Hi everyone,

I'd like to build an electronic circuit that can detect a train of repeated tone bursts in noise and would really appreciate some help.

Some background: The circuit would be a study on snow leopards in the Mongolian mountains, where trap lines connected to radio transmitters send a train of tone bursts via radio when triggered. A circuit that would listen to the right radio frequency and make an alarm when a trap line is triggered would mean that the field operator wouldn't have to listen to the traps every three hours 24/7 so could get some well-earned rest!!

We don't know the received level of the tone bursts, so cannot rely on absolute levels. My idea is to instead use relative levels and signal a detection when we hear pulses that are strong enough compared to the noise.

From my limited knowledge on electronics, I imagine a circuit consisting of
1) a bandpass filter tuned to the frequency of the tones
2) a full-wave rectifier
3) two smoothers:
a) one with an averaging length of several pulse cycles, which measures the average level of bursts + noise
b) one with an averaging length equal to a burst length
4) some form of comparator, which compares the output of circuit 3a with that of 3b. It'd be great if this comparator could give one output voltage when 3a divided by 3b is above a certain value, and another voltage when it is not. Is there a circuit that can do this??
5) Assuming there is a circuit for 4, 5 is another smoother operating on the output of the comparator/detector of 4. if the output level of this smoother exceeds a certain value, we know that we must have sufficiently strong pulses at the right frequency and with at least the required repetition frequency, so can signal an alarm (siren/light)

Is there anyone out there who can help me with step 4 and 5 in my circuit? Or can you think of a better circuit? (not unlikely since I only know basic engineering electronics) Or am I going the wrong way?

Me and the field operator who doesn't get much sleep (my brother!) would be very grateful for any help or suggestions.

Cheers
Toby

 

To get the most accurate information it would be wise to post the RF carrier frequency used and what its mode of modulation is. It wouldn't hurt to also include the approximate frequency of the tone bursts.

 

The traps operate on different frequencies and there is a separate radio receiver. The circuit would operate on the radio reciever's output (audio). The tone burst frequency is around 1 kHz.

 

Are we to understand that the tone bursts are not transmitted until the trap lines are tripped? Are we also to understand that the remote transmitters do not transmit at all (no dead carrier) until it's tripped? I'm also curious.... Is this an AM, SSB or FM system? I'm also curious about the rep rate of the 1KHz tone bursts because you could probably employ a Pump-Charge circuit (post of the 1KHz BPF) to aid in reducing false triggering.

 

Thanks CDRIVE for your interest! I guess I should have given a bit more information from start, but I didn't want the first post to be too long...

As you imply, the traps do transmit also when they're not tripped. It's the same burst tone pulses but with a lower rate of repetition. It'd be great to have a circuit that could also indicate the presence of these slower pulse trains, so that we know we're still in contact with the traps. I guess I figured that the circuit that would detect the rapidly repeated pulses that correspond to a trap that's been tripped would just need some minor adjustment to detect the slowly repeated pulses that correspond to a non-tripped trap.

I don't know what kind of modulation the system uses, but it operates near 150 MHz and is made by Telonics, Inc: http://www.telonics.com/products/vhfSpecial/trapsite.php

The burst rep rate is about 2 Hz in when tripped and about 1 Hz otherwise.

 

Hello,

You could try the NE567 as a tone detector.
See datasheet.
The output pulses could be fed into a NE555 as a missing pulse detector.
The output will start pulsing when the lower frequency starts.
See fig 10A on this page: http://www.sentex.ca/~mec1995/gadgets/555/555.html

Greetings,
Bertus

 

znauzer, the information you posted in post #5 is helpful but the Telonics website BITES! There is nothing more frustrating for a Techie than a website devoid of anything that resembles a Spec! I couldn't even find simple information like what the signal is received by. Is it a Telonics receiver or other? BTW, I'm going to assume that this is a FM system.

When Bertus posted his suggestion using a 567 & a 555 I questioned why the need for the 567(PLL), as I was going to go in a slightly different direction. After some thought I now realize that the 567 will satisfy one of your basic requirements. That being... much increased Noise Immunity and the ability to differentiate your tones from extraneous signals.

Excellent call Bertus!

 

CDRIVE, yeah the Telonics website really sucks! I just don't get people like that, I mean, they must send the specs with the stuff so why not put a pdf of them on the website?!? I don't have the receiver with me but I think it's non-Telonics.

Bertus, thanks for your excellent ideas!! The 567 and 555 look like a great way to solve the problem. I'll try to make a full circuit design and will upload it as soon as it's done.

Thanks again for all your help