Microchip Technology SA5 Series, Руководство пользователя

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MAC-SA5X User’s Guide
DS50002938A-page 22

2019 Microchip Technology Inc.
The analog frequency control is derived from the analog voltage applied to Pin 1. This
voltage is digitized, scaled, and applied to the SA5X frequency servo. Analog tuning
(AT) voltages above mid-point (2.5VDC) increase the output frequency, and, con-
versely, tuning voltages below midpoint decrease the output frequency. Because the
tuning range is ±1x10
-8
from 0V to 5V, this corresponds to roughly 1x10
-11
for every
2.5 mV change in AT voltage.
If the analog frequency tuning is disabled (by default), the analog frequency control
value is zero. It may be enabled with the
AnalogTuningEnabled
Parameter. The
Analog Tuning input voltage may be read via the
AnalogTuning
parameter, regard-
less if it is enabled or not.
Recommendation: If Analog Tuning is the desired method, it is highly recom-
mended that it be used exclusively. Do not try to implement Analog Tuning with Digi-
tal Tuning or 1PPS Disciplining simultaneously into one system.
3.3 DIGITAL TUNING
The MAC RF (and 1PPS) output may be adjusted by the user via the
DigitalTuning
parameter. Relative steering values are entered in (integer) units of parts in 10
15
,
though the resolution realized by the MAC hardware is approximately 1 part in 10
14
.
Digital adjustment is clamped to ±2 parts in 10
8
. Consult Section 3.3.1 “Calibration”
if a larger correction is desired.
Note: Steering commands may be entered during acquisition (
Locked
= 0) but
will not take effect until lock is achieved.
Frequency steering is volatile (unless the
store
command is used). Upon reboot, the
MAC returns to its nominal (calibrated) frequency setting. To update the non-volatile
calibration, see
Section 3.3.1 “Calibration” .
3.3.1 Calibration
In the unlikely event that the adjustment range of
DigitalTuning
has been
exhausted beyond the limits of ±2x10
-8
(perhaps due to cumulative frequency aging off-
sets) it may be desirable to update the calibration. When this is the case, manual cali-
bration of the MAC is accomplished in the following manner. First, the MAC output
should be compared to a superior Frequency Reference with a frequency counter or
other suitable test equipment. Next, the MAC is steered onto frequency by adjusting
the
DigitalTuning
parameter, (see Section 3.3 “Digital Tuning” or
Section 3.4 “1PPS Disciplining” ). Finally, the present value of the
DigitalTuning
parameter is summed into the non-volatile calibration register via the
latch
command.
This command will simultaneously reset
DigitalTuning
to zero at the new center
frequency. The user may then adjust the output ±2x10
-8
from its current value.
Note: Total net steering range is limited to ±1x10
-6
. See data sheet for exact
specification.
Note: The
latch
command is only valid when the MAC is locked (
Locked
= 1).
Note: Once a calibration is overwritten, it cannot be recalled.