User's Manual

AT7120 Owner Manual Rev. 1.1 – March 2009 38/76
Board CIP8431 – Adjustment Procedure
The tests on Module 4442 can be done with the output points connected to the rest of the RF structure of
the equipment, or with the output connected to a coaxial charge 50Ω/100W (minimum) that has a good
response in UHF TV (470MHz to 860MHz). However, whatever is the use of Module 4442, it is
important to make the adjustments with the module RF output connected to a coaxial charge.
Never operate Module 4442 without making sure its output is properly charged. The absence of a
coaxial charge, or if it doesn’t meet specifications, might cause severe damage not just to the
transistors but also to the test equipment.
In case it is decided to adjust Module 4442 outside the transmitter, it will be necessary to use a +28V
power supply. The output current of this power supply should not be smaller than 2.5A, as long as the
tests that need to be done don’t require maximum power output from the module. To adjust currents and
the response curve, the +28V/2.5A power supply is sufficient.
ID Current Adjustments
To adjust ID drain currents of both transistors, T1 and T2, Module 4442 must be idle.
Never adjust ID drain currents of Module 4442 LDMOS transistors while there is an input signal in the
module. To adjust currents, make sure Module 4442 input signal is in standby, otherwise the transistor
might be damaged.
To perform a more accurate adjustment it is best to wait at least 5 minutes after module has been
powered up, to make sure there is a thermal stability in the module when in standby.
The measuring instrument to be used during testing procedures is a digital multimeter with scale in
mVdc. Avoid the use of multimeter with auto-range.
Transistors T1 and T2
1. Adjust trimpots TPO-1 and TPO-2 totally anti-clock wise, thus zeroing T1 and T2’s drain
currents.
2. To adjust T1’s quiescent current, insert the digital multimeter with a maximum scale of
100mVdc to measure loss of voltage in resistors R34 and R35 (drain resistors for T1).
3. Note: These two resistors are in parallel, thus the voltage can be measured on either of them (R34
or R35).
4. Slowly adjust trimpot TPO-1 clock wise until T1 is set; then some loss in voltage (few mV) will
be observed. Continue adjusting TPO-1 until its voltage measures 60mVdc, what translates to a
drain current of 900mA for the BLF861A transistor.