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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-06-139
Date: October 2006
Appendix G describes the derivation of the digital ratioed frequency-shift electronics unit sensitivity, which is shown to be independent of the electronics unit operating frequency. The response time is proportional to the electronics unit oscillator frequency.
A simplified block diagram of a single-channel, digital ratioed frequency-shift electronics unit is illustrated in Figure G-1.
The ratioed frequency-shift electronics unit utilizes the change in inductance caused by a vehicle passing over a loop to measure the altered electronics unit oscillator frequency by using a frequency counter with a selected crystal clock frequency and frequency multiplier that make the sensitivity independent of the oscillator frequency.
Figure G-1. Digital ratioed frequency-shift electronics unit block diagram.
The sensitivity SfD of a digital ratioed frequency-shift electronics unit is given by
where the parameters in equation G-1 are defined in Figure G-1.
the sensitivity is independent of the electronics unit oscillator frequency fD.
The frame time is automatically adjusted so that the no-call frequency count is constant. The sensitivity is changed by adjusting the frame time for a different number of frequency counts.
The response time of the electronics unit is primarily dependent on the time required to fill the frequency counter and is equal to
Since Nft and SDf are constant, the response time changes in proportion to the electronics unit oscillator frequency.