Modulation Modes
The NI 5650/5651/5652 RF signal generator can generate three types of modulated signals, including:
Refer to NI 5650/5651/5652 Modulation Implementation for more information about frequency modulation.
Frequency-Shift Keying
Frequency-shift keying (FSK) is a type of frequency modulation which assigns bit values to discrete frequency levels. The NI 5650/5651/5652 is capable of producing a 2-FSK signal. The user 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 ≤ 1022
Symbol rate = 50 MHz / (n×m)
where
5 ≤ n < 216
1 ≤ m ≤ floor(1022/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)
OOK is a modulation scheme that varies the power level of the carrier signal between two discrete power levels. The user can control the following modulation parameters:
- Internal modulation waveform type: 1,024-bit user-defined bit-stream or PRBS
- Symbol rate
|
Note The maximum recommended symbol rate is 100 kHz. Higher symbol rates are possible but produce degraded modulation accuracy. |
A bit value of 1 sets the carrier signal power to the value configured by the Power Level (dBm) 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. Refer to the specifications document for your device for minimum and maximum power levels.
During a bit value of 1 (high power level), the NI 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 ≤ 1024
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
Frequency Modulation
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 ≤ 2038 or
10 ≤ m ≤ 1020 (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.