SRS Labs SR445A, Manual

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Calibration
The purpose of the calibration is to verify operation of the unit and:
1. Adjust the high frequency compensation for best pulse response.
2. Adjust the offset to null the dc voltage at the output with no input.
3. Adjust the gain to 10× for an unloaded output so that the nominal gain for an
amplifier driving a 50
Ω load will be 5× (or +14dB.)
Since the adjustments are interdependent, it is important that the adjustments be done in
the prescribed order, and that all of the adjustments be done. For example, adjusting the
high frequency compensation will affect the output offset. Calibration pots are accessible
from the underside of the unit.
Required equipment:
1. Pulse generator, splitter and attenuator to produce two ±100mV square waves
with a rise time of less than 1nS. (ECL reference output from SR620 Time
Interval Counter into an unmatched tee driving two 50
Ω cables with a 20dB
attenuator on the one that goes to the SR445A.)
2. Digital voltmeter with 4-wire ohm measurement capability (Agilent 34401)
3. Digital oscilloscope with at least 300MHz bandwidth.
High Frequency Compensation
The SR445A uses an AD8009 current feedback amplifier in the output stage. The gain of
the amplifier is controlled by the ratio of resistors in the feedback network and the
bandwidth is controlled by the Thevenin equivalent source impedance of the feedback
network. The ratio is fixed (by R115 & R116, R215 & R216, R315 & R316, R415 &
R416) to provide a gain of 5× and the source impedance may be adjusted (by P102, P202,
P302 and P402.)
The bandwidth is set to optimize the pulse response of the amplifier. This is done by
applying a fast pulse at the input and adjusting P102 (or P202, P302 or P402 for channels
2, 3 & 4) so that the output rise time and overshoot most closely match the rise time and
overshoot of the fast input pulse as observed on a 300MHz oscilloscope with 50
Ω input
impedance.
Note that adjusting P102 will affect the offset for channel 1, as there is a large input bias
current (150µA, max) to the inverting input of the AD8009. The offset will need to be
adjusted after the HF compensation is adjusted.
1. Split the ECL reference output from SR620 with a coax tee. Take one cable from
the tee to channel 1 of the oscilloscope (set to 50
Ω input termination) and the
other to the top channel of the SR445A via a 20dB coaxial attenuator.