Harris Broadcast Flexiva FAX 10KW, Technical Manual

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Section-4 Theory
October 28, 2013
4 ‐ 8
Copyright ©2013, Harris Broadcast WARNING: Disconnect primary power prior to servicing. 888 ‐ 2720 ‐ 001
In the event of a failure and the transmitter goes to Backup control, the Backup Control Ref pot would become the
power reference. The raise/lower power buttons will not function, however the APC still functions and the power
can be adjusted using the pot.
The APC is affected by several control lines in the system, forward power, reflected power foldback, forward power
limit and mute. When the forward power changes the APC changes in the opposite direction, ie if forward power
increases the APC voltage decreases thus lowering the exciter power which in turn lowers the transmitter power
back to its setting.
If the transmitter has an elevated reflected power that is above a user set threshold of 1.3 to 1.5:1, the voltage on
the VSWR Foldback line increases, this causes APC to lower the exciter power. If the forward power limit, which 10%
above calibrated power is hit, the Forward Power Limit voltage is increased which decreases the exciter power. If the
instantaneous reflected power exceeds 1.5:1, the VSWR Fault circuit will send a Fault OFF the controller as well as a
Mute to the APC. The controller will make three attempts to bring the transmitter back to power. After the third
attempt the transmitter will stay off and will require user intervention to turn it back on. In Backup control the
three ‐ strike is no longer available and the transmitter will remain OFF in the event of a VSWR fault.
The APC is shown driving the exciter, if a product such as the Harris Broadcast Micromax is used, the APC switch to
the exciter is opened and the APC switch to the IPA is closed. The APC still functions the same but the extra Op ‐ Amp
sets the correct level for the FAX IPA input.
4.1.7 PA Module Description
Each PA Module contains 2 RF Power Amplifiers referred to as "A" and "B." A FAX 10 kW Transmitter and Power
Block uses 8 of these modules. One of the eight modules is used as an IPA module.
Figure 4-8 PA Module
Figure 4 ‐ 8 above right side shows the PA Module with the cover off. There are 2 PWAs. On the left is the PA pallet
which contains amplifiers A and B. On the right is an additional circuit card that contains the module control logic
and interface functions.
The PA circuit board is mounted on a thick solid copper substrate. The RF Power transistors are soldered down
directly to the copper substrate. Copper is used because it is the best conductor of heat and will dissipate the heat
away from the RF power transistors quickly and evenly. The copper substrate then mounts to the aluminum heatsink
assembly to dissipate the heat away from the copper. There is a thin film between the copper spreader and the
aluminum heatsink for optimum heat transfer.
The RF signal is amplified in the PA modules by specially designed LDMOS Power FETs with a nominal gain of 14 – 22
dB, depending on frequency and operating mode. Both sides are identical so only one path will be described. RF
drive power enters Amplifier via J1. RF passes through a forward power coupler for overdrive detection, then to an
input matching network to the FET Q1. The FET operates in push ‐ pull to produce approximately 800W FM power
into a 50 ohm load. Their low impedance outputs are transformed via the output network to 50 ohms and passed to
a reflected power coupler for VSWR protection. RF Output exits the board via edge connector pins on J1.