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ISL58214
Oscillator Control
A high frequency component can be added to the laser current to
reduce speckle.
The HFM block has two states determined by whether the
oscillator is enabled or disabled. The HFM block outputs RF
modulation when the oscillator is enabled and a DC level when
the oscillator is disabled. The DC level is PHFMOFF in HFMOFF
mode.
TABLE 23. OSCILLATOR CONTROL LOGIC
X-01-3 OSCILLATOR OUTPUT
HFM Frequency Counter
HFM frequency can be monitored via internal counter. The way to
monitor the HFM frequency is to measure how many cycles occur
during a serial clock period. If the serial clock period is known,
this enables the HFM to be measured and adjusted by the drive
firmware. To enable this measurement, follow the following
steps:
? First clear Reg X-01 Bit 7 = 0. This allows the serial port to use
the control register at Reg 1-0E.
? Set Reg 1-0E bit 1 = 1. This enables the serial clock to gate the
counter.
0
1
Off
On
? Set Reg1-0E bit 0 = 1. This step is optional, but recommended.
This would divide the serial clock by 2, hence increase the
HFM I OUT Equations
The oscillator current is approximately determined by
interpolation from the data sheet.
IOUT OSC-ON ? A ON,max x PHFMON/255 x K REF OR Zero.
where:
? A ON,max = for example: 100mA (typ).
? K REF = (RSET SPEC /RSET)
? RSET SPEC = for example 620 Ω .
When the HFM oscillator is turned on, the oscillator current
reaches a peak value set by PHFMON. The relation between
PHFMON and average optical power is difficult to derive
deterministically. First, the AC current reaching the laser is
attenuated by parasitics. The higher the HFM frequency, the
larger the loss. Second, if the laser is biased near threshold, the
turn-on delay reduces the optical pulse width. Third, laser
relaxation oscillation further shapes the optical pulse.
An empirical procedure is to first find the best conditions for the
AC current (PHFMON) and frequency (HFMFREQ) in terms of read
noise reduction.
21
number of count of cycles by 2. Since SEN is not synchronized
with the HFM, higher number of counts means lower the error.
? Set Reg X-01 bit 7 = 1. This allows the control logic to measure
the burst of HFM between serial clock edges. The control logic
will continue to do this on any serial transfer, either a write or a
read. The measurement is done during the address portion of
the serial cycle.
? By reading Reg 1-0E, the measurement is done.
? The HFM frequency then can be calculated as HFM freq =
HFMCNT(dec) x known SCLK/2.
The amount of HFM current that reaches the laser and produces
light modulation is a very complex transfer function. Although
the HFM circuit simply switches between zero and the specified
current, the frequency is so high that the result is heavily
influenced by the chip, packages and layout.
FN6944.1
July 29, 2013
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