AZT71_1301 Datasheet, PDF (3/14 Pages) Arizona Microtek, Inc – Programmable Capacitive Tuning IC

Arizona Microtek, Inc.
AZT71
Programmable Capacitive Tuning IC
ENGINEERING NOTES
CAPACITOR STRUCTURE
The AZT71 capacitance value is composed of four parallel capacitors banks, C_F is a fixed capacitor value of 6.6pF and
C_hi, C_mid & C_lo are variable capacitors of differing ranges and resolutions as seen in Table 4. Capacitors composing C_hi,
C_mid and C_lo are set with a binary control word through an 11-bit shift register described in PROGRAMMING THE AZT71.
The values of each C_hi, C_mid and C_lo stepping are detailed in the complete Nominal Capacitance Binary Mapping
spreadsheet.
C_Total = C_F + C_hi + C_mid + C_lo
Table 4 - AZT71 Capacitor Structure
Internal
Capacitor
C_F
C_hi
C_mid
C_lo
Total
Min Value
(pF)
6.6
0
0
0
6.6
Max Value
(pF)
6.6
19.2
9.8
1.953
37.553
Step Size
(pF)
n/a
6.4
1.4
0.063
FUNCTIONAL MODES
The AZT71 has two methods for setting the capacitance value on the X₁ pin.
• READING THE CONTROL WORD DIRECTLY FROM THE SHIFT REGISTER
In tunable filter applications, reading from the shift register will be desirable as the control word can be constantly
varied. New control words can be serially inputted as required to change the capacitance value in real time. (Note:
With a serial data input, the capacitance value during transitions between control words is deterministic upon their
differences.) The shift register is also useful for testing the capacitance and subsequent oscillator frequency. This
mode is active when the CLK pin is left logic high. For the shift register, capacitors are selected when bits are active
HIGH.
• READING THE CONTROL WORD FROM THE VALUE CONTAINED IN THE EEPROM
If a certain control word needs to be stored, it can be written to the nonvolatile EEPROM memory. This is useful in
oscillator applications where it prevents customer adjustment and retains factory programming. This mode is active
when the CLK pin is at logic low or not connected. For the EEPROM, capacitors are selected when bits are active
LOW.
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Jan 2013, Rev 1.3