Specifications

4-22 888-2857-004 5/9/13
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
Platinum VAX-C Series
factors on a change of frequency and provide reasonable accuracy without requiring
re-calibration. A full two-point calibration should be performed at first turn-on at
mid-band (205 MHz).
• On-board switching is performed so that the forward power sample will automati-
cally be used as a signal source for reflected calibration. An attenuated forward sam-
ple is switched in while the system is running to establish a calibration point
representing a -10dB reflection. The reflected calibration is automatically performed
“in the background” by the system during forward calibration, and VSWR hardware
trip points are automatically calculated and programmed for the VSWR fault com-
parator. This eliminates the need to perform reflected calibrations.
• Although the design intent is to allow frequency changes and maintain reasonably
accurate power measurement without requiring re-calibration, the user can perform a
calibration if they desire. Since the detector slopes were established in the one-time
two-point calibration and are stored, re-calibration can be streamlined to a single
point.
• Forward calibration can be performed while operating (this of course assumes that
the end user has a well-calibrated directional coupler and a power meter connected
externally). The measured high point, calculated low point (based on the stored
slope) and frequency of calibration are stored in EEPROM for the new frequency of
operation.
4.5.0.3 +50 VDC Power Supply Interface
An AC/DC converter is used to provide the +50VDC required by the BLF881 LDMOS
devices in the PA assembly. This unit was used because it incorporates a logic-level
enable function as opposed to one which which requires the AC to be switched to
provide On/Off function. The PA Interface uses a 2N7002 in an open drain
configuration. When turned ON the 7002 provides the ground to enable the AC/DC
Converter. The AC/DC Converter has an open collector status output which is grounded
when the supply is on and no faults are present. The /PS_PRESENT pin is grounded on
the power supply end, with a 10k pull-up on the PA Interface, to indicate a cable
connection to the supply.