Advanced Amateur Radio Exam
Section A-005-004 SSB
Section A-005-004 SSB
Single-sideband (SSB) modulation is a cornerstone technique in modern communication systems, especially in radio transmission. It offers distinct advantages over traditional amplitude modulation (AM) by being more spectrum and power-efficient. This article delves into the mechanics of SSB transmission, exploring how balanced modulators function, the importance of carrier suppression, and methods used to test and ensure the quality of SSB signals.
A balanced modulator, a key component in SSB transmission, is responsible for producing a double sideband, suppressed carrier signal. This means that it effectively eliminates the carrier frequency while retaining the sidebands that carry the information.
Single-sideband phone signals are typically generated using a balanced modulator followed by a filtering stage. This filter is crucial as it removes one of the sidebands, leaving only a single sideband, which is then transmitted.
In SSB transmitters, carrier suppression primarily occurs in the balanced modulator stage. This process is vital for efficient transmission as it reduces power waste and minimizes bandwidth usage.
Compared to conventional AM transmission, SSB offers a 6 dB gain in the transmitter and a 3 dB gain in the receiver. These gains stem from the more efficient use of power and bandwidth in SSB.
When testing the peak power output of a single-sideband transmitter with a two-tone generator, the peak output is twice the RF power output of any one of the tones. This test ensures that the transmitter can handle simultaneous high-power outputs without distortion.
The amplitude linearity of a single-sideband phone transmitter can be tested using two audio-frequency sine waves. This is typically observed with an oscilloscope. For an effective test, two non-harmonically related tones are fed into the microphone input, and their interaction is analyzed.
In a two-tone test for linearity, any two audio tones may be used, as long as they fall within the transmitter's audio passband and are not harmonically related. This choice ensures that the test accurately reflects the transmitter's performance across its operating range.
By performing a two-tone test using an oscilloscope, one can measure the linearity of a single-sideband phone transmitter's amplifier. This test is crucial for ensuring that the transmitter accurately reproduces the input signals without distortion.
In a single-sideband phone transmission, the carrier is typically suppressed to at least 40 dB below peak output power. This suppression level is key to achieving the efficiency benefits of SSB.
"Flat topping" in a single-sideband phone transmission refers to signal distortion caused by excessive drive. This phenomenon can lead to reduced signal clarity and increased interference, making it important to monitor and control signal levels.
Single-sideband modulation, with its efficient use of power and bandwidth, represents a significant advancement over traditional AM. Understanding the function of balanced modulators, the importance of carrier suppression, and the methods for testing SSB transmitters is essential for anyone involved in radio communication. By ensuring proper modulation techniques and monitoring for issues like flat topping, operators can maintain high-quality, efficient transmission, crucial in modern communication systems.