Amplitude Modulation (AM) and Frequency Modulation (FM)

A modulated waveform consists of a carrier waveform and a modulating waveform. In AM, the carrier amplitude is varied by the voltage level of the modulating waveform. In FM, the carrier frequency is varied by the voltage level of the modulating waveform. The instrument accepts an internal or external modulation source. On a two-channel instrument, one channel can modulate the other.

Select AM or FM before setting up any other modulation parameter. For more information on modulation, see Modulation.

To Select AM or FM

The instrument allows only one modulation mode to be enabled on a channel. When you enable AM or FM, all other modulations are off. On two-channel models, the two channels’ modulations are independent from one another, and the instrument can add modulated waveforms from two channels. See PHASe:SYNChronize and COMBine:FEED for details.
The instrument will not allow AM or FM to be enabled with sweep or burst. Enabling AM or FM, turns off sweep and burst.
To avoid multiple waveform changes, enable modulation after configuring the other modulation parameters.
Front Panel:

or

Then turn modulation on:

The waveform is output using the present carrier and modulating waveform settings.
SCPI:
[SOURce[1|2]:]AM:STATe{ON|1|OFF|0}

[SOURce[1|2]:]FM:STATe {ON|1|OFF|0}

Carrier Waveform Shape

AM or FM carrier shape: Sine (default), Square, Ramp, Pulse, Triangle, Noise (AM only), PRBS, or Arbitrary waveform. You cannot use DC as the carrier waveform.
For FM, the carrier frequency must always be greater than or equal to the frequency deviation. Attempting to set a deviation greater than the carrier frequency will cause the instrument to set the deviation equal to the carrier frequency.
The carrier frequency plus the deviation cannot exceed the selected function's maximum frequency plus 100 kHz. If you attempt to set the deviation to an invalid value, the instrument adjusts it to the maximum value allowed with the present carrier frequency. The remote interface also generates a "Data out of range" error.
Front Panel:

Then select a waveform shape.
SCPI:
[SOURce[1|2]:]FUNCtion <function>

The APPLy command configures a waveform with one command.

Carrier Frequency

The maximum carrier frequency varies by function, model, and output voltage, as shown here. The default is 1 kHz for all functions other than arbitrary waveforms. Arbitrary waveform "frequency" is also set using the FUNCtion:ARBitrary:SRATe command.

Front Panel:
SCPI: [SOURce[1|2]:]FREQuency {<frequency>|MINimum|MAXimum|DEFault}

The APPLy command configures a waveform with one command.

Modulating Waveform Shape

The instrument accepts an internal or external AM or FM modulation source. On a two-channel instrument you can modulate one channel with the other.

Note

You cannot modulate noise with noise, PRBS with PRBS, or an arbitrary waveform with an arbitrary waveform.

The modulating waveform shape (internal source) may be:
Sine wave
Square with 50% duty cycle
UpRamp with 100% symmetry
Triangle with 50% symmetry
DnRamp with 0% symmetry
Noise - white gaussian noise
PRBS - Pseudo Random Bit Sequence (polynomial PN7)
Arb - Arbitrary waveform
Front Panel:

or

Then choose the modulating shape:
SCPI:
[SOURce[1|2]:]AM:INTernal:FUNCtion <function>

[SOURce[1|2]:]FM:INTernal:FUNCtion <function>

Modulating Waveform Frequency

The instrument accepts an internal or external modulation source.

Modulating frequency (internal source): minimum is 1 µHz, and the maximum values vary by model, function, and output, as shown here.
Modulating frequency (external source): 0 to 100 kHz
Front Panel:
(33500 Series only)

or

Then enter the AM or FM frequency with the knob and keypad:
SCPI:
[SOURce[1|2]:]AM:INTernal:FREQuency {<frequency>|MINimum|MAXimum|DEFault}

[SOURce[1|2]:]FM:INTernal:FREQuency {<frequency>|MINimum|MAXimum|DEFault}

Modulation Depth (AM)

The modulation depth is a percentage that represents the amplitude variation. At 0% depth, the amplitude is one-half of the carrier’s amplitude setting. At 100% depth, the amplitude varies according to the modulating waveform, from 0% to 100% of the carrier’s amplitude.

Modulation depth: 0% to 120%. The default is 100%.
Even at greater than 100% depth, the instrument will not exceed ±5 V peak on the output (into a 50 Ω load). To achieve modulation depth greater than 100%, output carrier amplitude may be reduced.
Front Panel:
SCPI: [SOURce[1|2]:]AM[:DEPTh] {<depth_in_percent>|MINimum|MAXimum}

Double Sideband Suppressed Carrier AM

The instrument supports two forms of amplitude modulation, "Normal" and Double Sideband Suppressed Carrier (DSSC). In DSSC, the carrier is not present unless the modulating signal has an amplitude greater than zero.

Front Panel:
SCPI: [SOURce[1|2]:]AM:DSSC{ON|1|OFF|0}.

Frequency Deviation (FM)

The frequency deviation setting represents the peak variation in frequency of the modulated waveform from the carrier frequency.

When the carrier is PRBS, frequency deviation causes a change in the bit rate equal to one-half of the set frequency. For example, a 10 kHz deviation is equivalent to a 5 KBPS change in bit rate.

Frequency deviation: 1 µHz to (carrier frequency)/2, default 100 Hz.
For FM, the carrier frequency must always be greater than or equal to the frequency deviation. Attempting to set a deviation greater than the carrier frequency will cause the instrument to set the deviation equal to the carrier frequency.
The carrier frequency plus the deviation cannot exceed the selected function's maximum frequency plus 100 kHz. If you attempt to set the deviation to an invalid value, the instrument adjusts it to the maximum value allowed with the present carrier frequency. The remote interface also generates a "Data out of range" error.
Front Panel:
SCPI: [SOURce[1|2]:]FM[:DEViation] {<peak_deviation_in_Hz>|MINimum|MAXimum|DEFault}

Modulating Source

The instrument accepts an internal or external modulation source. On a two-channel instrument you can modulate one channel with the other.

On the 33600 Series, you can configure the modulation source to accept either a ±5 V signal or a ±1 V signal. See INPut:ATTenuation[:STATe] for details.

Modulating source: Internal (default), Other Channel, or External. The External modulation source may be up to 100 kHz.
With the External source, an external waveform modulates the carrier waveform. The modulation depth (AM) or frequency deviation (FM) is controlled by the signal level on the rear-panel Modulation In connector. The external modulation input has -3 dB bandwidth of 100 kHz.
AM example: with modulation depth 100%, when the modulating signal is at +5 V, the output will be at the maximum amplitude. When the modulating signal at -5 V, the output will be at minimum amplitude.
FM example: with deviation of 10 kHz, then a +5 V signal level corresponds to a 10 kHz increase in frequency. Lower external signal levels produce less deviation and negative signal levels reduce the frequency below the carrier frequency.
Front Panel:
After enabling AM or FM, select the modulating source as shown:
SCPI:
[SOURce[1|2]:]AM:SOURce {INTernal|EXTernal|CH1|CH2}

[SOURce[1|2]:]FM:SOURce {INTernal|EXTernal|CH1|CH2}