12
Interfacing to the HSP45106 NCO-16
The HSP45106 is a 16-bit, Numerically Controlled Oscillator
(NCO). The HSP45106 can be used to generate various
modulation schemes for Direct Digital Synthesis (DDS)
applications. Figure 28 shows how to interface an HI5731 to
the HSP45106.
Interfacing to the HSP45102 NCO-12
The HSP45102 is a 12-bit, Numerically Controlled Oscillator
(NCO). The HSP45102 can be used to generate various
modulation schemes for Direct Digital Synthesis (DDS)
applications. Figure 29 shows how to interface an HI5731 to
the HSP45102.
This high level block diagram is that of a basic PSK
modulator. In this example the encoder generates the PSK
wa vef orm by driving the Phase Modulation Inputs (P1, P0) of
the HSP45102. The P1-0 inputs impart a phase shift to the
carrier wa ve as defined in Table 2.
The data port of the HSP45102 drives the 12-bit HI5731
DAC which converts the NCO output into an analog
waveform. The output filter connected to the DAC can be
tailored to remove unwanted spurs for the desired carrier
frequency. The controller is used to load the desired center
frequency and control the HSP45102. The HI5731 coupled
with the HSP45102 make an inexpensive PSK modulato r
with Spurious Free performance down to -76dBc.
Definition of Specifications
Integral Linearity Error, INL, is the measure of the worst
case point that deviates fro m a best fit straight line of data
values along the transfer curve.
Differential Li neari ty Error, DNL, is the measure of the
error in step size between adjacent codes along the
conv erter’ s transf er curve. Ideally, the step size is 1 LSB from
one code to the next, and the deviation from 1 LSB is known
as DNL. A DNL specification of greater than -1 LSB
guara nt ee s mo notonicity.
Feedthru, is the measure of the undesirable s witching noise
coupled to the output.
Output Voltage Full Scale Settling Time, is the time
required from the 50% point on the clock input for a full scale
step to settle within an ±1/2 LSB error band.
Output Voltage Small Scale Settling Time, is the time
required from the 50% point on the clock input fo r a 100mV
step to settle within an 1/2 LSB error band. This is used by
applications reconstructing highly correlated signals such as
sine waves with more than 5 points per cycle.
Glitch Area, GE, is the switching transient appearing on the
output dur ing a code transition. It is measured as the area
under the curve and exp r essed as a picoVolt-time
specification (typically pV-s).
Differential Gain, ∆AV, is the gain error from an ideal sine
wa ve with a normalized amplitude.
Differential Phase, ∆Φ, is the phase error from an ideal sine
wave.
Signal to Noise Ratio, SNR, is the ratio of a fundamental to
the noise floor of the analog output. The first 5 harmonics
are ignored, and an output filter of 1/2 the clock frequency is
used to eliminate alias products.
Total Harmonic Di stortion, THD, is the ra tio of the DAC
output fundamental to the RMS sum of the harmonics. The
first 5 har monics are included, and an ou tput filter of 1/2 the
clock frequency is used to eliminate alias products.
Spurious Free Dynamic Range, SFDR, is the amplitude
difference from a fundamental to the largest harmonically or
non-harmonically related spur . A sine wa ve is loaded into the
D/A and the output filtered at 1/2 the cl ock frequency to
eliminate noise from clocking alias terms.
Intermodulation Distortion, IMD , is the measure of the
sum and difference products produced when a two tone
input is driven into the D/A. The distortion products created
will arise at sum and difference frequencies of the two tones.
IMD can be calculated using the following equation:
TABLE 3. PHASE MODULATION INPUT CODING
P1 P0 PHASE SHIFT (DEGREES)
00 0
01 90
1 0 270
1 1 180
IMD 20Log (RMS of Sum and Difference Distortion Products)
RMS Amplitude of the Fundamental()
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HI5731