EFJohnson 001-9800-001 Manuel D’Utilisation

3-24

February 2001

Part No. 001-9800-001

VCO to the center of the operating band when the 
control voltage is at its midpoint.

The output signal on the collector of Q902 is 

coupled by C911 to a buffer amplifier formed by Q901 
and Q903. This is a shared-bias amplifier which 
provides amplification and also isolation between the 
VCO and stages which follow. C918 provides imped-
ance matching on the input, and the resistors in the 
circuit provide biasing and stabilization (R913 also 
provides current limiting). C901, C902, and C908 are 
RF decoupling capacitors, and C909 and C919 provide 
an AC ground on the emitters of Q901 and Q903. 

The output signal on the collector of Q903 is fed 

through C914 to synthesizer U804. A 50-ohm load is 
provided by R855, and C841, C842, and C849 provide 
impedance matching. Likewise, the output signal on 
the collector of Q901 is fed through C904 to buffer 
amplifier Q801 (see Section 3.10.5). Resistor R902 
lowers the Q of L903 to make is less frequency 
selective.

VCO Frequency Shifting

In the applicable 800 or 900 MHz band, the VCO 

must be capable of producing frequencies from the 
lowest receiver first injection frequency up to the 
highest talk-around mode transmit frequency. At 800 
MHz, the lowest VCO frequency is 798.0625 MHz 
(52.950 MHz below the lowest receive frequency of 
851.0125 MHz) and the highest talk-around frequency 
is 869.9875 MHz. Therefore, the VCO frequency band 
spread is approximately 72 MHz. If this large 
frequency shift was achieved only by varying the VCO 
control voltage, the VCO gain would be undesirably 
high. Instead, capacitance is switched in and out of the 
tank circuit to provide a coarse frequency shift. 

This switching is provided by PIN diode CR901 

which is controlled by the logic signals from the Q0 
and Q1 outputs (pins 4 and 5) of shift register U800. 
Q803 and Q804 function as inverters and drivers. 
When a PIN diode is forward biased, it presents a very 
low impedance to RF signals. Conversely, when it is 
reverse biased, it presents a very high impedance. 

Forward biasing of PIN diode CR901 adds capac-

itance to the tank circuit which lowers its resonant 
frequency. The diode is forward biased in the normal 
mode and reverse biased in the talk-around mode (both 

800 and 900 MHz). The logic levels on pins 3 and 4 of 
the VCO are as follows:

H (5.5V)

L (0V)

L (0V)

H (5.5V)

In the normal mode, CR901 is forward biased by 

current flowing through R903, L905, CR901, and 
L902. Capacitors C907 and C912 are then effectively 
AC grounded through CR901 and C905. The control 
lines are isolated from tank circuit RF by L902/C903, 
L905/C906, C811, C812, C823, and C824. 

Frequency Control and Modulation

Fine VCO frequency control is performed by 

varying the DC voltage across varactor diodes CR903 
and CR904 (coarse control is provided as described in 
the preceding description). As the DC voltage applied 
across a reverse-biased varactor diode increases, its 
capacitance decreases. Therefore, the VCO frequency 
increases as the control voltage increases and vice 
versa. The amount of frequency change produced by 
CR903 and CR904 is set by series capacitor C922. A 
DC ground is provided by L908, and the control line is 
isolated from tank circuit RF by L909 and C925.

The VCO is frequency modulated in a similar 

manner. Another capacitance leg of the tank circuit is 
formed by C915, C920, and varactor diode CR902. 
The audio and data modulation signal is applied across 
CR902, and a fixed bias from a voltage divider formed 
by R853 and R854 is applied through R851 to pin 5. 
Isolation and filtering of this DC bias is provided by 
C838, C839, C840, and R852. Refer to the next 
section for more information on modulation.

3.10.3 VCO AND TCXO MODULATION

Both the reference oscillator and VCO are modu-

lated in order to provide the required frequency 
response. If only the VCO was modulated, the phase 
detector in U804 would sense the frequency difference 
and change the control voltage to counteract it, espe-