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Publication Number: FHWA-HRT-06-139
Date: October 2006

Traffic Detector Handbook:Third Edition—Volume II

APPENDIX G. DIGITAL RATIOED FREQUENCY-SHIFT ELECTRONICS UNIT ANALYSIS

ABSTRACT

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.

ANALYSIS

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. Figure G-1 shows that the inductance at the detector terminals L sub D feeds into the detector oscillator, which in turn feeds into the frequency multiplier. The measurement frame time is increased or decreased by the variable frame time logic as a function of the detector oscillator frequency small F sub capital D. The frequency counter counts the number of oscillations during the measurement frame time and sends the value N sub small FC super small V to the comparator to compare it to the frequency reference memory N sub small FC with no vehicle present. The difference, capital delta capital N sub small FC is sent to the second comparator. If the second comparator finds that capital delta capital N sub small FC is in excess of the threshold sensitivity, N sub small FT, then a call is output.

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

Equation G-1. Capital S subscript Capital D superscript F equals the quotient of parenthesis negative 2 times Capital N subscript F C times F subscript N C parenthesis divided by the product of M times F subscript Capital D. (G-1)

where the parameters in equation G-1 are defined in Figure G-1.

If

Equation G-2. F subscript N C divided by the product of M times F subscript Capital D equals constant proportional to 1 divided by Capital N subscript F C. (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

Equation G-3. T superscript F equals F subscript N C divided by the product of M times F subscript Capital D which in turn equals the product of 2 times Capital N subscript F T divided by parenthesis M times F subscript Capital D times Capital S subscript Capitol D superscript F parenthesis. (G-3)

Since Nft and SDf are constant, the response time changes in proportion to the electronics unit oscillator frequency.

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