Modulation Schemes

Modulation Schemes

The PXI/PXIe-5650/5651/5652 RF Analog Signal Generator can generate the following types of modulated signals:

• Frequency modulated (FM) signals
• Frequency shift keying (FSK) signals
• On-off keying (OOK) signals
• Phase modulated (PM) signals
• Phase-shift keying (PSK) signals

Frequency Modulation (FM)

Frequency Modulation (FM) is a form of modulation in which changes in the frequency of the carrier wave correspond directly with changes in the baseband signal. You can control the following modulation parameters:

• Internal modulation waveform types: square, sine, triangle
• Modulation waveform frequency
• FM deviation

The valid modulation waveform frequency (modulation rate) range is expressed in the following equation:

Modulation Rate = 50 MHz / n × m

where

1 ≤ n < 2¹⁶

10 ≤ m ≤ 2,038 or
10 ≤ m ≤ 1,020 (for square modulation waveforms)

m is the number of samples per waveform, which is optimized by the driver to produce the most linear waveform for a desired rate. Also, m is always an even number for the FM rate calculation.

Note  The maximum recommended waveform frequency is 100 kHz. Higher frequencies are possible but produce degraded modulation accuracy.

Frequency-Shift Keying (FSK)

Frequency-shift keying (FSK) is a type of frequency modulation that assigns bit values to discrete frequency levels. The PXI/PXIe-5650/5651/5652 is capable of producing a 2-FSK signal. You can control the following modulation parameters:

• Internal modulation waveform types: user-defined bit stream or pseudorandom bit sequence (PRBS)
• Symbol rate
• FSK deviation

NI-RFSG calculates the exact symbol rate and FSK deviation, which can differ slightly from the input parameters.

For user-defined bit streams, the valid symbol rate range and bit stream length are determined by the following relations:

1 ≤ bit stream length ≤ 1,022

Symbol rate = 50 MHz / (n × m)

where

5 ≤ n < 2¹⁶

1 ≤ m ≤ floor(1,022/bit stream length)

For PRBS:

Symbol rate = 10 MHz/n

where

1 ≤ n < 2¹⁶

Note  The maximum recommended symbol rate is 100 kHz. Higher symbol rates are possible but produce degraded modulation accuracy.

On-Off Keying (OOK)

On-off keying (OOK) is a modulation scheme that varies the power level of the carrier signal between two discrete power levels. You can control the following modulation parameters:

• Internal modulation waveform type: 1,024-bit user-defined bit-stream or PRBS
• Symbol rate

A bit value of 1 sets the carrier signal power to the value configured by the Power Level property or the NIRFSG_ATTR_POWER_LEVEL attribute. A bit value of 0 sets the carrier signal power to the lowest possible level for this frequency.

During a bit value of 1 (high power level), the PXI/PXIe-5650/5651/5652 bypasses the main attenuators. Therefore, the high power level range is limited by the power level range of the ALC. The supported OOK high power level range is from the maximum specified power level for this frequency to approximately 10 dB lower than the maximum specified power. Refer to the specifications document for your device for minimum and maximum power levels.

For user-defined bit streams, the valid symbol rate range and bit stream length are related in the following relations:

1 ≤ bit stream length ≤ 1,024

Symbol rate = 10 MHz / (n × m)

where

5 ≤ n < 2¹⁶

1 ≤ m ≤ floor(1,022/bit stream length)

For PRBS:

Symbol rate = 10 MHz/n

where

1 ≤ n < 2¹⁶

The symbol rate is determined by a programmable 16-bit register, as indicated in the following equation:

PRBS symbol rate = 10 MHz/n

where

1 ≤ n < 2¹⁶

Note  The maximum recommended symbol rate is 100 kHz. Higher symbol rates are possible but produce degraded modulation accuracy.

Phase Modulation (PM)

Phase Modulation (PM) is a form of modulation in which changes in the phase of the carrier wave correspond directly with changes in the baseband signal. You can control the following modulation parameters:

• Internal modulation waveform types: square, sine, triangle
• Modulation waveform frequency
• PM deviation

The valid modulation waveform frequency (modulation rate) range is expressed in the following equation:

Modulation Rate = 50 MHz / n×m

where

1 ≤ n < 2¹⁶

10 ≤ m ≤ 2,038 or
10 ≤ m ≤ 1,020 (for square modulation waveforms)

m is the number of samples per waveform, which is optimized by the driver to produce the most linear waveform for a desired rate. Also, m is always an even number for the PM rate calculation.

Note  The maximum recommended waveform frequency is 100 kHz. Higher frequencies are possible but produce degraded modulation accuracy.

Phase-shift Keying (PSK)

Phase-shift keying (PSK) in digital transmission refers to a type of angle modulation in which the phase of the carrier is discretely varied to represent data being transmitted—either in relation to a reference phase or to the phase of the immediately preceding signal element. The PXI/PXIe-5650/5651/5652 is capable of producing a 2-PSK signal. You can control the following modulation parameters:

• Internal modulation waveform types: user-defined bit stream or pseudorandom bit sequence (PRBS)
• Symbol rate

NI-RFSG calculates the exact symbol rate and PSK deviation. The driver-calculated symbol rate can differ slightly from the input parameter.

For user-defined bit streams, the valid symbol rate range and bit stream length are determined by the following relations:

1 ≤ bit stream length ≤ 1,022

Symbol rate = 50 MHz / (n × m)

where

5 ≤ n < 2¹⁶

1 ≤ m ≤ floor(1,022/bit stream length)

For PRBS:

Symbol rate = 10 MHz/n

where

1 ≤ n < 2¹⁶

Note  The maximum recommended symbol rate is 100 kHz. Higher symbol rates are possible but produce degraded modulation accuracy.

Related Topic

Modulation Implementation

Power Level Property

NIRFSG_ATTR_POWER_LEVEL Attribute