AD9520-5/PCBZ Analog Devices Inc, AD9520-5/PCBZ Datasheet - Page 32

• Manufacturer Part Number: AD9520-5/PCBZ
• Description: 12/24 Channel Clock Gen 2,0GH
• Manufacturer: Analog Devices Inc

Specifications of AD9520-5/PCBZ

• Design Resources: Synchronizing Multiple AD9910 1 GSPS Direct Digital Synthesizers (CN0121) Phase Coherent FSK Modulator (CN0186)
• Main Purpose: Timing, Clock Generator
• Utilized Ic / Part: AD9520-5
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Lead Free Status / Rohs Status: Compliant

AD9520-5
There are several configurable modes of reference switchover.
The switchover can be performed manually or automatically.
Manual switchover is performed either through Register 0x01C
or by using the REF_SEL pin. The automatic switchover occurs
when REF1 disappears. There is also a switchover deglitch feature
which ensures that the PLL does not receive rising edges that are far
out of alignment with the newly selected reference.
There are two automatic reference switchover modes (0x01C):
• Prefer REF1. Switch from REF1 to REF2 when REF1 disappears.
• Stay on REF2. Automatically switch to REF2 when REF1
In automatic mode, REF1 is monitored by REF2. If REF1
disappears (two consecutive falling edges of REF2 without an
edge transition on REF1), REF1 is considered missing. On the
next subsequent rising edge of REF2, REF2 is used as the reference
clock to the PLL. If 0x01C[3] = 0b (default), when REF1 returns
(four rising edges of REF1 without two falling edges of REF2
between the REF1 edges), the PLL reference switches back to
REF1. If 0x01C[3] = 1b, the user can control when to switch
back to REF1. This is done by programming the part to manual
reference select mode (0x01C[4] = 0b) and by ensuring that the
registers and/or the REF_SEL pin is set to select the desired
reference. Automatic mode can be reenabled when REF1 is
reselected.
Manual switchover requires the presence of a clock on the reference
input that is being switched to or that the deglitching feature be
disabled (0x01C[7]).
Reference Divider R
The reference inputs are routed to the reference divider, R. R (a
14-bit counter) can be set to any value from 0 to 16,383 by writing
to 0x011 and 0x012. (Both R = 0 and R = 1 give divide-by-1.) The
output of the R divider goes to one of the PFD inputs to be
compared with the VCO frequency divided by the N divider.
The frequency applied to the PFD must not exceed the maximum
allowable frequency, which depends on the antibacklash pulse
setting (see Table 2).
The R divider has its own reset. The R divider can be reset using
the shared reset bit of the R, A, and B counters. It can also be
reset by a SYNC operation.
VCO/VCXO Feedback Divider N: P, A, B, R
The N divider is a combination of a prescaler (P) and two counters,
A and B. The total divider value is
where P can be 2, 4, 8, 16, or 32.
Return to REF1 from REF2 when REF1 returns.
disappears, but do not switch back to REF1 when it reappears.
The reference can be set back to REF1 manually at an
appropriate time.
N = (P × B) + A
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Prescaler
The prescaler of the AD9520 allows for two modes of operation:
a fixed divide (FD) mode of 1, 2, or 3, and a dual modulus
(DM) mode where the prescaler divides by P and (P + 1) {2 and
3, 4 and 5, 8 and 9, 16 and 17, or 32 and 33}. The prescaler
modes of operation are given in Table 48, 0x016[2:0]. Not all
modes are available at all frequencies (see Table 2).
When operating the AD9520 in dual modulus mode, P/(P + 1),
the equation used to relate the input reference frequency to the
VCO output frequency is
However, when operating the prescaler in FD Mode 1,
FD Mode 2, or FD Mode 3, the A counter is not used (A = 0)
and the equation simplifies to
When A = 0, the divide is a fixed divide of P = 2, 4, 8, 16, or 32.
By using combinations of DM and FD modes, the AD9520 can
achieve values of N all the way down to N = 1. Table 25 shows
how a 10 MHz reference input can be locked to any integer
multiple of N.
Note that the same value of N can be derived in different ways,
as illustrated by the case of N = 12. The user can choose a fixed
divide mode P = 2 with B = 6, use the dual modulus mode 2/3
with A = 0, B = 6, or use the dual modulus mode 4/5 with
A = 0, B = 3.
A and B Counters
The B counter must be ≥3 or bypassed, and unlike the R
counter, A = 0 is actually zero.
The maximum input frequency to the A/B counter is reflected
in the maximum prescaler output frequency (~300 MHz) specified
in Table 2. This is the prescaler input frequency (external VCO
or CLK) divided by P. For example, dual modulus P = 8/9 mode
is not allowed if the external VCO frequency is greater than
2400 MHz because the frequency going to the A/B counter is
too high.
When the AD9520 B counter is bypassed (B = 1), the A counter
should be set to zero, and the overall resulting divide is equal to
the prescaler setting, P. The possible divide ratios in this mode
are 1, 2, 3, 4, 8, 16, and 32.
Although manual reset is not normally required, the A/B counters
have their own reset bit. Alternatively, the A and B counters can be
reset using the shared reset bit of the R, A, and B counters. Note
that these reset bits are not self-clearing.
R, A, and B Counters: SYNC Pin Reset
The R, A, and B counters can be reset simultaneously through the
SYNC pin. This function is controlled by 0x019[7:6] (see
The
SYNC pin reset is disabled by default.
f
f
VCO
VCO
= (f
= (f
REF
REF
/R) × (P × B + A) = f
/R) × (P × B) = f
REF
× N/R
REF
× N/R
Table 48
).