MC13110A/B MC13111A/B
24
MOTOROLA ANALOG IC DEVICE DATA
First Local Oscillator
The 1st LO is a multi–vibrator oscillator. The tank circuit is
composed of a parallel external capacitance and inductance,
internal programmable capacitor matrix, and internal
varactor. The local oscillator requires a voltage controlled
input to the internal varactor and an external loop filter driven
by on–board phase–lock control loop (PLL). The 1st LO
internal component values have a tolerance of
±
15%. A
typical dc bias level on the LO Input and LO Output is 0.45
Vdc. The temperature coefficient of the varactor is
+0.08%/
°
C. The curve in Figure 33 is the varactor control
voltage range as it relates to varactor capacitance. It
represents the expected internal capacitance for a given
control voltage (VcapCtrl) of the MC13110A/B and
MC13111A/B. Figure 32 shows a representative schematic of
the first LO function.
LO1 In
LO1 Out
Vcap Ctrl
Programmable
Internal
Capacitor
1st LO
Varactor
Varactor
Cext
Lext
Figure 32. First Local Oscillator Schematic
To select the proper Lext and Cext we can do the following
analysis. From Figure 34 it is observed that an inductor will
have a significant affect on first LO performance, especially
over frequency. The overall minimum Q required for first LO
to function as it relates to the LO frequency is also given in
Figure 34.
Choose an inductor value, say 470 nH. From Figure 34,
the minimum operating Q is approximately 25. From the
following equation:
Q Coil = Rp/X Coil
where: Rp = parallel equivalent impedance (Figure 35).
Cext can be determined as follows:
fLO
2
LextCext
where: Lext = external inductance
,
Cext = external
capacitance.
1
Figure 34 clearly indicates that for lower coil values, higher
quality factors (Q) are required for the first LO to function
properly. Also, lower LO frequencies need higher Q’s. In
Figure 35 the internal programmable capacitor selection
relative to the first LO frequency and the parallel impedance
is shown. This information will help the user to decide what
inductor (Lext) to choose for best performance in terms of Q.
Refer to the Auxiliary Register in the Serial Interface
Section for further discussion on LO programmability.