Motorola P100 Series, Detailed Service Manual, Page: 44 / 336

5-6 435 – 480 MHz UHF2 Theory of Operation: Keypad
5.4.4 Loop Filter
The loop filter contains C751 – C754, and R726 – R728. It reduces the residual side-band noise to
get the best signal-to-noise ratio. The output signal from loop filter is applied to VCO.
5.4.5 Dual VCO
The dual VCO module contains a RX VCO and a TX VCO. They are configured as colpitts oscillators
and connected to power up through transistor switches. Only one VCO is selected at a time. A
steering line voltage between 0.35 V and 9.7 V at varactor CR701 tunes the full RX frequency range
from 389.9 MHz to 434.9 MHz, and varactor CR702 tunes the full TX frequency range from 435 MHz
to 480 MHz.
In Receiver mode, high signal of RX_EN from Pin 71, U101 activates Q305. When Q305 is activated,
current flows through the base of Q304 and thus activates the Q705. The varactor CR701 sets the
resonance frequency. When there is a change in voltage supplied by loop filter, there is a change in
the resonance frequency. L703 is the resonating coil, which forms the tank circuit together with
variable cap C710.
In Transmit mode, high signal of TX_EN1 from Pin 84, U101 enables current flows through collector
of Q503 and thus activates Q706. The varactor CR702 sets the resonance frequency. When there is
a change in voltage supplied by loop filter, there is a change in the resonance frequency. L706 is the
resonating tuning coil, which forms the tank circuit together with variable cap C722.
5.5 Keypad
Left, Right and P1 to P3 keys are directly connected to microprocessor via 22 pin connector. When
any of these keys is pressed, the voltage goes "low" and microprocessor detects it.
For full keypad models, the number keys are in matrix type which consisted of 3 rows and 4 columns.
When any of these keys is pressed, the voltage goes "low" and microprocessor interprets the voltage
for each key press.
Figure 5-4. Keypad Block Diagram
KEY P AD
L e f t
N UMB ER S
R ig h t
P1 P2 P 3
K ey
K ey ro w
K ey
co lu mn
22pin
connector
K
eypa
d
boar
d
22pin
connector
Main
boar
d
MCP
d etect