Frequency comparator

[kautilya]

Main Question or Discussion Point

does anyone know a frequency comparator is designed?

also what are the elements of programmable logic control in a speed radar?

 

[berkeman]

does anyone know a frequency comparator is designed?

also what are the elements of programmable logic control in a speed radar?

The function of determining a signal's frequency content can be accomplished several different ways. If you just have a single frequency that you want to compare an input signal's spectrum against, you can use a simple bandpass filter to tell you how large the signal content is at that frequency in the input waveform.

If you want more information about multiple frequencies in the input waveform, you can use an analog spectrum analyzer circuit (mixer + LPF), or you can digitize the waveform, and use digital signal processing (DSP) and/or Fast Fourier Transform (FFT) processing to give you the signal energy at different frequencies.

As to the RADAR gun question, it is using the Doppler shift in frequency of the return signal to calculate the speed of the reflecting object.

http://en.wikipedia.org/wiki/Radar_gun

 

[deakie]

In addition to Berkeman's answer in paragraph 1, i found the following circuit that highlights a method of achieving frequency comparison.
there is a simple explanation that goes along with it but generally the idea is clear.
http://www.newcircuits.com/circuit.php?id=msr006