XR-2207 Datasheet, PDF (13/24 Pages) Exar Corporation – Voltage-Controlled Oscillator

XR-2207
Frequency Control (Sweep and FM)
The frequency of operation is controlled by varying the
total timing current, I_T, drawn from the activated timing
pins 4, 5, 6, or 7. The timing current can be modulated by
applying a control voltage, V_C, to the activated timing pin
through a series resistor R_C. As the control voltage
becomes more negative, both the total timing current, I_T,
and the oscillation frequency increase.
The circuits given in Figure 15 and Figure 16 show two
different frequency sweep methods for split supply
operation.
Both binary keying inputs are grounded for the circuit in
Figure 15. Therefore, only timing pin 6 is activated.
The frequency of operation, normally
f
+
1
R3C
is now
proportional to the control voltage, V_C, and determined
as:
ƪ ƫ _f
+
1
R3C
1
*
VCR3
RCV-
Hz
If R₃ = 2MΩ, R_C = 2kΩ, C = 5000pF, then a 1000:1
frequency sweep would result for a negative sweep
voltage V_C  V-.
The voltage to frequency conversion gain, K, is controlled
by the series resistance RC and can be expressed as:
K
+
Df
DVC
+
1
RCCV-
HzńV
The circuit of Figure 15 can operate both with positive and
negative values of control voltage. However, for positive
values of V_C with small (R_C/R₃) ratio, the direction of the
timing current I_T is reversed and the oscillations will stop.
Figure 16 shows an alternate circuit for frequency control
where two timing pins, 6 and 7, are activated. The
frequency and the conversion gain expressions are the
same as before, except that the circuit will operate only
with negative values of V_C. For V_C > 0, pin 7 becomes
deactivated and the frequency is fixed at:
f
+
1
R3
The circuit given in Figure 17 shows the frequency sweep
method for single supply operation. Here, the oscillation
frequency is given as:
ƪ ǒ Ǔƫ f
+
1
R3C
1
)
R3
RC
1
*
VC
VT
where VT = Vbias + 0.7V.
This equation is valid from VC = 0V (RC is in parallel with
R3) to
ǒ Ǔ VC + VT
1
)
RC
R3
Caution
Total timing current I_T must be less than 6mA over the frequency
control range.
Rev. 2.02
13