3.3 Fast servo loop
21
• On the oscilloscope, the
FAST
output of the FSC should show
a 100 Hz sine wave.
• Adjust the
FAST GAIN
knob to vary the proportional gain of the
fast servo until the output is the same amplitude as the input.
• To measure the proportional feedback frequency response, ad-
just the frequency of the function generator and monitor the
amplitude of the
FAST
output response. For example, increase
the frequency until the amplitude is halved, to find the
−
3 dB
gain frequency.
3.3.3 Measuring the differential response
1. Set
FAST INT
to
OFF
to switch off the integrator loop.
2. Set the
FAST GAIN
to unity using the steps described in the
section above.
3. Set the
DIFF GAIN
to 0 dB.
4. Set
FAST DIFF/FILTER
to 100 kHz.
5. Sweep the frequency of the function generator from 100 kHz
to 3 MHz and monitor the
FAST
output.
6. As you sweep the error signal frequency, you should see unity
gain at all frequencies.
7. Set the
DIFF GAIN
to 24 dB.
8. Now as you sweep the error signal frequency, you should
notice a 20 dB per decade slope increase after 100 kHz that
will start to roll off at 1 MHz, showing the opamp bandwidth
limitations.
The gain of the fast output can be altered by changing resistor
values, but the circuit is more complicated than for slow feedback
(§3.2.2). Contact
MOGL
abs for further information if required.
