|
|
|
Model 198A
Mixed Beam Light Chopper
General
The model 198A is a dual frequency optical chopper, using a thin
rotating metal blade with an inner set of 11 apertures and an outer set of
18 apertures to simultaneously chop two light beams. The blade is driven
by a precision DC motor whose speed is controlled via a phase locked loop
that is referenced to either an internal quartz crystal oscillator or an
external reference signal. The chopper has three reference frequency
outputs, one at each of the frequencies generated by the inner and outer
apertures, and one at a frequency equal to their sum. The three
frequencies f1, f2 and f1 + f2
are chosen to be relative primes which significantly reduces mutual
harmonic interference.
It is ideal for use in measurements where two modulated beams give rise
to a third optical signal at a frequency which is the sum of the two
chopped frequencies. The model 198A acts as a chopper for the incident
light sources at frequencies f1 and f2 and generates
a reference signal at a frequency f1 + f2
which can be used to drive a subsequent instrument, such as a lock-in
amplifier.
In conjunction with the dual reference mode offered by the model 7265
or 7280 DSP lock-in amplifiers, the model 198A can
also be used to implement a very cost-effective dual-beam ratiometric
measurement system. This technique can eliminate variations in source
intensity over several orders of magnitude. It is described further in an
Applications Note AN1005 "Dual Beam Ratiometric Measurements using
the Model 198A Mixed Beam Light Chopper", available free of charge by
download from this site. Click
here to access the Applications Notes page
The chopper can also be used as a conventional single-beam unit since
it also generates reference signals at f1 and f2.
Quartz Crystal Frequency Accuracy and Stability
The model 198A, in common with all SIGNAL RECOVERY
light choppers, uses a quartz crystal oscillator as its primary frequency
standard. The oscillator signal is divided down to yield the required
chopper frequency, and then the motor speed is continuously adjusted to
phase lock the actual chopper frequency to this required value. The result
is a chopper with an output frequency as stable as any other modern
frequency source.
External Frequency Control
Like many other choppers, the frequency can be controlled externally.
However, unlike other units the control is via an applied reference signal
(TTL levels may be used) rather than an analog voltage. The chopper locks
to this applied reference, which is at the same frequency as that
generated by the outer set of apertures (f1), but for detection
purposes the reference outputs it generates ( i.e. at f1,
f2 and f1 + f2) are of course
also available.
Mechanical Dimensions
Blade Dimensions
Specifications
| Operation |
|
|
Frequency |
55 Hz to 1500 Hz |
|
outer
sector |
90 Hz to 1500 Hz,
18 apertures, f1 |
|
inner
sector |
55 Hz to 917 Hz, 11
apertures, f2 |
|
sum |
145 Hz - 2417 Hz, f1 +
f2 |
|
Control |
|
|
manual |
Digital push-button |
|
external |
Application of 0.5 V to 10 V
sine or squarewave, 90 Hz to 1500 Hz to Sync In f1
BNC connector |
|
Internal Frequency |
|
|
accuracy |
±20 ppm at 25 °C |
|
stability |
±30 ppm/ °C (range -10 °C
to 60 °C) |
|
Jitter (measured pk-pk and
presented as a % of a full cycle) |
|
|
outer
sector |
|
|
90 to 140 Hz |
blade only: 0.2%; blade +
electronics: < 6% |
|
140 to 1500 Hz |
blade only: 0.2%; blade +
electronics: <1.5% |
|
inner
sector |
|
|
55 to 100 Hz |
blade only: 0.2%; blade +
electronics: < 8% |
|
100 to 917 Hz |
blade only: 0.2%; blade +
electronics: <2.0% |
|
Lock indication |
Bicolor LED, showing Red when
unlocked and Green when locked |
|
Settling Time
|
7 s nominal for f1 +
f2 = 1 kHz from switch-on;
9 s nominal for f1 + f2 frequency change
from 150 to 2400 Hz;
30 s nominal for f1 + f2 frequency
change from 2400 to 150 Hz |
| Outputs |
Sync Out f1 |
10 V pk-pk squarewave at
outer sector chopping frequency (f1), 90 Hz to 1500 Hz |
|
Sync Out f2 |
10 V pk-pk squarewave at
inner sector chopping frequency (f2), 55 Hz to 917 Hz |
|
Sync Out f1 + f2 |
10 V pk-pk squarewave at sum
chopping frequency (f1 + f2), 145 Hz
- 2417 Hz |
|
Connectors |
BNC |
|
Impedance |
10 kW. Note that although the
output voltage is 10 V pk-pk, the high output impedance means
that the outputs can be directly connected to the Reference
Frequency input of any SIGNAL RECOVERY
lock-in amplifier without causing problems. |
| Power
Requirements |
Via power adapter for 110 V
AC, 50/60 Hz or 220/240 V AC, 50/60 Hz or external
isolated 17 V to 20 V DC or AC rms. supply. State which voltage is
required when ordering |
| Dimensions |
4" high x 4.8" wide
x 1.7" deep
(104 mm wide x 122mm wide x 44mm deep) |
| Weight |
1 lb (0.45 kg) excluding
power supply |
| |
|
|
is
a trademark of AMETEK, Inc.