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Implementation Of Modulation Techniques on SpinCore PulseBlasterDDS and RadioProcessor Boards

Introduction

    SpinCore's PulseBlasterDDS and RadioProcessor boards can be used to perform a variety of signal modulation techniques. Signal modulation allows for one signal to be transmitted using another signal. This procedure is useful because some signals cannot physically be transferred over certain media. The original signal  is referred to as the modulating signal, and the signal that it is packed inside of is referred to as the carrier. Signal modulation is used for everyday applications such as FM & AM radio, ethernet communications, and HDTV.

    These pages show how to implement a wide variety of signal modulation techniques on PulseBlasterDDS and RadioProcessor boards. They also show the use of these boards to generate carriers of various shapes that could be used in NMR experiments.

    A simple block diagram of the SpinCore PulseBlasterDDS-I-300 board is given in Figure 1. The PulseBlasterDDS-I-300 board consists of two major building blocks. Both of the blocks are controlled by the same master clock.

1. DDS Core

This block mainly consists of a numerically controlled oscillator (NCO), an arbitrary waveform generator (AWG) and an attenuator.  It also contains frequency, phase and amplitude registers that hold the parameters that are used by the NCO, AWG and the attenuator.

2. PulseBlaster Timing Core

This block controls the registers in the DDS core and is used for generating TTL outputs for triggering RF signals on the oscilloscope in this work.


Figure 1: PulseBlasterDDS Architecture.


The complete C code demonstrating the techniques described below will be provided with the next version of SpinAPI, but for now can be found here.

The modulation techniques are further explained in the sections given below.

Modulation Techniques:

1. Analog Modulation
                                                     AM2

Figure 2: Analog modulation refers to the process of transferring an analog baseband (low frequency) signal, like an audio or TV signal, over a higher frequency signal such as a radio frequency band.


Figure 2 shows an AM waveform on the first channel and the TTL output on the second channel of a dual-channel oscillocope. Here, the carrier frequency Fc was set to be equal to 1 MHz and a sine wave message signal having a frequency Fm=100 kHz was used with a modulation index of 50%.

Details about an implementation on the SpinCore PulseBlasterDDS board is given here.
2. Digital Modulation

ASK2

Figure 3: Digital modulation is used to transfer a digital bit stream over an analog channel at a high frequency. This enables us to transmit signals generated in a digital circuit across a physical medium. This is because digital signals can be handled with higher security and precision.

Figure 3 shows a type of digital modulation technique known as Amplitude Shift Keying (ASK) for a carrier frequency of Fc=500 KHz and a digital bit rate = 100KHz for the baseband signal. The digital signal had an amplitude of 0.5Vp-p for a logic 0 and 1.0 Vp-p for a logic 1.  In the figure, the first channel shows the result of ASK and the second channel shows the original digital signal.

          Details about an implementation on the SpinCore PulseBlasterDDS board is given here.

3. Pulse Modulation

PAM1


Figure 4: Pulse modulation methods are used to transfer narrowband analog signals such as voice signals over a wideband channel or, in some schemes, as a bit stream over another digital transmission system.

Figure 4 shows output pulse amplitude modulated result on the first channel and the TTL outputs on the second channel for a carrier frequency of Fc=1 MHz and a sine wave message signal with a frequency of Fm=100 kHz.

Details about an implementation on the SpinCore PulseBlasterDDS board is given here.






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