# PLL to detect changes in base frequency (about 100kHz)

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#### earckens

##### Member
A base frequency of about 100kHz is used in a coil to detect cars passing overhead. When a car (metallic object) passes, the frequency drops about 1% due to the change in inductance. I have no idea where/how to start on a design for a PLL circuit capable to perform the task to signal at its output whether the base frequency has changed.
Changes are rather sudden (within one second).
My question: I need a headstart, either design concept or references to literature about the subject so that I can start with a design; would that be possible? Thanks!

#### unclejed613

##### Well-Known Member

when i need reference material, i go to archive.org first... it's a lot better search experience than just googling something. if you put "phase lock loop theory" in google, the first few responses are guaranteed to be something like "find out where to buy phase locked loop theory"...

there are probably several approaches you could use. the simplest would be to tap off the loop filter output voltage and feed a window comparator (which is a pair of comparators, one for "high" and one for "low"). if the coil pulls the oscillator outside the limits represented by the comparator reference voltages, you get a logic "1" out of the comparator. or, you could use a single comparator if your oscillator always gets pulled in one direction when a car is detected.

#### MikeMl

##### Well-Known Member
I built a detector for magnetic particles (iron filings) passing through a pipe, being carried by a liquified slurry of coal dust. I placed two coils around the pipe about 30m apart. Each coil was the tank circuit for a Colpitts oscillator. The oscillators each ran at about 200kHz. I used a phase detector followed by a low-pass filter (loop filter) to steer Oscillator A to the same freq/phase as Oscillator B with a loop time constant of about 1sec. As the metallic particles moved through the pipe, they would effect the frequency of A first, followed by B. That produced a very nice positive going pulse , followed by an negative going pulse, out of the loop filter. You could tell exactly when the metallic particles passed each coil...

The advantage is that for 99% of the time, the two oscillators are phase locked. Only during a brief transient while the clump of particles was in the respective coil would the loop "unlock", but then when the particles were flushed out of the system, the two oscillators get back in step again...

In your case, one of the two oscillators could be crystal-controlled.

#### dknguyen

##### Well-Known Member

when i need reference material, i go to archive.org first... it's a lot better search experience than just googling something. if you put "phase lock loop theory" in google, the first few responses are guaranteed to be something like "find out where to buy phase locked loop theory"...

there are probably several approaches you could use. the simplest would be to tap off the loop filter output voltage and feed a window comparator (which is a pair of comparators, one for "high" and one for "low"). if the coil pulls the oscillator outside the limits represented by the comparator reference voltages, you get a logic "1" out of the comparator. or, you could use a single comparator if your oscillator always gets pulled in one direction when a car is detected.
Thanks. Did not realize such a site existed.

##### Well-Known Member
Phase locked loops are sometimes used as FM detectors in receivers. Have a look at an application note for the LM565 and you will see some examples of tone discrimination or FM detection.

#### ronsimpson

##### Well-Known Member
Block 1) I would start out looking for "metal detector schematics". The front end if many metal detectors is a one transistor + large coil oscillator.
Block 2) I have used the 4046 PLL on many projects.

Most metal detectors have two oscillators. One changes frequency with metal and one is very stable. Your ear detects the difference between the two frequencies. Depending on what type of PLL you have there can be a voltage that signals the difference between the two oscillators.