JAJSNQ4 March 2023 ADC34RF52
PRODUCTION DATA
Each digital down-converter (DDC) uses a 48-bit numerically controlled oscillator (NCO) to fine tune the frequency placement prior to the digital filtering. Different NCO frequencies for each DDC are programmed using SPI register writes and the desired NCO frequency can be selected using SPI or the GPIO pins. When using the GPIO pins for NCO frequency control, frequency hopping can be achieved in less than 1 µs. The digital NCO is designed to have a SFDR of at least 100 dB. The number of available, programmable NCO frequencies depends on # of DDC bands used as illustrated in Table 7-19.
# of DDCs used | # of Frequencies per NCO |
---|---|
1 | 4 |
2 | 4 |
There are two different NCO operating modes available - phase continuous and infinite phase coherent.
Phase Continuous NCO: During a NCO frequency change, the NCO phase gradually adjusts to the new frequency as shown in Figure 7-36. The 'dashed' line shows the phase of original f1 frequency.
Infinite Phase Coherent NCO: With a phase coherent NCO, all frequencies are synchronized to a single event using SYSREF. This enables an infinite amount of frequency hops without the need to reset the NCO as phase coherency is maintained between frequency hops. This is illustrated in Figure 7-36 (right). When returning to the original frequency f1, the NCO phase appears as if the NCO have never changed frequencies.
The oscillator generates a complex exponential sequence of:
where: frequency (ω) is specified as a signed number by the 48-bit register setting
The complex exponential sequence is multiplied with the real input from the ADC to mix the desired carrier to a frequency equal to fIN + fNCO. The NCO frequency can be tuned from –FS/2 to +FS/2 and is processed as a signed, 2s complement number.
The NCO frequency setting is set by the 48-bit register value given and calculated as:
where:
The NCO programming is illustrated with this example:
Table 7-20 shows the register writes to set frequency 1 of NCO1 of DDCA to that frequency:
ADDR | DATA | DESCRIPTION |
---|---|---|
0x05 | 0x08 | Select DDCA page |
0x105 | 0x5A | Frequency = 460 MHz with FS = 1.3 GSPS 99,598,837,913,001 = 0x5A95A95A95A9 where the MSB goes to address 0x105 and the LSB to 0x100. |
0x104 | 0x95 | |
0x103 | 0xA9 | |
0x102 | 0x5A | |
0x101 | 0x95 | |
0x100 | 0xA9 | |
0x181 | 0x00 | Load and update NCO1 with the new frequency. |
0x181 | 0x30 |