Alan HP106 - 2 CIRCUIT DESCRIPTION (HP406); 2.1 General information; 2.2 Microprocessor;control; 2.3 VCO; Synthesizer (PLL)

HP406 Service Manual
Page 4 of 12
2 CIRCUIT DESCRIPTION
2.1 General information
The HP-406 has three printed circuit boards. Circuitry is divided in the following sections:
§ Microprocessor/Control
§ VCO/Synthesizer
§ Transmitter
§ Receiver
§ Signalling
§ Battery
Refer to the Block Diagram and the Schematics.
2.2 Microprocessor/control
The microprocessor IQ9 is constantly operating when the radio is turned ON. It is continuously monitoring the
keyboard, the PTT line and other internal inputs such as the squelch detect, etc. When a change occurs, the
microprocessor makes the appropriate response. The microprocessor is used for control. The Radio emits a
beep on channel change and the synthesizer is loaded with the correct frequency information. The
microprocessor runs off a 8 MHz oscillator which is composed of X1 , C30 , C31 and R48 .
When the radio is first turned on, the microprocessor reads the radio status from the EEPROM Q8 .
The microprocessor determines the receive frequency codes, then loads the synthesizer via its pins 42 (PLL LE),
46 (PLL CLK) and 43 (PLL DATA).
The microprocessor is fitted with an internal program flash memory as well, therefore functions can be
customised, if necessary, upon specific request from the customer.
2.3 VCO / Synthesizer (PLL)
This section consists of the Temperature-Compensated Crystal Oscillator (TCXO), Voltage Controlled
Oscillator(VCO), Synthesizer and the Loop Filter.
2.3.a Temperature-Compensated Crystal Oscillator (TCXO)
The reference oscillator ( X401 ) is a temperature compensated crystal- oscillator, RV402 is used to adjust the
oscillator on frequency (14.4 MHz) at room temperature (22  ). The reference oscillator is held within the
specified ± 2.5 PPM from -30 to +60  .
2.3.b Voltage-Controlled Oscillators
Only one of the VCOs runs at a time, which is controlled by Q420 and Q17 . When the PTT is pressed, Q17 pin
34 goes low (approx. 0V) disabling the receive VCO by the Q420 and biases on Q919 to enable the transmitter
VCO.
The receive VCO consists of C482-C485 , CV901 , L926-L928 , and Q922-Q923 . This VCO oscillates at 45.1 MHz
above the programmed receive frequency. The VCO’s oscillating frequency is tuned by the varactors Q923 . The
tuning voltage is supplied from the output of the Loop Filter. The output of the VCO is AC coupled ( C475 and
R558 , C530 , C505 ) to the synthesizer and the output buffer Q414 respectively.
The transmitter VCO consists of C984 , C980-C982 , CV902 , L924 , L925 , L929 and Q916-Q917 .
This VCO oscillates on the programmed transmit frequency. The VCO’s oscillating frequency is tuned by the
varactors Q917 . The tuning voltage is supplied from the output of the Loop Filter. The output of the VCO is AC
coupled ( C486 ) to the synthesizer input buffer Q428 and the output buffer Q415 respectively.
The transmit voltage controlled oscillator is directly frequency-modulated and operates on the carrier frequency.
In the receive mode, the transmit VCO is disabled and the receive VCO is enabled, producing the receive local
oscillator signal at a frequency 45.1 MHz below the incoming receive frequency.