Motorola SSETM 5000 service manual Vocon Board, Loop Filter, VCO Buffer IC Vcobic

Theory of Operation: VOCON Board 2-17

2.4.4.2.3 Modulation

To support many voice, data, and signaling protocols, the SSE 5000 radio must modulate the transmitter carrier frequency over a wide audio frequency range, from less than 10 Hz up to more than 6 kHz. The FracN 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 modify 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.

2.4.4.2.4 Voltage Multiplier and Superfilter

Pins 12 (VMULT3) and 11 (VMULT4) together with diode arrays D201 and D202 and their associated capacitors form the voltage multiplier. The voltage multiplier generates 11.5 Vdc to supply the phase detector and charge-pump output stage at pin 47 (VCP).

The superfilter is an active filter that provides a low-noise supply for the VCOs and VCOBIC. The input is regulated 5 Vdc from Vdd5 at pin 30 (SFIN). The output is superfiltered voltage FSF at pin 28 (SFOUT).

The output from pin 15 (VMULT1) is used as a clock for the SCF IC, FL200.

2.4.4.3 Loop Filter

The components connected to pins 43 (IOUT) and 45 (IADAPT) form a 3rd-order, RC low-pass filter. Current from the charge-pump output, IOUT, is transformed to voltage VCTRL, which modulates the VCOs. Extra current is supplied by IADAPT for rapid phase-lock acquisition during frequency changes. The lock detector output pin 4 (LOCK) goes to a logic “1” to indicate when the phased-lock loop is in lock.

2.4.4.4 VCO Buffer IC (VCOBIC)

The VCOBIC (U250) is an analog IC containing two NPN transistors for use as oscillators, an active- bias circuit, transmitter and receiver buffer amplifiers, and switching circuitry. The VCOBIC has three RF outputs:

•TX_OUT (pin 10)—the modulated transmitter carrier

•RX_OUT (pin 8)—the receiver first LO

•PRESC_OUT (pin 12)—connected to FracN pin 32 (PREIN) through a matching circuit

Transmit/receive control is a single 5.0 Vdc logic input, TRB_IN (pin 19). When TRB_IN is low, the receiver buffer is active and the transmitter circuits are disabled. The converse is also true.

This radio uses two external, discrete, varactor-tuned, Colpitts VCOs based on transistors Q211 and Q215. Bias current to the VCOs is switched on and off by transistors Q214 and Q210, which are controlled by FracN outputs AUX2 and AUX4. Transistors packged in Q301 form a 3.3 Vdc-to-5 Vdc logic-level shifter for the signal from the FracN AUX3 pin to the VCOBIC.

2.5VOCON Board

This section provides a detailed circuit description of the SSE 5000 VOCON (vocoder and controller) board.

NOTE: Refer to Table 12-2for a listing of VOCON schematics that will aid in the following discussion.

The VOCON board block diagram (see Figure 2-7) contains three functional blocks and six connector symbols.

The functional blocks consist of the following:

6881094C12-A November 11, 2004