Experiment with Diode Ring Modulator Using ADALM2000 in StudentZone
In the realm of electronic communications, a diode ring modulator stands as a crucial component, playing a significant role in various applications such as amplitude modulation (AM), frequency mixing, and demodulation of radio signals. This device is also instrumental in generating and recovering single-sideband signals in communication systems.
The simplified diode ring modulator approach, as we shall see, eliminates the need for input and output transformers, relying instead on the ADALM2000 Active Learning Module to feed the sum and difference of the carrier and modulating signal to opposite junctions of the diode ring.
The diode ring modulator, comprising four diodes, four 100 Ω resistors, two 1 kΩ resistors, two trifilar winding transformers (if available), and the ADALM2000, operates by employing diodes as switches. These switches control whether the input signal is passed with or without a 180° phase reversal, with the carrier signal setting the diodes on and off at a high rate of speed.
In electronic communications, a balanced modulator is a circuit that produces double-sideband suppressed-carrier (DSBSC) signals, which suppress the radio frequency carrier and leave the sum and difference frequencies at the output. The diode ring modulator is one of the most extensively used circuits in electronic communications, and it is used in frequency and phase modulation systems, digital modulation systems like PSK and QAM, and for producing DSBSC signals.
To construct the circuit, one can use a 1N914 fast switching diode for the diode ring, set W1 as a 1 kHz sine modulating signal with 1 V amplitude peak-to-peak, W2 as a 10 kHz sine carrier with a 3 V amplitude peak-to-peak, and use a 1:2 turns ratio for the input and output transformers. A Hexa-Path Magnetics transformer with either an HP3, HP4, HP5, or HP6 winding layout is needed for the activity, but if not available, LTspice simulations can be used instead.
The output waveform of the diode ring modulator has the carrier signal suppressed and is made up of the sum and difference of the input frequencies, which are RF pulses that take the shape and amplitude of the modulating signal at the rate of the carrier signal. However, a small carrier component always goes with the output signal, a phenomenon known as carrier leak, which occurs due to reasons such as if the transformers are not exactly center tapped, and if the diodes are not perfectly matched.
To observe the waveform, set the oscilloscope's horizontal at 200 μs/div and the vertical at 500 mV/div. Run the signal generator and the oscilloscope and observe the waveform. It should have a similar result to the waveform in Figure 9.
For the modulator to operate effectively, the carrier's amplitude must be adequately greater than the modulating signal's, about six to seven times greater. Changing the resistor values in the simplified diode ring modulator setup can also affect the amplitude of the output waveform. For example, changing R1 and R2 to 1 kΩ resistors decreases the amplitude, while changing R3 and R4 to 1 kΩ resistors increases the amplitude.
In summary, the diode ring modulator, a four-diode circuit, plays a pivotal role in various communication applications. It is instrumental in AM and demodulation, frequency mixing, and single-sideband (SSB) generation and detection. Other uses include phase modulation and complex signal processing in communication systems. Understanding this device provides valuable insights into the workings of electronic communications.
The diode ring modulator, as demonstrated, is not solely limited to its use in traditional transformer-based configurations. Instead, it can leverage technology like the ADALM2000 Active Learning Module to eliminate the need for input and output transformers.
In a broader context, the diode ring modulator, despite its relatively simple composition, contributes significantly to advanced communication systems, such as digital modulation systems like PSK and QAM, and various complex signal processing applications.
