Analog Modulation Settings and Limits
RF Signal Generation Capabilities
Chris Armstrong
Last Update il y a un an
Analog modulation is a common function for RF signal sources. These include amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM or ΦM). We will also discuss pulse modulation here.
Amplitude modulation changes the amplitude of the wave over time. It appears like this in the time and frequency domain:
Time Domain – Oscilloscope view of modulation and carrier
Frequency Domain – Spectrum view of AM signal
Siglent generator models support amplitude modulation up to 100% depth (% change in amplitude). They are specified to enable modulation frequencies (speed of the modulation cycle) from 10 Hz to 100 kHz at up to 80% depth.
Amplitude modulation of a carrier is a key use case in immunity testing for radiated and conducted emissions work.
All Siglent RF sources can use either an internal or external source to drive these modulations.
Frequency modulation (FM) is changing of the signal frequency over time. Frequency and phase modulation (PM or ΦM) are not used together since the concept of a phase difference lacks meaning when comparing two waves at different frequencies. Here are the frequency domain views for frequency and phase modulation:
The specification limitations for these signals is dependent on the start or base frequency. Use these tables to determine the variable N for these specifications based on the model or series:
Frequency and phase modulation also operate from 10 Hz – 100 kHz. The key limitations are in deviation - that is plus and minus change from the base frequency in MHz or the plus and minus change from reference phase 0 in radians. Here are the setting limits for deviation:
Pulse modulation has several important specifications. First, the pulse modulation feature has both single and double pulse configurations with settable pulse widths, periods, and a delay between two pulses in double pulse mode. For more complex pulse configurations add the pulse train generator option available on any of the models.
The minimum pulse width can be set as low as 20 nanoseconds with the pulse period being set down to 40 nanoseconds. These values can be set at 10 nanosecond intervals and can be set as long as 300 seconds across all of the models. Here are examples of pulse and pulse train operation in the time domain:
Pulse Modulation with a SSG6082A-V
Pulse Train Modulation with a SSG6082A-V
The SSG6082A-V supports pulse modulation up to an 8 GHz carrier. It has high dynamic range with on-off ratio >80dBc and rise-fall time <15ns.
Pulse signal fidelity is typically specified by this on-off ratio. It specifies the level change during the off period. This table shows the ratio for different generators:
Often modulations can be used in combination to create even more complicated wave types. This may even include IQ signals. IQ signals incorporate two carriers at the same frequency in quadrature - 90° out of phase. For a more in-depth look at IQ signal generation go to the section titled: Vector Modulation Settings and Limits.
This table explains which modulation schemes can be used in tandem on Siglent RF generators:
IQ Modulation cannot be used with other modulations on the SSG3000X-IQE models. IQ modulation is not available on the SSG5000A and SSG6000A models (the SSG6082A-V is an exception).