Lab Report: Receiver System Integration

Abstract

In this laboratory experiment, we designed and analyzed a 915MHz Superheterodyne receiver. Our study focused on assessing the performance of individual subsystems and their integration to understand their collective impact on receiver performance. We successfully down-converted a 915MHz RF signal to an IF signal at 70MHz. Key aspects of our investigation included VCO characteristics, signal generation, and the impact of filtering on system behavior. Our findings provide valuable insights into the design and optimization of receiver systems.

1. Introduction

The Superheterodyne receiver is a fundamental component in modern wireless communication systems.

It functions by mixing the received RF signal with a Local Oscillator (LO) signal to produce an Intermediate Frequency (IF) signal. This laboratory experiment aimed to dissect the receiver system into its individual subsystems, study their characteristics, and assess their combined performance. The major subsystems include:

1. RF Amplifier: Amplifies the RF signal to raise its power above the noise floor.
2. RF Filter: Attenuates unwanted frequencies and reduces insertion loss.
3. Mixer: Mixes the RF and LO signals to produce various beat frequencies.
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4. Voltage-Controlled Oscillator (VCO): Generates a tunable LO signal.
5. IF Amplifier: Amplifies the IF signal to increase sensitivity and stability.
6. IF Filter: Provides selectivity by rejecting adjacent signals.

2. VCO Characterization

In this section, we characterized the Voltage-Controlled Oscillator (VCO), which plays a crucial role in generating the LO signal. VCOs utilize either YIG (Yttrium Iron Garnet) spheres or Varactor diodes for voltage tuning of the oscillation frequency. The characterization involved tuning the VCO to produce the desired 985MHz output frequency while varying the tuning voltage.

The results are summarized in the table below.

The data revealed that increasing the tuning voltage led to higher output frequencies but lower power levels. This behavior was consistent for harmonics as well.

3. Cable Loss Measurement

To account for cable loss in our measurements, we measured the transmission coefficient of output cable #1 using a Vector Network Analyzer (VNA). The insertion loss for various frequencies was recorded and is presented in the table below.

This measurement allowed us to correct the VCO output power for cable loss in subsequent calculations.

4. LO Power Determination

The Local Oscillator (LO) power level to be delivered to the mixer during subsystem operation was determined by measuring the corrected VCO output power under different conditions. The coupling factor of the VCO at 985MHz was found to be 18.4dB. Additionally, the "Aux out" of the VCO was utilized for monitoring and as part of a phase-locked loop.

5. Mini-Circuits Signal Generator

We characterized the Mini-Circuits signal generator, which provided a clean 915MHz input signal for our RF amplifier. This signal generator was chosen due to its low noise compared to real-world signals received through antennas. The generated input signal was then amplified by the RF amplifier, and the IF output at 70MHz was measured on a spectrum analyzer through output cable #1.

6. Impact of Filtering

Filtering is a crucial aspect of receiver design. In our experiment, we examined the effects of filtering by removing and replacing both the RF filter and the IF filter.

When the IF power exceeded the RF power, we measured the system conversion gain, which represents the ratio of IF output power to RF input power. In Part A, the conversion gain was calculated to be 23.09 dBm.

We also measured the power of the antenna-received input signal at 915MHz, which was found to be -20.53 dBm. This information allowed us to calculate the IF power corresponding to the antenna-received input signal.

IF Power Calculation:
Conversion Gain (CG) = -23.09 dBm
Antenna Input Power (P_antenna) = -20.53 dBm
IF Power (P_IF) = CG - P_antenna = 2.56 dBm
Measured IF Power = 2.32 dBm

We also tracked power levels at various points in the system using a spectrum analyzer.

In the next part of the experiment, we introduced real-world antenna-received input signals by connecting an antenna to our system. The spectrum analyzer displayed both the laboratory-generated signal and real-world wireless traffic from local cellphone and paging services.

When the IF filter was removed, we observed unwanted mixing products with the same modulation as our IF signal, leading to decreased receiver selectivity. Replacing the RF filter with the IF filter resulted in the image frequency being equidistant from the LO and RF signals, causing interference at the IF stage.

The importance of an image rejection filter was highlighted when we observed that detuning the VCO resulted in two signals overlapping, making demodulation challenging. Conversely, rejecting the image frequency led to chaotic interference, rendering the received signal unintelligible. Therefore, image rejection filters play a crucial role in receiver performance.

Maintaining a narrow bandwidth with filters after the first amplifier is challenging due to the conflicting requirements of RF and IF filtering. RF filters reject image frequency, while IF filters provide adjacent channel rejection. The behavior of the filter's passband changes with impedance matching.

In Part B of the lab, we conducted a comprehensive characterization of system components. This included measuring transmission coefficients, determining amplifier gain, quantifying mixer conversion loss, assessing the insertion loss of attenuators, filters, and cables, and studying the broadband and narrowband responses of filters, including bandwidth and insertion loss.

7. Transmission Coefficients

To assess the performance of system components, we measured their transmission coefficients. This analysis provided insights into the efficiency of signal transfer through the components.

8. Amplifier Gain

The gain of the RF and IF amplifiers was determined to evaluate their ability to boost signal amplitudes above the noise floor. Amplifier gain is a critical factor in achieving high sensitivity and improved signal stability.

9. Mixer Conversion Loss

Conversion loss of the mixer was quantified to understand the efficiency of the mixing process. It represents the power lost during the conversion of RF and LO signals into the IF signal.

10. Insertion Loss

The insertion loss of attenuators, filters, and cables was measured to assess their impact on signal integrity. Low insertion loss is essential for preserving the quality of the transmitted signal.

11. Broadband and Narrowband Response

We examined the broadband and narrowband responses of filters to understand their ability to pass desired signals while attenuating unwanted frequencies. Filter bandwidth and insertion loss were critical parameters for evaluating their performance.

12. Conclusion

In conclusion, this laboratory experiment provided valuable insights into the design and integration of a 915MHz Superheterodyne receiver system. We successfully characterized key components, including the VCO, measured cable losses, determined LO power levels, and studied the effects of filtering. Our findings underscored the importance of image rejection filters and the challenges of maintaining a narrow bandwidth in the presence of conflicting RF and IF filtering requirements. This knowledge is essential for designing efficient and high-performance receiver systems in wireless communication applications.