MCP37231 datasheet(60/144 Pages) MICROCHIP

60 / 144 page

MCP37231/21-200 AND MCP37D31/21-200

DS20005322D-page 60

 2014-2016 Microchip Technology Inc.

4.8.3.3

Numerically Controlled Oscillator

(NCO)

The on-board Numerically Controlled Oscillator (NCO)

provides the frequency reference for the in-phase and

quadrature mixers in the digital down-converter (DDC).

The NCO serves as a quadrature local oscillator,

capable of producing an NCO frequency of between 0

Hz and fS with a resolution of fS/232, where fS is the

ADC core sampling frequency.

Figure 4-18 shows the control signals associated with

the NCO. In octal- or dual-channel mode, the NCO

allows the output phase to be adjusted on a

per-channel basis.

FIGURE 4-18:

NCO Block Diagram.

• NCO Frequency Control:

The NCO frequency is programmed from 0 Hz to fS,

using the 32-bit-wide unsigned register variable

NCO_TUNE<31:0>

Addresses

0x82 – 0x85

(Registers 5-42 – 5-45).

The

following

equation

used

set

the

NCO_TUNE<31:0> register:

EQUATION 4-5:

NCO FREQUENCY

Mod() is a remainder function. For example,

Mod(5,2) = 1 and Mod(1.999, 2) = 1.999.

Example 1:

If fNCO is 100 MHz and fS is 200 MHz:

Example 2:

If fNCO is 199.99999994 MHz and fS is 200 MHz:

4.8.3.4

NCO Amplitude and Phase Dither

The EN_AMPDITH and EN_PHSDITH parameters in

Address 0x80 (Register 5-40) can be used for

amplitude and phase dithering, respectively. In

principle, these will dither the quantization error created

by the use of digital circuits in the mixer and local

oscillator, thus reducing spurs at the expense of noise.

In practice, the DDC circuitry has been designed with

sufficient noise and spurious performance for most

applications. In the worst-case scenario, the NCO has

an SFDR of greater than 116 dB when the amplitude

dither is enabled, and 112 dB when disabled. Although

the SNR (≈ 93 dB) of the DDC is not significantly

affected by the dithering option, using the NCO with

dithering options enabled is always recommended for

the best performance.

4.8.3.5

NCO for fS/8 and fS/(8xDER)

The output of the first down-conversion block (DDC1)

is a complex signal (comprising I and Q data) which can

then be optionally decimated further up to 128x to

provide both a lower output data rate and input channel

filtering. If fS/8 mode is enabled, a second mixer stage

(DDC2) will convert the I/Q signals to a real signal

centered at half of the current Nyquist frequency; i.e., if

the output data rate in I/Q mode is 25 Msps per channel

(12.5 MHz Nyquist), then in fS/8 mode the output data

rate would be 50 Msps (25 Msps each for I and Q), and

the signal would be re-centered around 12.5 MHz. In

single-channel mode, this is done at the output of the

decimation filters (if used). In dual-channel mode, this

must be done prior to the decimation.

When decimation is enabled, the I/Q outputs are up-

converted by fS/(8xDER), where DER is the additional

decimation rate added by the FIR decimation filters.

This provides a decimated output signal centered at

fS/8 or fS/(8xDER) in the frequency domain.

Note: The NCO is only used for DDC or CW octal-

channel mode. It should be disabled when

not in use.

CH(n) NCO_PHASE<15:0>

Amplitude Dither

EN_AMPDITH

EN_PHSDITH

EN_LFSR

NCO_TUNE<31:0>

Phase Offset Control

NCO Tuning

Sine/Cosine

Signal Generator

NCO Output

EN_NCO

Phase Dither

EN_LFSR

NCO_TUNE<31:0>

round 2

Mod f

NC O fS





------------------------------------







Where:

fS = sampling frequency (Hz)

fNCO = desired NCO frequency (Hz)

Mod (fNCO, fS)= gives the remainder of fNCO/fS

Mod f

NCO





Mod 100 200





100

N C O_TU NE<31:0>

rou nd 2

Mod 100 200





200

--------------------------------------







0 x8000 0000

Mo d f

NCO





Mod 199.99999994 200





199.99999994

N CO_TUN E<31:0>

round 2

Mod 199.99999994 200





200

---------------------------------------------------------------







0xFFFF FFFF

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