Harmonic Spur Reduction

AD9912

Note that the SYSCLK PLL bypassed and SYSCLK PLL enabled input paths are internally biased to a dc level of ~1 V. Care should be taken to ensure that any external connections do not disturb the dc bias because this may significantly degrade performance.

Generally, it is recommended that the SYSCLK inputs be ac-coupled, except when using a crystal resonator.

OUTPUT CLOCK DRIVERS AND 2× FREQUENCY MULTIPLIER

There are two output drivers provided by the AD9912. The primary output driver supports differential 1.8 V HSTL output levels, while the secondary supports either 1.8 V or 3.3 V CMOS levels, depending on whether Pin 37 is driven at 1.8 V or 3.3 V.

The primary differential driver nominally provides an output voltage with 100 Ω load applied differentially. The source impedance of the driver is approximately 100 Ω for most of the output clock period; during transition between levels, the source impedance reaches a maximum of about 500 Ω. The driver is designed to support output frequencies of up to and beyond the OC-12 network rate of 622.08 MHz.

The output clock can also be powered down by a control bit in the I/O register map.

Primary 1.8 V Differential HSTL Driver

The DDS produces a sinusoidal clock signal that is sampled at the system clock rate. This DDS output signal is routed off chip where it is passed through an analog filter and brought back on chip for buffering and, if necessary, frequency doubling. Where possible, for the best jitter performance, it is recommended that the frequency doubler be bypassed.

The 1.8 V HSTL output should be ac-coupled, with 100 Ω termi- nation at the destination. The driver design has low jitter injection for frequencies in the range of 50 MHz to 750 MHz. Refer to the AC Specifications section for the exact frequency limits.

2× Frequency Multiplier

The AD9912 can be configured (via the I/O register map) with an internal 2× delay-locked loop (DLL) multiplier at the input of the primary clock driver. The extra octave of frequency gain allows the AD9912 to provide output clock frequencies that exceed the range available from the DDS alone. These settings are found in Register 0x0010 and Register 0x0200.

The input to the DLL consists of the filtered DDS output signal after it has been squared up by an integrated clock receiver circuit. The DLL can accept input frequencies in the range of 200 MHz to 400 MHz.

Single-Ended CMOS Output

In addition to the high-speed differential output clock driver, the AD9912 provides an independent, single-ended output, CMOS clock driver that is very good for frequencies up to

150 MHz. The signal path for the CMOS clock driver can either include or bypass the CMOS output divider.

If the CMOS output divider is bypassed, the HSTL and CMOS drivers are the same frequency as the signal presented at the FDBK_IN pins. When using the CMOS output in this configu- ration, the DDS output frequency should be in the range of

30 MHz to 150 MHz. At low output frequencies (<30 MHz), the low slew rate of the DAC results in a higher noise floor. This can be remedied by running the DDS at 100 MHz or greater and using the CMOS divider. At an output frequency of 50 MHz, the best technique depends on the user’s application. Running the DDS at 200 MHz, and using a CMOS divider of 4, results in a lower noise floor, but at the expense of close-in phase noise.

At frequencies greater than 150 MHz, the HSTL output should be used.

CMOS Output Divider (S-Divider)

The CMOS output divider is 16 bits cascaded with an additional divide-by-two. The divider is therefore capable of integer division from 1 to 65,535 (index of 1) or from 2 to 131,070 (index of 2).

The divider is programmed via the I/O register map to trigger on either the rising (default) or falling edge of the feedback signal.

The CMOS output divider is an integer divider capable of handling frequencies well above the Nyquist limit of the DDS. The S-divider/2 bit (Register 0x0106, Bit 0) must be set when FDBK_IN is greater than 400 MHz.

Note that the actual output divider values equal the value stored in the output divider register minus one. Therefore, to have an output divider of one, the user writes zeros to the output divider register.

The most significant spurious signals produced by the DDS are harmonically related to the desired output frequency of the DDS. The source of these harmonic spurs can usually be traced to the DAC, and the spur level is in the −60 dBc range. This ratio represents a level that is about 10 bits below the full-scale output of the DAC (10 bits down is 2−10, or 1/1024).

Such a spur can be reduced by combining the original signal with a replica of the spur, but offset in phase by 180°. This idea is the foundation of the technique used to reduce harmonic spurs in the AD9912. Because the DAC has 14-bit resolution, a −60 dBc spur can be synthesized using only the lower 4 bits of the DAC full-scale range. That is, the 4 LSBs can create an output level that is approximately 60 dB below the full-scale level of the DAC (commensurate with a −60 dBc spur). This fact gives rise to a means of digitally reducing harmonic spurs or their aliased images in the DAC output spectrum by digitally adding a sinusoid at the input of the DAC with a similar magnitude as the offending spur, but shifted in phase to produce destructive interference.

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