Simoco EMEA Ltd SDB670AC01 Manual De Usuario
SDB670 – SERVICE MANUAL
TNM-M-E-0032
May 13
Page 36
TECHNICAL DESCRIPTION
5.3.3.2
Power Control
Output power is stabilised by a power control feedback loop. A printed circuit transmission line,
L521, R557, C557, D506 and associated components comprise the power detector. Comparator
U502A and associated components provide the power setting and control functions. Forward and
reverse power is sampled by the power detector and applied as a DC voltage to the inverting input
of the comparator.
L521, R557, C557, D506 and associated components comprise the power detector. Comparator
U502A and associated components provide the power setting and control functions. Forward and
reverse power is sampled by the power detector and applied as a DC voltage to the inverting input
of the comparator.
Power output settings are derived from alignment data stored in flash memory during the initial
factory alignment. The DSP processes this data to extrapolate the power output level relative to
any programmed channel frequency. The PA-TXPWR voltage is read from the flash and outputted
from the FPGA as a PWM signal. This is integrated by a low pass filter comprising R81k and
C43k. Further low pass filtering is provided through the buffer stage U502B. The Tx power set
voltage at U502B-7 is a DC voltage proportional to the programmed Tx power setting and is
applied to the non-inverting input of the comparator U502A.
factory alignment. The DSP processes this data to extrapolate the power output level relative to
any programmed channel frequency. The PA-TXPWR voltage is read from the flash and outputted
from the FPGA as a PWM signal. This is integrated by a low pass filter comprising R81k and
C43k. Further low pass filtering is provided through the buffer stage U502B. The Tx power set
voltage at U502B-7 is a DC voltage proportional to the programmed Tx power setting and is
applied to the non-inverting input of the comparator U502A.
The resulting output from U502A is a voltage that corrects for output power variations and also set
to the required programmed level. This is applied to the power module U505-2 and controls the
internal gate bias level on each stage.
to the required programmed level. This is applied to the power module U505-2 and controls the
internal gate bias level on each stage.
PA module output level changes due to supply voltage, load or temperature variations, are
detected and applied to the comparator. This proportionally adjusts the PA module bias supply to
compensate for these variations.
detected and applied to the comparator. This proportionally adjusts the PA module bias supply to
compensate for these variations.
High temperature protection is provided by thermistor R590, which progressively reduces the
power level if the PA module temperature becomes excessive.
power level if the PA module temperature becomes excessive.
PA current is monitored via comparator U500A, which checks the voltage drop across
L500/L501/L502 and compares this to its pre-programmed alignment setting PA-ICAL. If this is
exceeded, PA-ISENSE will increase, thereby causing the Tx output power to be proportionally
reduced via the PA-TXPWR voltage until the current is below the aligned setting.
L500/L501/L502 and compares this to its pre-programmed alignment setting PA-ICAL. If this is
exceeded, PA-ISENSE will increase, thereby causing the Tx output power to be proportionally
reduced via the PA-TXPWR voltage until the current is below the aligned setting.
Note.
Care should be taken during servicing for low output power. If the drive power is lost,
the power control voltage will go high, thereby possibly causing the current or DC
power into the PA to exceed its specification. Therefore, the power supply current
should be monitored at all times and preset to as low as required. The radio has
additional inbuilt safeguards, but these should not be relied on.
the power control voltage will go high, thereby possibly causing the current or DC
power into the PA to exceed its specification. Therefore, the power supply current
should be monitored at all times and preset to as low as required. The radio has
additional inbuilt safeguards, but these should not be relied on.
5.3.3.3
Antenna Switch and Harmonic Filter
The antenna switch circuit consisting of pin diodes D500/D502/D503, is controlled by
Q504/Q508/Q509 and associated circuitry via PA-ON1. This allows the Tx output to be coupled to
the antenna while providing isolation for the Rx input and vice versa. PA-ON1 goes high with the
Tx switched on, so that all diodes are forward biased allowing power to be coupled through to the
antenna and isolating the Rx by RF grounding its input at C542. The short circuit at the Rx input is
transformed to an effective open circuit at D500 by L528, which minimises Tx loading. With the Tx
switched off, PA-ON1 goes low so that the diodes are reverse biased allowing the Rx input signal
to reach the Rx front end with minimal loss. The harmonic rejection low pass filter comprises
L513/L514/L515 and associated capacitors and this is designed to give maximum rejection at
approximately twice the maximum carrier frequency. The sharp cut-off characteristic is aided by
resonant notch capacitors C576, C577 and C580.
Q504/Q508/Q509 and associated circuitry via PA-ON1. This allows the Tx output to be coupled to
the antenna while providing isolation for the Rx input and vice versa. PA-ON1 goes high with the
Tx switched on, so that all diodes are forward biased allowing power to be coupled through to the
antenna and isolating the Rx by RF grounding its input at C542. The short circuit at the Rx input is
transformed to an effective open circuit at D500 by L528, which minimises Tx loading. With the Tx
switched off, PA-ON1 goes low so that the diodes are reverse biased allowing the Rx input signal
to reach the Rx front end with minimal loss. The harmonic rejection low pass filter comprises
L513/L514/L515 and associated capacitors and this is designed to give maximum rejection at
approximately twice the maximum carrier frequency. The sharp cut-off characteristic is aided by
resonant notch capacitors C576, C577 and C580.
5.3.3.4
Temperature Sensor
A temperature sensor U503 has been provided to accurately measure the internal PCB
temperature in the vicinity of the Tx PA module. This temperature is automatically read by the
temperature in the vicinity of the Tx PA module. This temperature is automatically read by the