Economy Radar Detector

[John Larkin]

I don't know. If it is a polarized electret, IIRC heat de-polarizes it.
In Sensor Magazine I remember seeing an article about a (IIRC) PVDF RF
detector, in which IR was flashed on it to reset it, like a
chopper-stabilized amp uses to measure tiny DC values. I didn't find it
in a quick search of sensormag web site.

The dielectric in effect acted like the permeable core in a flux-gate
magnetometer. But instead of the permeable ferro-magnetic material being
switched by a saturating magnetic flux, the ferro-electric material is
switched by an IR flux. That's what I remember anyways.

Scott

There's a simpler explanation for the operation of this circuit: it's
bogus, and doesn't work.

John

 

[anthony wooldridge]

That "current pulse" is AC, right? That means that for a fraction of a ns
it's a current pulse in one direction, and then for equally long it's a
current pulse in the other direction. The two average out to zero. And
they average out a lot faster than any component in a 741 can respond - the
diodes and transistors in that 741 aren't even fast enough to rectify the AC
so as to get a DC shift. (Put differently: take a look at the input
capacitance of the 741; now compare it to your desired sub-pF detector
capacitor.) Basically, the 741 is pretty much transparent to microwaves,
I'm thinking.

And by the way, you mentioned a "high strength microwave field i.e. from a
radar camera." Umm, I don't know the specifics but I'm thinking that if
they're aiming that thing at me, it's not very high energy. And if you want
to detect it from far enough away that you can actually respond with your
brake pedal before it's too late - that is, from at least a few hundred
yards - then it's very weak indeed.

Consider this: It is in the cop's interest for the radar signal to be as
weak as possible while still permitting detection - after all, they don't
want to blanket the area with radar signals that might give them away. The
radar camera has high-quality, purpose-built, sophisticated detectors. So,
the radar camera can put out a signal that is just barely strong enough for
its very good detectors to detect a bounce off your big metal object. Now,
you want to detect that signal from about the same distance (the cop has to
detect the bounce, so double the distance; but you want to detect far enough
in advance to slam on the brakes; so let's call it even). So it's a pretty
safe bet that your detector needs to be about as good as the cop's, to
compete.

That's about right but the logic is a bit flawed.
the bounced signal falls of as 1/R^4
the direct signal only falls off at 1/R^2
That's one reason why most detectors get you from behind and not direct on.
that way you need to rely on a bounced signal also to detect the detector
ie much harder to do.
However if the bounced signal comes from a (static) object near the
radar unit you are in luck ( 1/R1^2 *1/R2^2 )
but it is a matter of luck (and careful placement by the cops)
so the chances are you will need to
detect at much longer distance than that,
and therefore need a detector equal or more sensitive than the cop's
The only caveat is that he needs to lock on and do precision measurements,
you only need to detect for presence of microwave.
Anthony

 

[John Larkin]

The only caveat is that he needs to lock on and do precision measurements,
you only need to detect for presence of microwave.
Anthony

But the cop has an advantage: he has his own local reference - his
transmitter - to use as a synchronous reference for his receive mixer,
so gets a very sensitive coherent receiver for free. That's the beauty
of doppler. Plus, he owns a nice directional antenna perfectly tuned
to both his transmit and receive frequencies. The target, on the other
hand, is trying to detect a not-very-stable CW signal somewhere in a
very wide band or three. That's why the good radar detectors are
superhets.

John