GC4116 datasheet(13/57 Pages) ETC

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GC4116 MULTI-STANDARD QUAD DUC CHIP

DATA SHEET REV 1.0

- 8 -

APRIL 27, 2001

This document contains preliminary information which may be changed at any time without notice

32 BITS

20 BITS

32 BITS

FREQUENCY

WORD

SINE/COSINE

OUT

23 MSBs

5 BITS

18 MSBs

DITHER

GENERATOR

PHASE

OFFSET

16 BITS

SINE/COSINE

LOOKUP

TABLE

Figure 8. NCO Circuit

3.3.6 The Numerically Controlled Oscillator

(NCO)

The tuning frequency of each down converter is

specified as a 32 bit word and the phase offset is specified

as a 16 bit word. The NCOs can be synchronized with NCOs

on other chips. This allows multiple down converter outputs

to be coherently combined, each with a unique phase and

amplitude. A block diagram of the NCO circuit is shown in

Figure 8.

The tuning frequency is set to FREQ according to the

formula FREQ = 232F/FCK, where F is the desired tuning

frequency and FCK is the chip’s clock rate. The 16 bit phase

offset setting is PHASE = 216P/2

π, where P is the desired

phase in radians ranging between 0 and 2

π. Note that a

positive tuning frequency should be used to upconvert the

signal. A negative tuning frequency can be used to spectrally

flip the spectrum of the desired signal. FREQ and PHASE

are set in addresses 16 through 31 of the frequency and

phase pages.

The NCO’s frequency, phase and accumulator can be

initialized and synchronized with other channels using the

FREQ_SYNC, PHASE_SYNC, and NCO_SYNC controls in

addresses

through

11.

The

FREQ_SYNC

and

PHASE_SYNC controls determine when new frequency and

phase settings become active. Normally these are set to

“always” so that they take effect immediately, but can be

used to synchronize frequency hopping or beam forming

systems. The NCO_SYNC control is usually set to never, but

can be used to synchronize the LOs of multiple channels.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-150

-100

-50

Frequency (Fs)

dlo.nodither.894784853.ppow

Ftune = 5/24 Fs

No dither

-105.28 dB

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-150

-100

-50

Frequency (Fs)

dlo.dither.894784853.ppow

Ftune = 5/24 Fs

-116.4 dB

With dither

a) Worst case spectrum without dither

b) Spectrum with dither (tuned to same frequency)

-105 dB

-116 dB

FREQ=5/24 FS

Figure 9. Example NCO Spurs

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-150

-100

-50

Frequency (kHz)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-150

-100

-50

Frequency (kHz)

-121 dB

-107 dB

a) Plot without dither or phase initialization

b) Plot with dither and phase initialization

Figure 10. NCO Peak Spur Plot

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