Voltage Multiplier; Superfilter; Modulation; Figure 3-36. Waveform Representation During Programming Of The Lv Frac-N Ic (Vhf) - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

Theory of Operation: Frequency Generation Unit (FGU)
U3751 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 L3757 and L3756 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 U3750, 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 four 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 U3751
(Figure 3-36).

Figure 3-36. Waveform Representation During Programming of the LV Frac-N IC (VHF)

3.7.1.3 Voltage Multiplier

Pin 12 (VMULT3) and pin 11 (VMULT4) of U3751, together with diode arrays D3757 and D3758 and
their associated capacitors C3763, C3764, C3765 and C3767, form the voltage multiplier. The
voltage multiplier generates 12.0 Vdc from the 3.0-V supply to supply the phase detector and
charge-pump output stage at pin 47 (VCP) of U3751. 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 U3751.

3.7.1.4 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 L3752, C3811, C3751, and C3820. This
voltage is applied to pin 30 (SFIN) of U3751 and the emitter of Q3756. The output is a superfiltered
8.2 Vdc at the junction of pin 28 (SFOUT) of U3751 and the collector of Q3756. Filtering is
accomplished with capacitors C3821, C3753, and C3752 at the output of this circuit and C3823 at
pin 26 of U3751.

3.7.1.5 Modulation

To support many voice, data, and signaling protocols, XTL 5000 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.
6881096C74-B
Pin 9 (Chip Select)
Pin 7 (Data)
Pin 8 (Clock)
MAEPF-27805-O
3-47
May 25, 2005