Filtering...

[sethy]

I was curous to know what the term Filtering really means... Is it basically just cutting sensetivity? or is there a way to really establish threats from non-threats

[jimbonzzz]

I was curous to know what the term Filtering really means... Is it basically just cutting sensetivity? or is there a way to really establish threats from non-threats

Basically, there is no way to tell 100% threats from non-threats.
In general, radar is radar, and it must be reported as an alert.
But, there are a few methods detectors use to attempt to minimize them:

1. Cut sensitivity

2. Don't report alerts shorter than a certain duration
I read a post where Radar Roy talks about the PNI doing this, maybe other detectors do this as well in city mode.

3. DSP:

I believe the Escort splits the radar carrier into 32 "levels", then ignores the strongest three, and determines if a signal is above the threshold of an alert based on the rest of the "levels". They use signal averaging here. i.e. If a signal is detected on one sweep over the threshold, but not in a specified number of subsequent sweeps, then they might suppress the alert. Most if not all detectors nowdays use DSP, and there are some different approaches and algorithms that they use to determine whether to report the alert or not, and to pick out radar from the noise.

They have to be careful not to go too far, without possibly suppressing actual police radar alerts. Some detectors might suppress alerts they aren't sure of, some report them to be cautious.

4. To prevent Ka falsing from other detectors, they can detect harmonics:

The problem was that the old detectors had a local oscillator of 11.55 GHz, and the third harmonic of this is in the Ka band (11.55 GHz x 3=34.65 GHz). This caused Ka false alerts. So, in order to suppress alerts from this, detectors look for the leaked LO frequency of 11.55 GHz, or the second harmonic at ~23.1 GHz (11.55x2), which would also normally be present with an interfering detector, but not police radar. But, because of X and K bandpass filtering in some of these detectors, the harmonics might be too weak to detect, so in some cases detectors must err on the side of caution and report an alert anyway. Detector makers have various ways to approach this, and some simply compare the harmonic relationship between different carriers deteced in these ranges, and if there IS a relationship then the alert is suppressed. The interfering signals don't always have to be present at the same time as the harmonic for the alerts to be suppressed, if these signals appear within a specified timeframe then the alert is suppressed.

5. I believe I read somewhere that they can determine if a Ka signal is false by watching to see if it changes freq i.e. if the other detector is changing the LO when sweeping to mix with the received signals.

Some detectors suppress false alerts naturally by not having a rear antenna. It would make sense that a detector with only a front antenna likely has 1/2 the falsing as one with both front and rear.

I also suspect that some detectors might be more directional than others i.e. so they are less likely to detect motion sensors that are well off to the side of the road you are driving on, but I'm not sure about this.

Well, those are the only methods I know of, if anyone else knows some methods please chime in...


Jim

[Iknox]

Thanks for the GREAT INFO

[compu44]

Tons of info there. Much appriciated jimbonzzz

[jimbonzzz]

I also forgot about the Cobra "Intellimute", which senses engine RPMs and can tell if you are stopped or going slow, and then mutes the alerts (since you probably aren't speeding)

Jim

[happya$$]

Jim,

What do you do for a living? You seem to know a lot about radar

[jimbonzzz]

Jim,

What do you do for a living? You seem to know a lot about radar

Well my job has nothing to do with Radar...

I'm no radar expert by any means, my interest in radar is really just a hobby like a lot of the other people who post here, so take anything I say with a grain of salt

Radar and detectors fascinate me, and so does the technology. I'm a ham and a scanner enthusiast, so I like messing around with radio and gadjets in general. I also like fast cars

But since you asked, I am an EE, I currently work with Engine/Transmission Control Modules at the GM Proving Ground in Michigan.


Jim

[compu44]

I also forgot about the Cobra "Intellimute", which senses engine RPMs and can tell if you are stopped or going slow, and then mutes the alerts (since you probably aren't speeding)

Jim

Good thing I don't have one, wouldn't want to have trouble on those 200km/h in neutral times....

It's a feature that's selectable on/off. Never used it myself as I have no tachomoter.

[joop1987]

It's a feature that's selectable on/off. Never used it myself as I have no tachomoter.

It uses the frequency of your alternator; so no need to hook it up to a tach.

[SETITOFF]

On this Cobra INTELLIMUTE, it actually picks up the RPM directly from the engine. You're right, it doesn't need to be hooked to a tach due to the electronics built in. The engine RPM is sensed directly thru the power cord connected to the lighter. I have the Cobra 9400 and it has this feature.

I've had it set and it does work, eliminating any alerts while you are sitting still. Once you begin in motion again it goes into active scanning.

Let's say you're stopped near a WalMart with an auto-door opener: it will not alert on that door until you begin moving. It's an okay feature.

The way it is set is you activate it when the engine is in idle so it picks up the lowest RPM after the engine is at nromal operating temp of course, say 700RPM. Anything over that the unit is in active scanning.