Datasheet

LM97593
www.ti.com SNWS019B –JULY 2007–REVISED APRIL 2013
Figure 61. Complex NCO Output Phase Dither Disabled
Example of NCO spurs due to phase truncation
(Before Phase Dithering)
The LM97593 provides two sets of coefficient memory for both F1 and F2. These coefficient memories can be
independently routed to channel A, channel B, or both channel A and B with a crossbar switch. The coefficients
can be switched on the fly but some time will be required before valid output data is available.
The Numerically Controlled Oscillator
The tuning frequency of each down converter is specified as a 32 bit word (.02Hz resolution at CK=52MHz) and
the phase offset is specified as a 16 bit word (.005
o
). These two parameters are applied to the Numerically
Controlled Oscillator (NCO) circuit to generate sine and cosine signals used by the digital mixer. The NCOs can
be synchronized with NCOs on other chips via the sync pin SI. This allows multiple down converter outputs to be
coherently combined, each with a unique phase and amplitude.
The tuning frequency is set by loading the FREQ register according to the formula FREQ = 2
32
F/F
CK
, where F is
the desired tuning frequency and F
CK
is the chip’s clock rate. FREQ is a 2’s complement word. The range for F is
from -FCK/2 to +FCK(1-2
-31
)/2.
If a sub-sampled signal is in an even Nyquist zone the sampling process causes the order of the I and Q
components to be reversed. Should this occur simply invert the polarity of the tuning frequency F.
Figure 62. Complex NCO Output Phase Dither Enabled
Example of NCO spurs due to phase truncation
(After Phase Dithering)
The 2’s complement format represents full-scale negative as 10000000 and full-scale positive as 01111111 for
an 8-bit example.
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