how to make a circuit to detect a certain frequecny?

[karmue]

Hello and thanks for your time.
I am a 3rd year electrical engineering student. My friends and I are having a little problem when doing our project course.
For example, I have 4 same ultrasound transmitters powered by 38khz, 39khz, 40khz and 41khz ac powersupply. They will send different frequencies of ultrasound to one ultrasound receiver, and the receiver will transfer that ultrasound signal back to AC current which has the same frequency of the sender. How do i fliter out other signals and only let the one we are interested pass (let's say it's the 41khz one). I tried a 2nd order band-pass filter and it's not that good because the V input magnitude is not fixed and we cannot set up a thrushold.

what should I look at? thanks again

 

[(*steve*)]

You could put a notch filter in the feedback of a op-amp and get a very narrow band bandpass filter.

But that will not eliminate the problem that if you are detecting the 38kHz and 39kHz, the various products of that will produce a signal at 40kHz and 41 kHz. Very hard to determine if you're seeing a real signal or an intermodulation product.

At least that's how I understand it.

 

[trobbins]

Look into PLL techniques, and post back if find something usefull.

 

[Externet]

I do not have your explanation clear enough.

If all 4 transmitters are active, you want to know which are reahing the receiver, or want to decode the modulation of one of them ?

In other words, the receiver is getting one or more signals; do you want to discern which is/are being received or to extract data/amplify only one ?

Four NE567 can tell the presence of each being received. If there is modulation to recover, it is another story.

 

[karmue]

I do not have your explanation clear enough.

If all 4 transmitters are active, you want to know which are reahing the receiver, or want to decode the modulation of one of them ?

In other words, the receiver is getting one or more signals; do you want to discern which is/are being received or to extract data/amplify only one ?

Four NE567 can tell the presence of each being received. If there is modulation to recover, it is another story.

basically the case is:
The design of an autonomous vehicle capable of navigating a track with other moving vehicles present. The vehicle must be able to identify its opponents candles, and blow them out. Once all of the opponent's candles have been successfully extinguished, the vehicle must exit the track to complete the race. the course is a 12ft by 12 ft square with four one-foot-high walls.

we plan to use ultrasound to measure distances but worried about other car using the same frequency. Since most people in this class just use normal ultrasound sensors( working ac frequency is between 38k-42k), we would like to setup a certain frequence to send and receive. I was thinking to use a filter to get rid of any frequency we dont want but after some test it did not turn out to be that simple.

thanks for the imformation on PLL and the ne567 chip, it looks like a solution. I have found a circuit on the internet, but I don't know how to change the pre-defined frequence of it. The PLL l learned last year is used for controlling such as motor speed.

 

[(*steve*)]

Ah, now I see the problem.

How many cycles of whatever frequency are you using? The 567 will not latch on to the signal immediately. Indeed, the narrower the band that you want it to lock on to, the longer it will take. This delay may not be constant and will affect your accuracy. The delay itself will need to be taken into account when you calculate the distance.

 

[karmue]

Ah, now I see the problem.

How many cycles of whatever frequency are you using? The 567 will not latch on to the signal immediately. Indeed, the narrower the band that you want it to lock on to, the longer it will take. This delay may not be constant and will affect your accuracy. The delay itself will need to be taken into account when you calculate the distance.

thanks for the information

we plan to use 5~6 cycles as a brust.

one more question, is lm567L the same as lm567 or lm567C? I cannot find any information of lm567L on the internet.

 

[(*steve*)]

thanks for the information
one more question, is lm567L the same as lm567 or lm567C? I cannot find any information of lm567L on the internet.

Quite likely. The suffixes tend to indicate temperature ranges, case type, etc.

You may need to test whether transmitters near your operating frequency will impair its accuracy.

Of course, if you're allowed to be offensive, it may be a good idea to broadcast pulsed ultrasonic noise over a wide spectrum (say 35 to 45 kHz) when your unit is not actively using its sonar. It might stop the opponents from detecting obstacles