tuning), using Advanced Design System (ADS). The objective was to compare their performance based on conversion loss and mixing products.

From the simulation results, it is evident that Test B, with harmonic tuning, exhibits lower conversion loss compared to Test A. The harmonic tuning stub in configuration B effectively blocks harmonics, allowing more power to go into the fundamental IF frequency.

3. RF Choke Characterization

In this section, we characterized RF choke inductors to understand their behavior in blocking RF and LO signals while permitting DC current flow. The inductors were tested on a dedicated test board with a series-mounted inductor of 820 nH between two 50-ohm lines. The S-parameters were measured using a Vector Network Analyzer (VNA).

At lower frequencies, the inductor allows signals to pass through, but as frequency increases, the reflection coefficient increases, and the transmission coefficient decreases, indicating effective blocking of RF and LO frequencies.

In the case of non-ideal inductors with parasitic capacitance, a parallel resonance may occur. At this point, the impedance of the inductor increases, resulting in self-resonance. After resonance, it acts as a capacitor, allowing more signals to pass through at higher frequencies.

4. Quadrature Hybrid Coupler

A quadrature hybrid coupler was used in the mixer design to split power equally between input signals, providing a 90° phase difference at its output ports. This component plays a crucial role in mixer performance.

5. Measurement of RF and LO Signals

In this section, a Mini-Circuits signal generator served as the RF source, and a Voltage-Controlled Oscillator (VCO) acted as the LO source for the mixer. Output powers of both signals were measured using a spectrum analyzer, resulting in RF power at the end of the coaxial cable being 5.2 dBm, and LO power at 8.7 dBm.

Measurement of different mixing products for two types of mixer circuits, Type A (with a DC return path but no harmonic tuning) and Type B (with a DC return path and harmonic tuning), was performed. Comparison between simulated and fabricated results indicated that both configurations exhibited higher conversion loss when no harmonic tuning stub was present.

The conversion loss (Lc) can be calculated as:

Lc = P_if - P_rf

Simulation conversion loss for Test A and B were 16.389 dB and 10.685 dB, respectively. Circuit-fabricated conversion loss for Test A and B were 14.58 dB and 12.8 dB, respectively.

Harmonic tuning stubs effectively suppressed harmonics, leading to a decrease in conversion loss and an increase in power directed to the IF signal. Additionally, the 1st harmonic of RF and LO signals changed from -19.78 dBm to -18 dBm when the tuning stub was connected, resulting in a reduction in conversion loss.

The shorted stub at the diode input, being a quarter-wavelength long, acts as a shunt open circuit for RF and LO frequencies while providing a DC connection between the diode and ground. This arrangement effectively blocks RF and IF signals from interfering with the diode operation.

6. Diode as a Video Frequency Detector

In the final part of the experiment, we demonstrated how a diode can function as a video frequency detector. Measurements of DC voltages at different drive levels on the Vector Network Analyzer (VNA) were taken, and responsivity was calculated as the ratio of DC voltage to the power of the drive level (in mW).

It was observed that as the input power level doubled, the voltage at the mixer output also doubled, especially at low RF input power levels. This behavior is consistent with the diode operating as a square-law detector. However, at higher input power levels, the diode reached saturation, and the DC voltage no longer varied with the input RF power level.

7. Conclusion

In conclusion, this experiment allowed us to gain insight into the performance of single-ended mixers. The presence of harmonic tuning stubs significantly improved conversion loss by blocking unwanted harmonics and enhancing the power directed to the fundamental IF frequency. Additionally, the behavior of RF choke inductors was characterized, demonstrating their effectiveness in blocking RF and LO signals while allowing DC current to pass.

Furthermore, the experiment highlighted the application of a diode as a video frequency detector, showcasing its square-law behavior at low input power levels and saturation at higher power levels.