DETAILED DESCRIPTION
2-14
2.2
LF1223A Low Frequency Generator
General Description:
LF1223A provides the audio signals to be modulated onto the carrier signal in the transmitter
(AC 1226A). The levels of the 90Hz, 150Hz, RF level and ident signals and the morse code of
the ident signal are programmable from the RMS.
Block Diagram:
Block Description:
DIGITAL SECTION
The LF_DIG block provides the interface to the RMS, the parameter storage (EEPROM) and
the analog section. All sequencing and local parameter update is performed by this block.
Most of the functionality of the DIGITAL SECTION is handled by the NMP110A FPGA
described in chapter 2.2.1.
ANALOG SECTION.
All levels are controlled digitally through multiplying DACs (MDACs), and the AC levels are
stabilized through AGC amplifiers. The fine tuning of the phase between the 90 and 150Hz
signals is done manually with potentiometers. The morse code is keyed in an analog multi-
plexer, and can be synchronized with a DME.
RF level and ident (DC_IDENT), SDM and DDM (90/150Hz) are generated in the MDAC
chain. Multiplication (m) in the MDACs are 0
≤
m<1.
•
The reference voltage is multiplied in the first MDAC to form the DC portion of DC_IDENT.
The keyed 1020Hz sine wave is multiplied in an other MDAC to control the ident amplitude,
this signal forms the IDENT portion of DC_IDENT signal.
•
The DC portion of DC_IDENT is multiplied in the third MDAC to form the modulation sum
reference .
•
This modulation sum reference is split in two signals where one is modified by a fourth
MDAC before they are combined again to form the modulation difference.
2.2.1
NMP110A Low Frequency Generator Control
General description:
NMP110A is a FPGA within the LF_DIG block. It provides the interface between the RMS and
the local parameter storage and sequencing of the LF signals are performed here. NMP110A
is based on the Actel ACT1020 FPGA. For electrical specifications see the ACT1020
datasheet.
