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Theory of Operation
3-74
2715 Spectrum Analyzer Service Manual
GPIB, RS- 232, Real Time Clock, NVRAM, and Gated Measurement Functions
The Digital Options module provides both the RS--232 and GPIB interface and
gated sweep capability. The features are:
H Gated measurement functions to provide in service measurement capability
H A standard IEEE 488.1 GPIB interface
H Two RS--232 interfaces which includes:
A full 7 wire interface (RXD,TXD,DTR,DSR,RTS,CTS,CD)
A simple 3 wire (RXD,TXD,DTR) interface
H A real time clock with battery back up
H NVRAM for storing waveforms and instrument settings
In-service CATV measurements require the ability to look for very weak noise or
distortion components. For example, the C/N measurement will commonly have a
noise floor 50 to 65 dB below the visual carrier (in a 300 kHz resolution band-
width), and the CSO measurement will have similar amplitudes but in a 30 kHz
resolution bandwidth. Unfortunately, the sidebands resulting from video modulation
of the carrier are generally much stronger that this.
To overcome this measurement difficulty, the analyzer’s data acquisition is shut off
except during quiet scan lines. These quiet scan lines occur during vertical retrace.
The temporary absence of video modulation during these lines means that sidebands
are not present (provided that the resolution filter being used has a settling time
shorter than the horizontal line time). This function is performed by the acquisition
control facility on the Display Storage board.
In practical application the settling time of the resolution filter is not entirely
predictable. The abrupt edges that occur during the horizontal sync pulse immedi-
ately preceding a given quiet line shocks the resolution filter into ringing even if it
is tuned some distance away from the visual carrier. To overcome this, the sharp
edges associated with the sync pulse are replaced by soft edges having rise and fall
times of a few microseconds. The shock excitation of the filter is negligible even if
the filter is tuned as close as 500 kHz from the visual carrier. The IF Gate performs
this edge softening function, by blocking the IF path during the sync pulse time.
The Address B uffering and Decoding section consists of three 74HC T541 bus
receivers (U24, U25, and U26) to receive 20 bits of address and 4 control signals
(BRD_L, BWR_L, BIO/W_L and BCLK), a GAL26V12 (U7) for address decoding
and a 74HCT245 bus transceiver (U1) to buffer the 8 bit data bus. Part of U10 is
also used to generate the bus wait signal BWAIT_L required for TTY write cycles.
Gated Measurement
Function
Instrument Bus Interface