LTC5599IUFPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC5599IUFPBF' PDF : 42 Pages View PDF
LTC5599
Applications Information
Register 0x05 contains two parts (see Table 11); the five
least significant bits IQPHF realize a fine phase adjustment,
while the three most significant bits IQPHE are used for
coarse adjustments. The fine phase adjustment realized
by IQPHF can be approximated as:
jIQ = –((Nph –16)/15) • ln(fLO/50) (degrees)
for 30MHz < fLO < 1300MHz
where Nph is the decimal value of IQPHF and fLO is the
frequency of the LO signal in MHz. A positive value for
jIQ means that the I-channel LO phase is more than 90°
ahead of the Q-channel LO phase. Notice from the expres-
sion that the phase adjustment range and resolution are
coupled, and dependent on the LO frequency. At low LO
frequencies the the smallest adjustment range and highest
resolution is achieved, while high LO frequencies exhibit
the largest range and lowest resolution.
The extension bits IQPHE provide a larger phase adjust-
ment range, particularly useful at lower LO frequencies,
and overcome another trade-off; between phase adjustment
range and the maximum center frequency of the poly-
phase filter. The latter trade-off is due to the fact that the
capacitances in the I-channel, CppI, and Q-channel, CppQ, of
the poly-phase filter control both these parameters. Their
difference sets the phase shift, while their sum determines
the center frequency of the filter.
The extension bits IQPHE introduce a large phase offset in
addition to the fine adjustment realized by the IQPHF bits.
The sign of this large offset can be positive or negative,
controlled by IQPHSIGN (bit 7 in register 0x00). Including
these bits, the total phase shift from quadrature can be
expressed as:
jIQ = –(MPH/15) • ln(fLO/50) (degrees) with
MPH = NCOARSE + NPH –16 and
NCOARSE = 32 • (–1)IQPHSIGN + 1 • NEXT
where Next equals the decimal value of the IQPHE bits. The
valid range of values for (Nph –16) is thus expanded from
{–16, –15, ... , +15} to {–240, –239, ... , +239}. Table 9 in
the Appendix lists all the possible combinations. The cod-
ing ranges for IQPHSIGN = 0 and IQPHSIGN = 1 overlap
between Mph = –16 and Mph = +15, such that IQPHSIGN
only needs to be changed for larger phase shifts.
As a side effect, the extension bits slightly detune the
center frequency of the poly-phase filter, after crossing the
boundary to a new NCOARSE value. This can be observed
as a large step in the actual phase shift. A solution for this
is to decrease the value in the frequency register 0x00
(increase the poly-phase filter center frequency) at the
NCOARSE value boundaries. The result is a smooth phase
adjustment. In the demo board QuikEval GUI, this LO fre-
quency register adjustment is automatically taken care of.
Whenever the poly-phase filter center frequency is adjusted
to improve the smoothness of the phase adjustment, it is
recommended to manually program the LO port impedance
match using the CLOO bits in register 0x06. By default,
changing the filter center frequency also automatically
adjusts the matching of the LO port (when CLOEN, bit 4
in register 0x06 is set). However, since the LO carrier
frequency does not change, automatic adjustment of the
LO match is undesirable in this case; it may add another
large step to the phase adjustment. Instead, the LO match
should remain unchanged while the filter center frequency
is adjusted. This can be achieved as follows. First, the
current LO matching configuration is read from the CLO
bits in register 0x1D, and written to the CLOO override
bits in register 0x06. Subsequently, the CLOEN bit (bit 4,
register 0x06) is cleared to disable automatic LO match
adjustment. As a result the center frequency can be ad-
justed in register 0x00 without changing the LO match.
At 100MHz the maximum phase shift is about ±9.8°, while
at 1GHz it is about ±3°. The extension bits are not useful
above 988.2MHz since the poly-phase center frequency
register 0x00 value cannot be adjusted low enough to
ensure a smooth transition to a new NCOARSE value.
Square Wave LO Drive
Harmonic content of the LO signal adversely affects
quadrature phase error and gain accuracy, whenever a
poly-phase filter is used for quadrature generation. The
LTC5599 can correct for phase and gain errors due to har-
monics in the LO carrier (e.g. in a square wave) by setting
appropriate values in the I/Q gain and I/Q phase registers.
Such adjustments are typically needed when the 3rd-order
harmonic of the LO signal exceeds the desirable side-band
suppression minus 13dB. Although the poly-phase filter
is less sensitive to the second harmonic content of the LO
5599f
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For more information www.linear.com/LTC5599
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