What is modulation? For example, two people, Lao Wang and uncle Liu, were invited to attend the symposium together. Because there were many noisy people in the venue and the two people were not seated together, Lao Wang wanted to talk to uncle Liu at this time, so Lao Wang raised his hand and motioned uncle Liu to say hello to him from a distance. It happened that uncle Liu turned his head and saw it, and also raised his hand and waved. At this time, the two sides have realized the transmission of information. Next, uncle Liu found that there was an empty seat around him. He waved his hand back and forth to signal the old throne. When Lao Wang saw it, he made an OK gesture to indicate receipt. At this time, uncle Liu continues to swing his hands back and forth on his raised hands, which means that the modulation signal is loaded on the basis of the carrier wave. The purpose of transmitting different information can be achieved through different gestures. At the same time, if the two hands make different gestures at the same time, the amount of information that can be transmitted will also increase. This is the charm of modulation.

Sine wave is often used as carrier in modulation, but sinusoidal signal is not the only carrier form. The discrete pulse train in time can also be used as a carrier. At this time, the modulation is achieved by changing some parameters of the pulse with the baseband signal. This modulation is called pulse modulation. Pulse modulation refers to the process of modulating RF carrier with pulse signal. Pulse modulation is divided into pulse amplitude modulation (PAM), pulse width modulation (PWM) and pulse position modulation (PPM) according to the different pulse parameters (amplitude, width, time position) changed by baseband signal.

So how to output and view the pulse modulation signal? We need a signal source with pulse modulation option and a spectrum analyzer. For example, RIGOL’s dsg3000b series (dsg3000b-pug pulse modulation, pulse generator and pulse sequence generator options need to be added), and a keysight n9030apxa.

First, turn on the pulse modulation function of the instrument on the signal source. You can see that the source is divided into internal source and external source. When the internal modulation source is turned on, the internal pulse generator of the instrument provides the modulation signal, and the pulse type, pulse period, pulse width and other parameters of the modulation signal can be set. If the external source is selected, the pulse type, cycle, pulse width, trigger mode and pulse output menu will be grayed out and disabled. The RF signal source receives the external pulse signal input from the rear panel [pulse in / out] connector as the modulation signal.

The external source signal interface is usually at the back of the instrument, which is a BNC interface.

How do I output and view a pulse modulated signal?

For example, I use an internal source to output a pulse signal with a frequency of 420MHz, an amplitude of 0dbm, a period of 10ms and a pulse width of 1ms. The pulse period represents the time interval between two adjacent pulses in a periodic repetitive pulse sequence.

The pulse type is single pulse. A single pulse is a pulse cycle that produces a single pulse signal. Multi pulse is a pulse cycle that generates multiple pulse signals. At this time, the cycle and pulse width menus are grayed out and disabled.

If the polarity is positive phase, the waveform is reversed.

Turn on the modulation key [mod] of the instrument, and then make some settings on the spectrum analyzer. First, set the sweep width to zero.

Finally, change the center frequency to 420MHz and check the amplitude value after modulation through peaksearch.

The trigger mode of RF source can be “automatic”, “external trigger”, “external gating”, “key trigger” or “bus trigger”.

The default is automatic trigger, and the RF signal source meets the trigger conditions at any time.

In case of external trigger, the RF signal source receives the external trigger signal input from the [triggerin] connector on the rear panel. Pulse modulation is started every time a TTL pulse with a specified polarity is received. To specify the polarity of TTL pulse, press the [trigger edge] key and select “rising edge” or “falling edge”, which is “rising edge” by default.

In the external gating mode, the RF signal source receives the external gating signal input from the [triggerin] connector on the rear panel. Each time a gating signal of specified polarity is received, pulse modulation is started once within its effective level. To specify the polarity of the external gating signal, change the “positive phase” or “inverse phase” setting, which defaults to “positive phase”.

Editor in charge: PJ

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