AK1590 Datasheet, PDF (16/36 Pages) Asahi Kasei Microsystems – 1GHz Delta-Sigma Fractional-N Frequency Synthesizer

[AK1590]
5.2 Digital Lock Detect
In digital lock detect, [LD] pin outputs ”Low” when the frequency is set. The output of [LD] pin turns from “Low” to “High”
(which means “locked state”) when a phase error smaller than T is detected for 63 times consecutively. If the phase error
that is larger than T is detected for 63 times consecutively during [LD] pin outputs “High”, the output of [LD] pin turns to
“Low”(which means “unlocked state”).
The accuracy of the phase detect is set by {LDCKSEL[1:0]}.
{LDCKSEL[1:0]} is set to “00”: T = REFIN cycle (This is not available for the reference dividing ratio ≤ 3.)
{LDCKSEL[1:0]} is set to “01”: T = REFIN cycle × 2 (This is not available for the reference dividing ratio ≤ 5.)
{LDCKSEL[1:0]} is set to “10”: T = REFIN cycle × 3 (This is not available for the reference dividing ratio ≤ 6.)
Since AK1590 is a Delta-Sigma Fractional-N type, a phase error up to 7 times larger than the VCO period frequency may
occur in the phase detector. Therefore {LDCKSEL[1:0]} setting should be large enough to cover the amplitude of the
Delta-Sigma Fractional frequency. However, if the VCO frequency does not satisfy either of the following formula, the
digital lock detect is not available. In such case, the analog lock detect should be used.
{DITH} = D[14] in <Address3> is set to “1” (DITH ON):
VCO frequency > [REFIN] pin input frequency / [{LDCKSEL[1:0]} + 1] × 7
{DITH} = D[14] in <Address3> is set to “0” (DITH OFF):
VCO frequency > [REFIN] pin input frequency / [{LDCKSEL[1:0]} + 1] × 4
Example 1)
When [REFIN] input frequency = 33.6MHz, {DITH} = 1, {LDCKSEL[1:0]} = ”10”;
33.6MHz / (2+1) × 7 = 78.4MHz
As a result, the digital lock detect is not available if the VCO frequency is equivalent to or smaller than
78.4MHz.
Example 2)
When [REFIN] input frequency = 33.6MHz, {DITH} = 0, {LDCKSEL[1:0]} = “01”;
33.6MHz / (1+1) × 4 = 67.2MHz
As a result, the digital lock detect is not available if the VCO frequency is equivalent to or smaller than
67.2MHz.
MS1478-E-01
16
2013/2