108
Chapter 5 Theory of Operation
Main Output Circuitry
5
Attenuators and/or the amplifier are switched in to set the output
amplitude range according to the following table:
Note that adjacent ranges overlap, so relay chatter is eliminated when
small amplitude changes are made near range edges.
U1901, U1902, and associated components form a high-current amplifier
that injects DC Offset into the signal path through the parallel
combination of R1912, R1913, and R1914. Injecting the offset after the
attenuators allows small signals to be offset by relatively high dc
voltages. The impedance of the system up to the offset injection point is
62.5Ω, but the parallel combination of R1912 through R1914 (260Ω)
lowers the impedance to 50Ω. V_OFFSET from the System DAC (±2.5 V
range) is amplified by 5.2 (to ±13 V at U1902’s output), but is then
attenuated by R1912 through R1914. The range of offset available from
this circuit is, therefore, ±1.2 V into 50Ω.
When the peak ac voltage or dc offset of the signal exceeds 1 V, the output
amplifier consisting of U1903, U1904, and associated components is
switched in. U1904 is a hybrid circuit capable of driving 10 V
pp
signals
into 50
Ω at 80 MHz. U1903 is a “servo” amplifier that improves U1904’s
accuracy at frequencies below 1 MHz. R1959 must be adjusted to match
U1903’s gain to that of U1904. In a special adjustment mode (described
on page 90), the peak-to-peak voltage of MEAS_OA_ADJ is measured by
the Modulation A/D and displayed as a bar on the display. R1959 is then
set to minimize the width of the bar.
Offset < 1 V Offset > 1 V
Amplitude Range
(V
pp
into 50Ω)
–10 dB –20 dB Amp –10 dB –20 dB Amp
< 31.6 mV In In Out N/A N/A N/A
30 mV to 100 mV Out In Out N/A N/A N/A
96 mV to 316 mV In Out Out In In In
300 mV to 1 V Out Out Out Out In In
0.96 V to 3.16 V In Out In In Out In
3 V to 10 V Out Out In Out Out In
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