Filter
Top Products
Theory of Operation
2715 Spectrum Analyzer Service Manual
3-51
The comparators are contained in an LM319, U9. The top (UP) comparator
determines if the summing node is greater than the 49. 5 mV reference point. The
bottom (DN) comparator determines is the summing node is less than the 0.0 V
reference. The outputs of these two comparators, UP and DN, are fed into the
Horizontal LCA. Inside the Horizontal LCA the UP and DN signals are
debounced to prevent false changes in the internal counter. Also inside the
Horizontal LCA is a small state machine which decodes the debounced UP and
DN signals, the SWPGATE signal, and the internal counter, to determine
whether to increment or decrement the internal counter.
A typical sequence of events for the tracking A/D converter are described in the
following paragraph.
First assume that the summing node starts a stable point and a valid sweep is
running. The SWP signal begins to increase. Once the output of U36 has
increased enough to force the summing node above the positive reference point
of 49.5 mV, the output of the top comparator, UP signal, goes high. This signals
the state machine inside the Horizontal LCA to increment its internal counter.
The outputs of this internal counter are fed back to the U11, increasing its sink
current, bring the summing node back inside the comparator window. At the
transition of the internal counter to a new value, the signal EOB (End Of Bin)
goes high until all data has been written into waveform memory. The reverse
procedure occurs for a SWP signal that is decreasing.
The timing diagram show in Figure 3--18 depicts a section of a typical sweep for
the Horizontal Tracking A/D Converter.
BCLK
UP
DN
HACQ<8..0>
HAD<8..O>
EOB
0ns 500ns 1000ns 1500ns 2000ns 2500ns 3000ns
Figure 3- 18: Timing Diagram of a Typical Sweep for the Horizontal Tracking A/D Converter
Vertical Input. The purpose of the Vertical Input section is to provide gain and
offset to the incoming LOGVID signal prior to a flash A/D Converter (ADC).
The LOGVID signal is amplified by operational amplifier U2, an NE5534. U2 is
configured as an inverting amplifier with a gain of about --3.5. Input signal gain
is adjusted using R24 (VIG), while input offset is adjusted using R20 (VIO). The
output of U2 is then passed through a resistive divider, which reduces the output