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GC5318

HIGH-DENSITY DIGITAL UPCONVERTER

SLWS187 – JULY 2006

The tuning frequency is specified as a 32-bit frequency word and is programmed as two sequential 16-bit

words over the control port. The NCO operates at the same speed as the txclk/(tadj_interp_decim + 1).

The frequency resolution is simply the F

CLK/ 2

32. The NCO frequency resolution is simply the F

CLK/ 2

32. As

an example, at an input clock rate of 61.44 MHz, the frequency step size would be approximately 14 MHz.

The frequency word is determined by the formula:

Frequency word (in decimal) = 232 x Tuning Frequency/F

CLK

NOTE

Frequency tuning words can be positive or negative valued. Specifying a positive

frequency value translates baseband frequencies upward. Specifying a negative tuning

frequency translates baseband frequencies downwards.

Table 2-13. Programming

VARIABLE

DESCRIPTION

phase_add_a(31:0)

32-bit tuning frequency word for the A signal when in CDMA mode. Also for UMTS mode.

phase_add_b(31:0)

32-bit tuning frequency word for the B signal when in CDMA mode. Not used in UMTS mode.

The phase of the NCO Sin/Cos output can be adjusted relative to the phase of other channel NCOs by

specifying a phase offset. The phase offset is programmed as a 16-bit word, yielding a step size of about

5.5 milliDegrees. The phase offset word is determined by the following formula:

Phase Offset Word = 216 x Offset_in_Degrees / 360; or,

Phase Offset Word = 216 x Offset_in_Radians / 2

Table 2-14. Programming

VARIABLE

DESCRIPTION

phase_offset_a(15:0)

16-bit phase offset word for the A signal when in CDMA mode. Also for UMTS mode.

phase_offset_b(15:0)

16-bit phase offset word for the B signal when in CDMA mode. Not used in UMTS mode.

Various synchronization signals are available that are used to synchronize the NCOs of all channels with

respect to each other. Frequency sync and phase offset sync determine when frequency and phase offset

changes occur. For example, generating a frequency sync after programming the two frequency words

causes the NCO (or multiple NCOs) to change frequency at that time, rather than after each of the three

frequency words is programmed over the control bus. The NCO accumulator reset sync signal is used to

force the sine and cosine oscillators to their zero phase state. Dither sync can be used to synchronize the

dither generators of multiple NCOs.

Table 2-15. Programming

VARIABLE

DESCRIPTION

ssel_nco(2:0)

Sync source for NCO accumulator reset

ssel_dither(2:0)

Sync source for NCO dither reset

ssel_freq(2:0)

Sync source for NCO frequency register loading

ssel_phase(2:0)

Sync source for NCO phase register loading

GC5318 Operation

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