Preston A. Jackson and Dr. Brad L. Hutchings, Electrical and Computer Engineering

Introduction

The ability to implement a commercial AM and FM radio receiver on a field programmable gate array (FPGA) has only recently become possible. FPGAs, the bottleneck technology, have made recent gains is speed and size that allow a signal demodulator to be designed completely in software. As shown below, two additional conversion systems are needed beyond the software demodulator, namely the analog to digital conversion system and the digital to analog conversion system.

This project proposed to design and implement the analog to digital conversion system. Unfortunately the project proved more difficult than anticipated and was not completed at the time of this report.

Procedure

Several key elements are necessary for the design of the analog to digital conversion system. First an antenna is used to convert the electromagnetic radiation broadcast by the commercial AM and FM transmitters into a voltage. This voltage is very small, on the order of -70dBm or in the tens of microvolts. To be of any use, the signal must be amplified. Then the amplified signal is modulated or shifted to a lower frequency much more suited for conversion to a digital signal. All of this preparation takes place on the “front-end” portion of the above schematic.

Once the signal is prepared it is fed into an analog to digital converter. This converter accepts a wire with the prepared signal and outputs a digital signal with twelve bits of precision. In order to avoid aliasing in the signal, the rate at which these digital snapshots must be taken is two times the highest frequency that we would like to sample. This gives us a sample rate of nearly 60 million samples per second.

This digital signal is then fed directly into the pins of the custom demodulator implemented on the FPGA, which is a flexibly designed microprocessor that can alter its functionality depending on the needs of the user.

Difficulties and Results

After designing and testing two antennas the coil antenna was found to be best for the AM spectrum. We purchased a commercial AM and FM antenna which was built of completely passive components and offered signal filtering of frequencies outside the broadcast radio range. The fact that it was built using passive components simplified the power requirements for the end-to-end project.

The “front-end” and “interface” portions of the design were originally designed on the same circuit board using microstripping techniques from RF engineering. The microstripping was used mainly to alleviate the routing and soldering problems. In the end, the microstripping did provide a simple way to attach all of the surface mount components onto the circuit board. On the other hand, the inflexibility of the design made repairing problems much more difficult.

The front-end amplification stage was designed using inexpensive operational amplifiers. This method might have worked well for the AM frequencies but amplification at FM frequencies was close to none. High frequency components were purchased that promised more consistent gain across high frequencies. They also promised to be cascadable, meaning that several could be configured in series amplifying each stage by the advertised amount. However, the cascading portion proved to be the most difficult problem and eventually exhausted the remaining time and effort.

The rest of the interface stage was completed and tested. It consisted of shifting the signal down from the high FM range of frequencies to lower frequencies much more suited for sampling with our analog to digital converter. Also, further filtering and amplification of the signal was needed to prepare the signal for conversion.

The analog to digital converter was purchased according to specifications. It operated at 12-bit precision and a sample rate of 65 mega samples per second. This met our requirements of precision and sample rate.

Finally, on the FPGA an interface for the demodulator was designed and implemented. It was to process the signal from the input pins and buffer it ready for the demodulation stage. This completed the proposed work.

Conclusion

Over all, this research provided a good experience in design of circuits. But more importantly, it was a problem of system design. From antenna to FPGA, the problem involved interfacing different components together. The overall scope of the design was completed and many of the portions reached maturity. However, the design was not completed due to the difficulty in amplifying the tiny broadcast radio frequency to acceptable levels for input into the FPGA.