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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.
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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. |
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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
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(G-1) |
where the parameters in equation G-1 are defined in Figure G-1.
If
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(G-2) |
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
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(G-3) |
Since Nft and SDf are constant, the response time changes in proportion to the electronics unit oscillator frequency.
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FHWA-HRT-06-139 |
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