XR-2207 Datasheet, PDF (17/24 Pages) Exar Corporation – Voltage-Controlled Oscillator

A. Squarewave and Triangle Outputs
B. Pulse and Sawtooth Outputs
C. Frequency Shift Keyed Outputs
Figure 19. Output Waveforms
Rev. 2.02
XR-2207
On-Off Keying
The XR-2207 can be keyed on and off by simply activating
an open circuited timing pin. Under certain conditions, the
circuit may exhibit very low frequency (<1Hz) residual
oscillations in the “off” state due to internal bias currents. If
this effect is undesirable, it can be eliminated by
connecting a 10MΩ resistor from pin 3 to V_CC.
Two-Channel FSK Generator (Modem Transmitter)
The multi-level frequency shift-keying capability of
XR-2207 makes it ideally suited for two-channel FSK
generation. A recommended circuit connection for this
application is shown in Figure 20.
For two-channel FSK generation, the “mark” and “space”
frequencies of the respective channels are determined by
the timing resistor pairs (R₁, R₂) and (R₃, R₄). Pin 8 is the
“channel-select” control in accord with Figure 11. For a
“high” logic level at pin 8, the timing resistors R₁ and R₂
are activated. Similarly, for a “low” logic level, timing
resistors R₃ and R₄ are enabled.
The “high” and “low” logic levels at pin 9 determine the
respective high and low frequencies within the selected
FSK channel. When only a single FSK channel is used,
the remaining channel can be deactivated by connecting
pin 8 to either V_CC or ground. In this case, the unused
timing resistors can also be omitted from the circuit.
The low and high frequencies, f₁ and f₂, for a given FSK
channel can be fine tuned using potentiometers
connected in series with respective timing resistors. In
fine tuning the frequencies, f₁ should be set first with the
logic level at pin 9 in a “low” level.
Typical frequency drift of the circuit for 0°C to 75°C
operation is $0.2%. Since the frequency stability is
directly related to the external timing components, care
must be taken to use timing components with low
temperature coefficients.
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