AN303 데이터 시트보기 (PDF) - Silicon Laboratories
부품명
상세내역
일치하는 목록
AN303 Datasheet PDF : 4 Pages
| |||
AN303
2.2. Control Voltage Temperature Behavior
The control voltage of the Si55x is converted by the VCADC and applied to the internal synthesizer in real-time.
The Si55x does not pull the crystal to achieve frequency tuning, so, in this way, differs from traditional VCXOs. The
key advantage of this digital approach is to provide customer orderable tuning slopes (Kv). Lower Kv’s can be
advantageous when designing a PLL, but this topic is beyond the scope of this application note. (See AN255 for
additional detail).
The VCADC relies on an internal voltage reference to define the full-scale voltage (conversion range). The internal
voltage reference changes with temperature causing the Kv to vary at a rate of ~0.1%/C° (e.g., 0.045 PPM/V per
C° for Kv = 45 PPM/V). The VCADC is ground referenced and therefore provides no frequency adjustment for VC =
0.0 V.
The full-scale reference is also reset after each powerup. The reset attempts to calibrate the Kv back to its nominal
value. Together, the variation and reset bound the allowed Kv range. The range of Kv is specified in the data sheet
as a minimum and maximum percent change from the nominal value.
The resulting frequency for a given VC voltage for various start-up and operating temperature conditions is shown
in Figure 2.
VCXO Transfer Gain Temperature Dependency for Kv = 135PPM/V (Nominal)
(assumes zero crystal temperature dependency)
400
350
Family of transfer curves showing the
dependency on startup temperature
300
and running temperature
250
To see this
curve:
start at cold then
run at hot
Calibration
Accuracy
200
150
100
To see this
curve:
50
start at hot then
0
run at cold
-50
-100
Vc range that the PLL
will hold to maintain
-150
-200
0
0.5
1
1.5
2
2.5
3
3.5
Nominal Control Voltage (V)
Vc =
Worst case “Nominal” Vc
due to reduced slope @
Figure 2. VCXO Transfer Gain Temperature Dependency
2.3. Measuring the Crystal’s Temperature Stability
Because of the secondary effect of Kv variation, measurement of the crystal temperature stability must be made
under a specific condition. Because the VCADC is ground referenced, applying 0.0 V to the VC input effectively
disables the output frequency tuning. Once inactive, the VCADC temperature dependency (and Kv variation) is
removed, and the crystal’s temperature stability can be observed.
2
Rev. 0.1
Share Link: 