Voltage Multiplier; Superfilter; Modulation; Figure 3-37. Waveform Representation During Programming Of The Lv Frac-N Ic (Uhf Range 1) - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Frequency Generation Unit (FGU)
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 3-37).

Figure 3-37. Waveform Representation During Programming of the LV Frac-N IC (UHF Range 1)

3.13.2.3 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.

3.13.2.4 Superfilter

The superfilter is an active filter that provides a low-noise supply for the VCOs, receiver and
transmitter injection amplifiers. Regulator U0950, located in the controller section, supplies 9.3 Vdc
to the FGU section thru the filtering network consisting of L5750, C5751, C5753, and C5755. This
voltage is applied to pin 30 (SFIN) of U5752 and the emitter of Q5752. The output is a superfiltered
8.2 Vdc at the junction of pin 28 (SFOUT) of U5752 and the collector of Q5752. Filtering is
accomplished with capacitors C5766, C5769, and C5772 at the output of this circuit and C5770 at
pin 26 of U5752.

3.13.2.5 Modulation

To support many voice, data, and signaling protocols, XTL 1500 radios must modulate the
transmitter carrier frequency over a wide audio-frequency range, from less than 10 Hz up to more
than 6 kHz. The LV Frac-N IC supports audio frequencies down to zero Hz by using dual-port
modulation. The audio signal at pin 10 (MODIN) is internally divided into high- and low-frequency
components, which modulate both the synthesizer dividers and the external VCOs through signal
MODOUT (pin 41). The IC is adjusted to achieve flat modulation frequency response during
transmitter modulation balance calibration using a built-in modulation attenuator.
The Digital-to-Analog Converter (DAC) IC (U0900), and switched-capacitor filter (SCF) IC (FL0900)
form the interface between the radio's DSP and the analog input of the LV Frac-N IC.
6815854H01-A
Pin 9 (Chip Select)
Pin 7 (Data)
Pin 8 (Clock)
MAEPF-27805-O
3-51
June 15, 2005