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Tuning Fork Chopper With Integrated Drive Electronics

Features and Advantages

  • ONE FIXED FREQUENCY from 200 Hz to 1,000 Hz
  • Integrated electronics
  • Aperture up to 5mm
  • Suitable for long life dedicated applications
  • Simple to use
  • Low cost
  • Small size/lightweight
  • Withstands shock and vibration
  • Low power drive electronics
  • Rugged, no wearing parts
  • High reliability, long life
  • High frequency stability
  • High amplitude stability
  • Ultra high vacuum operation1
  • High temperature operation1
  • Operates at cryogenic temperatures1
  • Meets military specifications1
  • Used in aerospace and atmospheric missions1
  • Jitter free operation
  • No radiated electromagnetic interference (EMI)
  • Reference signal available
  • Dark vanes are standard, bright vanes or mirrors are optional1
  • Single vane for alternate beam chopping1
1 Available on special order
  • Description
  • Drawings
  • Documents

The model CH-25 tuning fork chopper with integrated drive electronics is low cost, compact size, lightweight and has virtually unlimited life; this makes it a device most suitable for small instruments and hand-held or portable systems. It is a great solution when working in tight spaces and especially suitable for long life, dedicated applications from prototypes to high volume applications.

This ONE FIXED FREQUENCY resonant optical chopper (modulator) is an electromagnetically driven tuning fork device, which uses vanes, of different shapes and surface characteristics, attached to moving tines, to chop a light beam with a sinusoidal motion. A range of factory set fixed frequency modulation waveforms (sine, half sine and pulse) is available. The modulating frequency range of the CH-25 chopper is from 200 Hz to 3 KHz, fixed at any one value within the range. The aperture is inversely proportional to the frequency, and is a function of the size of the vanes and the type (duty cycle). Operation at the natural resonant frequency is sustained by a feedback amplifier. The driver controls the aperture and provides a TTL output as a reference signal.

The most common type of aperture is the half open in the rest position, 90% duty cycle. It is used for a large beam diameter. When the modulator is operating in this configuration the maximum aperture is produced. The vanes are factory adjusted so that when fully illuminated the maximum excursion condition (controlled by the drive electronics) generates an almost complete sine wave as shown in table. Typically, the light is transmitted for 90% of each cycle. The 50% duty cycle modulation is used for a small beam diameter. The vanes are factory adjusted to be just closed in the rest position. Full aperture for 50% duty cycle is one half of the 90% duty cycle. Balanced operation and high "Q" insure frequency stability, low electrical drive power and low reaction forces. High flexural stiffness provides good resistance to shock and vibration. The standard operating temperature is 0°C to +65°C.

Tuning fork choppers are especially suitable for long life, dedicated applications, OEM, built into an instrument/system, and for portable systems. When working in tight spaces tuning fork choppers are the best solution.

Applications include: test equipment, noise detection (with a lock-in amplifier), pollution and gas detection, radiometry, radiation pyrometry, military systems, scientific and medical research, small size and hand held instruments. An important use of the chopper is in the communication and data acquisition environments, where optical signals are obscured by noise. A fixed frequency modulated signal can be filtered from background noise much more readily than an unmodulated signal. This is best achieved using a lock-in amplifier. In this arrangement, the detected signal and the reference signal, mixed with the frequency of the tuning fork chopper, cause the frequency of interest to appear as a pure dc output level. All other signals are filtered with the low pass filter. Once noise is removed, proper signal measurements can be obtained.

Tuning fork choppers are the most reliable choppers. They have an infinite lifetime and are used for chopping light, plasma, X-ray and laser light (VIS to DUV). The choppers are used in systems and portable instruments in industrial, scientific, medical, aerospace and military applications worldwide.

Special pricing for OEM applications.

The following should serve as guide lines only:


(Hz) (mm) (Inches) (mm) (Inches) (mm) (Inches)
200 2.5 0.10 0 5.0 0.20 2.5 0.10
400 1.5 0.06 0 3.0 0.12 1.5 0.06
800 1.0 0.04 0 2.0 0.08 1.0 0.04
1000 0.85 0.03 0 1.7 0.07 0.85 0.03
1500 0.70 0.03 0 1.4 0.06 0.70 0.03
2000 0.40 0.02 0 0.80 0.03 0.40 0.02
2500 0.25 0.01 0 0.50 0.02 0.25 0.01
3000 0.20 0.008 0 0.40 0.016 0.20 0.008

CH-10 Duty Cycle
1) For 50% DUTY CYCLE: The opening at rest is zero.
2) The standard base holds a single coil. A base with two coils is recommended for low frequencies and high amplitude stability (with the AGC driver). For a wide temperature range, a base with three coils is recommended for high frequencies and high amplitude stability (with the AGC driver).
3) The vane motion is sinusiodal.


LENGTH (Inches) WIDTH (Inches) HEIGHT (Inches) LENGTH (Inches) WIDTH (Inches)
200-400 1.70 0.68 0.35 .78 0.625
400-800 1.40 0.68 0.35 .78 0.625
800-1000 1.30 0.68 0.35 .78 0.625
1000-1200 1.62 0.68 0.30 1.13 0.625
1200-1500 1.52 0.68 0.30 1.13 0.625
1500-1700 1.33 0.68 0.30 1.13 0.625
1700-2000 1.25 0.68 0.30 1.13 0.625
2000-2500 1.25 0.68 0.30 1.13 0.625
2500-3000 1.06 0.68 0.30 1.13 0.625

Input power: 24 V dc, +/-4V, 50 mA
Frequency range: 200 Hz to 3 KHz
Monitor output: A square wave TTL reference output
Connector: 3 pin Molex connector P/N 22-05-2021

TYPE [CH-25]; DUTY CYCLE[%]; VANE [B=bright or D=dark]; FREQUENCY [Hz]
Example: PART NO. CH25-90D-400. This part number specifies the model CH-25 chopper, with 90% duty cycle, dark vanes and a 400Hz operating frequency.
Special vane configurations, modulating waveforms and shapes are available on special order. Consult factory. Drive electronics with different packages, regulation, and reference signal and power supply options are available. Special pricing for OEM applications.