Boston Electronics Corporation
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Tuning Fork Chopper Tutorial
Tuning Fork Choppers, unlike our rotating disk types, are FIXED FREQUENCY devices. They are based on exactly the same principles as musical tuning forks, and their frequency is fixed to the desired frequency during manufacture by adjusting the mass of the fork tines,
Customer options are –
-
Frequency (10 Hz to 5000 Hz)
-
Duty Cycle (50/50 and 90/10 are the usual choices)
-
Aperture (trades off against duty cycle and frequency)
-
Vane coating (may be dark or reflective)
Frequency and Aperture
The maximum aperture of a tuning fork chopper depends on its frequency. The frequency of a tuning fork is inversely proportional to its amplitude of vibration. The higher the frequency, the smaller the amplitude and hence the smaller the aperture that can be blocked by the vanes at the tine ends. For example, see the following table -
|
Frequency (Hz) |
Aperture at 50% Duty Cycle (mm) |
Aperture at 90% Duty Cycle (mm) |
|
10 |
~5 |
~10 |
|
200 |
4 |
3 |
|
400 |
2 |
5 |
|
800 |
1 |
2 |
|
1600 |
0.5 |
1 |
|
3200 |
0.15 |
0.3 |
The vane motion is sinusoidal. Opening at rest for a chopper with a 50% duty cycle is 0% of the full aperture (closed), while opening at rest for a chopper with a 90% duty cycle is about 50% of the full aperture (partly open).
Duty Cycle
The duty cycle we are used to thinking of with traditional rotating disk choppers having equal width disk spokes and openings is 50/50 or "50%" as expressed above. In tuning fork choppers, the duty cycle reflects the proportion of time that the vanes part and overlap, so for a 50% duty cycle the vanes are open for 50% of the time and closed for the other half. However, the given duty cycle of a chopper is rarely if ever achieved, and the actual duty cycle is accounted for in subsequent signal processing.
Beam width has a profound effect on actual duty cycle in both rotating and resonating choppers. A chopper only functions at nearly exactly its given duty cycle when the beam is very small by comparison to the size of the disk opening (aperture). When the beam fills, for example, 2/3 of the aperture, then some light from that beam gets through for most of the time -- from the time the rotating blade lets through the first sliver of light or the vanes move apart slightly, to the time the rotating blade fully covers the beam or the vanes overlap, cutting off the last of the light.
For rotating choppers, the amount of light which passes through is always equal to or greater than the given duty cycle. The beam is fully blocked only when the disk blade covers all of the beam, which occurs for less time as the beam width increases. For tuning fork choppers, the amount of light which passes through is never greater than the given duty cycle. The beam is fully blocked for the entire time the vanes overlap. With both rotating and tuning fork choppers, the amount of time that the full beam width passes through decreases with increasing beam width.
Applications
Tuning fork choppers are especially suitable for dedicated applications such as embedded or built into an instrument rather than for general laboratory use. This is a consequence of their fixed frequency, small size, long life and low power consumption.
Variable speed rotating blade types like the 3OOCD, in contrast, are versatile regarding frequency and aperture, making them just the thing for the experimenter's bench. However, they are often too large to build into equipment and, with typically 8000 hour motor life, may not have sufficient lifetime for continuous long-term operation in the field.
Drivers
The standard drivers used with the tuning fork choppers are the ED-series. The AGC-series driver circuits should be chosen over the ED-series in applications in which external vibration of the tuning fork mount due to external shock or vibration etc. might cause the mechanical amplitude of the fork to be enhanced or suppressed. The AGC-series devices improve amplitude stability in a shock and vibration environment by up to ten times. Many customers will feel that they are capable of making their own drive circuits. We encourage this in OEM applications. We supply drive circuit schematics on request.
Similar Products
Our choppers are made in USA by Electro-Optical Products Corporation, for whom we are an authorized distributor. We know of no competition. Companies like American Time Products and Bulova who once offered the product have gone out of business. These choppers are nearly identical to those of American Time Products and may usually be used as direct replacements.