Nautel XR6, Troubleshooting Manual, Page: 75 / 291

XR6/XR3 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-15 Page 2-7
Balanced drive
The balanced drive buffer is a switching circuit that ensures the rise and fall times of its output square
wave are minimal. The f
c
signal is a 15 V peak-to-peak square wave that is the low level RF drive
signal for the RF power stage of the transmitter.
Balanced drive matching
Provision is made for several different values on the balanced drive output, depending on the host
transmitter's requirements. The positioning of shorting shunt post E5 on seven-position header XE5
can be changed to choose between different capacitor and resistor values. E5 should be installed in
the
D
position (shorting pins 7 and 8) for an XR6/XR3 transmitter.
N divider
The N divider circuit is a cascade counter made up of integrated circuits U6, U7, and inverters U8:A
and U8:F. The 4f
c
/B signal is divided by N, to provide a 2f
PDM
frequency which is between 126 kHz
and 134 kHz (when E3 is set in
LOW PDM
mode) or between 245 kHz and 276 kHz (when E3 is set
in
HIGH PDM
mode). E3 should be set to
HIGH PDM
mode when used in an XR6/XR3 transmitter.
The value of N is supplied by microprocessor U4 (see “N divider control information” on page 2-5 ).
The 2f
PDM
output, which is nominal 0 - 4 V peak pulses, is applied to the transmitter's pulse duration
modulation (PDM) generation circuit, and ultimately determines the PDM frequency.
RF drive source selection
The RF drive source may be provided by the integral numerically controlled oscillator, an external RF
generator at f
c
or a 10 MHz reference. The positioning of shorting shunt posts E1, E2 and E4
determines which RF drive source is selected.
Internal f
c
source
When the integral numerically controlled oscillator's output is to be used as the RF drive source, the
shorting shunt posts must be configured as follows:
- E1 in
INT
position (pins 2 and 3 shorted)
- E2 in
INT
position (pins 2 and 3 shorted)
- E4 in
INT
position (pins 2 and 3 shorted)
External f
c
source
When an externally generated carrier frequency is to be used as the RF drive source, the shorting
shunt posts must be configured as follows: