Balanced Modulators in Analog Designs

The ability of birds to maintain their balance while perched on a high voltage line or a tree branch has always fascinated me. Recent research has disclosed that the physiology of birds includes a balance-sensing organ named the lumbosacral organ or LSO that complements the vestibular system found in the inner ear. Located within the pelvis of birds, the LSO detects body posture and motion while sending signals that allow the bird to maintain equilibrium.

The fact that birds have a balancing system in the hip region may provoke some thought about improving the design and balance of bipedal robots. Before we allow our imaginations to run too far with the concept of robotic lumbosacral organs, we should step back and look at another type of balance needed for communication systems.

Amplitude Modulated Signals Don’t Belong in Museums

Any communication signal sent to a desired receiver is a voltage that varies with time. Without any modification, the transmission of the signal can get skewed and distorted. All of us know that the modulation process combines the signal that contains information with another signal called the carrier. Combining the signals yields a signal at frequencies both in a desired part of the spectrum and closer to the needs of the application. Without modulation, the communication signal would have an overly large wavelength that would require giant antennas and complex circuitry at the receiver.

Modulation allows a small signal to control a larger signal. Communications systems transmit the modulated signal because the signal fits within the usable spectrum and power needed by the receiving circuit. Amplitude modulation occurs through an information signal that causes the amplitude of a carrier signal and the information signal to vary over time in proportion to the instantaneous magnitude of the sum of the two signals.

Double or Nothing for Your Signals

Unfortunately, AM signals have bandwidths twice the size of the two identical sidebands that make up the modulating signal. Along with that negative factor, AM signals have all the power transmitted in the carrier. In most applications, we seek to conserve bandwidth and reduce power usage. To offset the bandwidth and power issues, we can reduce or eliminate the carrier signal and achieve double-sideband suppressed-carrier AM (DSB-SC). Rather than losing two-thirds of available power in the carrier signal, DSB-SC places nearly all power into the sidebands.

We know that a circuit that handles amplitude modulation (AM) allows the information-laden signal to vary the amplitude of the carrier. Any non-linear circuit based on a diode, bipolar junction transistor, or a Junction Field Effect Transistor (JFET) can function as an amplitude modulator. In a non-linear circuit, the nonlinearities of the circuit determine the relationships between original signals and any newly produced frequencies. With ideal amplitude modulation, the circuit should produce sum and difference values while removing undesired sum and difference harmonics.

We can build a simple AM circuit by applying modulation to the base of a single NPN bipolar transistor. Biasing the transistor into its nonlinear operating region causes the transistor to mix the carrier and information signals applied to its base. Non-linear mixing of the signals produces numerous sum and difference signals at the carrier frequency.

Figure One - Simple Amplitude Modulation Circuit

The LC circuit shown at the top of the diagram and at the collector of the transistor resonates at the carrier frequency and becomes a high impedance. As a result, frequencies at or near the carrier frequency remain available at the output. Because the LC circuit appears as a low impedance to undesired sum and difference frequencies, it functions as a short circuit to those frequencies.

Another Kind of Balance

While the circuit shown in figure one generates a regular AM signal and then filters the undesired carrier, the solution presented by the circuit does not offer a practical approach. Because the carrier signal remains close in frequency to the sidebands, the circuit would require a very precise filter that remains stable in terms of time and temperature. In addition, reducing a very larger carrier signal by filtering may still leave a strong carrier signal.

Within your work on RF circuits, you may have used a balanced modulator to generate and detect signals. Yet, the operation of a balanced modulator goes slightly deeper than generating and detecting signals. As we take a closer look, we find that balanced modulators suppress the unwanted carrier in an amplitude modulated (AM) wave and only produce sidebands. Along with eliminating the carrier, a balanced modulator also provides efficient use of power.

Referring to the block diagram shown in figure two, the carrier and modulating signals apply to the inputs of the balanced modulator. Let’s stop here for a moment. Balanced modulators operate as a non-linear resistance. Applying two signals at different frequencies through a non-linear resistance produces an AM signal with a suppressed carrier at the output.

Figure Two - Block Diagram of Balanced Modulator Operation

Moving to the output of our block diagram, we find a double side-band signal with a suppressed carrier. In contrast to the regular AM circuit shown in figure one, a perfect balanced modulator with perfect DSB-SC has an output of zero when either input is zero. In actual operation, though, a slight voltage called feedthrough may occur at the output even with zero voltage at the input. Since feedthrough can produce a small undesired carrier signal, the balanced modulator circuits either use an integrated circuit with an internal trimming resistor or add an external trimming resistor that reduces the feedthrough to a minimal value.  

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