SNAS740B October 2017 – January 2019 LMX2572
PRODUCTION DATA.
In fractional mode, the finest delta frequency difference between two programmable output frequencies is equal to Equation 8:
In other words, when the fractional numerator is incremented by 1 (one step), the output frequency will change by Δfmin. A two steps increment will therefore change the frequency by 2 × Δfmin.
In FSK operation, the instantaneous carrier frequency is kept changing among some pre-defined frequencies. In general, the instantaneous carrier frequency is defined as a certain frequency deviation from the nominal carrier frequency. The frequency deviation could be positive and negative.
Equation 9 and Equation 10 define the number of steps required for the desired frequency deviation with respect to the nominal carrier frequency output. Assume ΔfDEV is the frequency deviation,
In FSK SPI mode, registers R116 – R123 are used to store the desired FSK steps as defined in Equation 9 and Equation 10. The order of the registers, 0 to 7, depends on the application system. A typical 4FSK definition is shown in Figure 177. In this case, the FSK_DEV0 and FSK_DEV1 are calculated using Equation 9, while the FSK_DEV2 and FSK_DEV3 are calculated using Equation 10.
FSK SPI mode assumes the user knows which symbol to send. The user can directly write to FSK_SPI_DEV_SEL to select the desired frequency deviation. For example, to enable the device to support 4FSK modulation in FSK SPI mode, set:
DESIRED SYMBOL | WRITE REGISTER FSK_SPI_DEV_SEL | REGISTER SELECTED |
---|---|---|
10 | 2 | FSK_DEV2 |
11 | 3 | FSK_DEV3 |
10 | 2 | FSK_DEV2 |
11 | 3 | FSK_DEV3 |
01 | 1 | FSK_DEV1 |
00 | 0 | FSK_DEV0 |
… | … | … |
FSK SPI mode supports up to eight levels of FSK. To support an arbitrary-level FSK, use FSK SPI FAST mode. Constructing pulse-shaping FSK modulation by over-sampling the FSK modulation waveform is one of the used cases of this mode.
Analog-FM modulation can also be produced in this mode. For example, with a 1-kHz sine wave modulation signal with peak frequency deviation of ±2 kHz, the signal can be oversampled, say 10 times. Each sample point corresponding to a scaled frequency deviation.
In FSK SPI FAST mode, write the desired FSK steps directly to FSK_SPI_FAST_DEV. To enable this mode, set:
TIME | FREQUENCY DEVIATION (Hz) | CORRESPONDING FSK STEPS(1) | WRITE TO FSK_SPI_FAST_DEV |
---|---|---|---|
t0 | 618.034 | 396 | 396 |
t1 | 1618.034 | 1036 | 1036 |
t2 | 2000 | 1280 | 1280 |
… | … | … | … |
t6 | –1618.034 | 64500 | 64500 |
t7 | –2000 | 64256 | 64256 |
… | … | … | … |
Block Programming is possible with FSK SPI FAST mode programming as long as ADD_HOLD = 1, which will freeze the register address after the first register write. The same programming sequent as shown in Figure 37 applies.