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This two-part series investigates various types of transmitters that are found within myriad applications. Part 1 provided a general overview before examining classical AM and FM transmitters. In Part 2, we discuss single-sideband transmitters and then examine more modern transmitter types.
Single-Sideband Transmitters
With AM modulation, both an upper and a lower sideband are transmitted. The upper sideband frequency is equal to the sum of the carrier signal frequency and the modulating signal frequency, while the lower sideband frequency is equal to the carrier signal frequency minus the modulating signal frequency. A single-sideband (SSB) transmitter differs from an AM transmitter in that it only transmits either the upper or lower sideband—not both. Thus, an SSB transmitter uses less bandwidth than an AM transmitter.
Figure 1 shows one implementation of a SSB transmitter. An oscillator generates the carrier signal, which is then amplified before entering a balanced modulator. In addition, the audio signal is amplified and processed before also entering the balanced modulator.
1. This SSB transmitter makes use of a filter to remove the unwanted sideband.
Subsequently, the signal generated at the output of the balanced modulator enters a sideband filter. This filter allows the desired sideband to pass while rejecting the unwanted one. After the filter, the signal—which is now an SSB signal—enters a mixer, along with a local-oscillator (LO) signal. Next, at the mixer’s output, a higher-frequency signal is generated; then it gets amplified and launched.
Modern Wireless Transmitters
The modulating signal in AM and FM transmitters is purely analog. However, more modern transmitters utilize digital technology. In essence, today’s transmitters often take advantage of digital-signal-processing (DSP) technology to process the information to be transmitted.
I/Q Signals
Before discussing transmitters any further, it is helpful to explain in-phase/quadrature (I/Q) signals (also simply known as quadrature signals). I/Q signals are at the core of the complex modulation techniques implemented in many transmitters. Essentially, I/Q signals can be defined as a pair of signals that differ in phase by 90 degrees. The in-phase (I) signal is the reference signal, while the quadrature (Q) signal shifts 90 degrees in phase from the I signal.
A cosine wave and a sine wave differ in phase by 90 degrees. The cosine wave would be considered the I signal (phase equal to 0), while the sine wave represents the Q signal. When adding together a cosine wave and a sine wave with equal amplitudes, the result is a sinusoid that shifts in phase by 45 degrees from the I signal. Combining I and Q signals is an important concept with regard to complex modulation.
2. Shown is a simple representation of QPSK modulation.
Figure 2 is an illustration of quadrature phase-shift-keying (QPSK) modulation, including the I/Q signals as well as the RF carrier signal. The I and Q signals shown are actually digital bit streams. The table denotes that the phase shift of the output signal is determined by the I and Q values. As can be seen, QPSK has a total of four states.
Many other modulation techniques exist, but describing them all would go beyond the scope of this article. However, the concept discussed here demonstrates that a carrier signal can be modulated by controlling the amplitude of the I/Q signals. It is an essential factor in understanding the functionality of many of today’s transmitters.
Direct-Conversion Transmitter
One often-used transmitter is the direct-conversion transmitter, which has the benefit of being simple and cost-effective (Fig. 3). Here, the digital data that contains the information to be transmitted is processed, resulting in baseband I/Q signals. The I and Q signals are then each fed to respective digital-to-analog converters (DACs). Next, the DAC output signals are each applied to respective lowpass filters. After passing through these filters, both signals subsequently enter corresponding mixers.
3. The direct-conversion transmitter is widely used in wireless communication systems.
Meanwhile, an LO generates an RF signal. This signal is then split into two signals that are 90 degrees out of phase. Each of these signals drive the other input port of the aforementioned mixers, respectively. At this stage, the output signals from both mixers are combined, and the resulting modulated signal is amplified, fed to an antenna, and launched. The transmitted signal eventually arrives at a receiver, which demodulates the received signal to recover the I/Q signals.
Superheterodyne Transmitter
Figure 4 shows a block diagram of a superheterodyne transmitter, which has greater complexity than the direct-conversion transmitter. Its process is similar to that of the direct-conversion transmitter up until the first bandpass filter, shown as Bandpass Filter 1. The signal that reaches this filter is known as the intermediate-frequency (IF) signal. After passing through Bandpass Filter 1, the IF signal is amplified and then upconverted to the final output frequency by a mixer. After that, the signal is filtered, amplified, and launched.
Referring to Fig. 4, one drawback of the superheterodyne transmitter is the generation of unwanted signals at the output of Mixer 3. To explain, the frequency of the desired output signal could be equal to the sum of the LO2 and IF frequencies. However, an unwanted signal with a frequency equal to the difference of the LO2 and IF frequencies will also appear at the output of Mixer 3.
4. A superheterodyne transmitter functions similarly to the direct-conversion transmitter until reaching the first bandpass filter.
Alternatively, the reverse could be true: The desired output frequency could be equal to the difference of the LO2 and IF frequencies; thus, an unwanted signal with a frequency equal to the sum of the LO2 and IF frequencies will appear at the output of Mixer 3. No matter the case, Bandpass Filter 2 is employed to remove the unwanted signals.
Conclusion
Transmitters come in various shapes and sizes. While AM and FM transmitters are still in play, current wireless communication systems extensively use other types—in particular, direct-conversion and superheterodyne implementations . And let’s not forget that DSP technology is a key enabler of the communications that prevail today.
References
1. Frenzel, Louis E., Principles of Electronic Communication Systems, Fourth Edition, McGraw Hill, 2016.
2. Tektronix, What’s Your IQ—About Quadrature Signals, April, 2013.
