Line Frequency Rejection Filter; Ac Gain And Dynamic Reserve - Ametek 7280 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION

3.3.03 Line Frequency Rejection Filter

3.3.04 AC Gain and Dynamic Reserve

3-4
single-ended signals, subject to the limitation that the user must allow for the signal
inversion (equivalent to a 180° phase-shift) which it introduces when reading the
outputs.
Input Connector Shell, Ground / Float
The input connector shells may be connected either directly to the instrument's
chassis ground or floated via a 1 kΩ resistor. When in the float mode, the presence of
this resistor substantially reduces the problems which often occur in low-level lock-in
amplifier measurements due to ground loops.
Input Coupling Mode, Fast/Slow
When the input coupling mode is set to Slow, the signal channel gain is essentially
flat down to 0.5 Hz, but recovery from input overload conditions may take a long
time. Conversely, when the coupling mode is set to Fast, recovery from overload is
much faster, but there will be a noticeable roll-off in magnitude response and
significant phase shifts at frequencies below typically 20 Hz.
Input Signal Selection, V / I
Although the voltage mode input is most commonly used, a current-to-voltage
converter may be switched into use to provide current mode input capability, in
which case the signal is connected to the B/I connector. High impedance sources
(> 100 kΩ) are inherently current sources and need to be measured with a low
impedance current mode input. Even when dealing with a voltage source in series
with a high impedance, the use of the current mode input may provide advantages in
terms of improved bandwidth and immunity from the effects of cable capacitance.
The converter may be set to low-noise, normal or wide bandwidth conversion
settings, but it should be noted that if the best possible performance is required a
separate current preamplifier, such as the SIGNAL RECOVERY models 181 or
5182, should be considered.
Following the signal input amplifier there is an option to pass the signal through a
line frequency rejection filter, which is designed to give greater than 40 dB of
attenuation at the power line frequencies of 50 Hz or 60 Hz and their second
harmonics at 100 Hz and 120 Hz.
The filter uses two cascaded rejection stages with "notch" characteristics, allowing it
to be set to reject signals at frequencies equal to either of, or both of, the fundamental
and second harmonic of the line frequency.
Instruments are normally supplied with the line frequency filter set to 60 Hz with the
filter turned off. If the prevailing line frequency is 50 Hz then the filter frequency
should be set to this value using the control on the Configuration menu (see section
2.3).
The signal channel contains a number of analog filters and amplifiers whose overall
gain is defined by the AC Gain parameter, which is specified in terms of decibels
(dB). For each value of AC Gain there is a corresponding value of the INPUT LIMIT