Reference Oscillator; Lv Frac-N Synthesizer Ic; Voltage Multiplier; Figure 3-39. Waveform Representation During Programming Of The Lv Frac-N Ic (Uhf Range 2) - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Frequency Generation Unit (FGU)

3.13.3.1 Reference Oscillator

The radio's frequency stability and accuracy is derived from the 16.8 MHz reference oscillator
(Y5750). The 16.8 MHz reference oscillator circuitry provides a 16.8 MHz reference to the LV Frac-N
(U5752), receiver back-end IC (U5002), and the controller section of the XTL 1500 radio. The
reference oscillator circuitry consists of the reference oscillator Y5750 and the inverter/buffer circuitry
containing the active device U5751. Y5750 is a voltage-controlled, temperature-compensated crystal
oscillator (VCTCXO). Circuitry internal to Y5750 compensates for frequency error over temperature.
Warping of the oscillator on frequency is accomplished via a programmable DAC in the LV Frac-N.
The warp voltage is present at pin 25 (WARP) of U5752 and is applied to pin 1 of Y5750. The
16.8 MHz output frequency of Y5750 is capacitor-coupled to pin 23 of the LV Frac-N (U5752) and
also to the inverter/buffer stage U5751. L5753 and C5768 at the output of U5751 filter the 16.8 MHz
signal, and R5768 along with C5763 set the appropriate amplitude of the signal for the receiver back-
end and controller sections.

3.13.3.2 LV Frac-N Synthesizer IC

The LV Frac-N IC (U5752) functions include frequency synthesis, modulation control, voltage
multiplication and filtering, and auxiliary logic outputs for VCO selection.
U5752 is a mixed-mode IC containing digital and analog circuits. Separate power supply inputs are
used for the various functional blocks on the IC. Inductors L5755 and L5756 provide isolation
between supply pin 20 (AVDD - analog supply input) and pin 36 (DVDD - digital supply input)
connected to F3.0v. This 3.0 V DC supply is provided by U5750, a 3-V regulator IC.
All programmable variables on the synthesizer IC, such as the synthesizer frequency; reference-
oscillator warping; adapt-timer duration; modulation-attenuator setting; and auxiliary-control outputs,
which select one of five voltage-controlled oscillators, can be programmed through a serial
peripheral interface (SPI). The SPI is connected to the controller microcomputer via three
programming lines, namely the data (pin 7), clock (pin 8), and the chip enable (pin 9) of U5752
(Figure 3-39).

Figure 3-39. Waveform Representation During Programming of the LV Frac-N IC (UHF Range 2)

3.13.3.3 Voltage Multiplier

Pin 12 (VMULT3) and pin 11 (VMULT4) of U5752, together with diode arrays D5750 and D5751 and
their associated capacitors C5775, C5776, C5777 and C5778, form the voltage multiplier. The
voltage multiplier generates 13.4 Vdc from the 5.0-V supply to supply the phase detector and
charge-pump output stage at pin 47 (VCP) of U5752. This voltage multiplier is basically a stacked,
multiple-diode capacitor network driven by two 1.05 MHz, 180 degrees out of phase signals from
pins 12 and 11 of U5752.
6815854H01-A
Pin 9 (Chip Select)
Pin 7 (Data)
Pin 8 (Clock)
MAEPF 2 80 O
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June 15, 2005