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As we can see in the following link: http://www.radartutorial.eu/13.ssr/sr06.en.html

SSR interrogations formats are differed by pulse width and spacing between them.

How is this measurement done in the circuit level? Can this be done by an ADC or microcontroller or is it done by software?

So, how do we identify if it is one kind of interrogation or another?

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Answering your questions in reverse order.

So, how do we identify if it is one kind of interrogation or another?

The answer is at the top of the page

  • "The SSR interrogation format ... consists of two pulses (P1 and P3) of 0.8 µs width which are separated by a certain time - this determines the mode of interrogation."

    ie Two pulses are transmitted.

    Each pulse is of 0.8 uS duration.

    The time between the end of the first pulse and the start of the second pulse tells the receiver what response mode is required.

Duration between pulse edges in uS .......Mode
3 ... Military 1
5 ... Military 2
8 ... Military 3 or Civilian - most common mode
17 .. Unused 21 .. Civilian Pressure & Altitude query 25 .. Never used.

How is this measurement done in the circuit level?
Can this be done by an ADC or microcontroller
or is it done by software?

It would be done by any suitable means that the user decided to use. The pulse separation measurement could easily be done with hardware. It could be done with software - and so a PC or dedicated computer system which may use a microcontroller may be used.

BUT an ADC = Analog Digital Converter is used for converting analog signals to digital (as the name suggests). An ADC is not usually used for time mearurment.

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On a PIC microcontroller one would program a function like this: 1. configure an interrupt on rising edge of the capture module 2. the rising edge copies the current value in a register, the interrupt service routine has to save that value in another variable t1 3. the second interrupt catptures a second timer value t2 4. the distance between the rising edges is t2-t1 But I'm not sure if for example a PIC 18F1320 would be fast enough to do all this in about 1us. ;-) – 0x6d64 Sep 24 '11 at 9:45
@Russell McMahon: When I asked how to identify whether it is one kind of interrogation or another I meant how to do this in circuit. I read the page, you didn't have to paste the obvious here. If this could easily be done with hardware, please post it here, it would be very much appreciated. – Cassio Sep 24 '11 at 11:49
@ox6d64: For what I could see, PIC18f1320 has a 40MHz clock. Don't you think that's enough? What is actually an interrupt? And do you think a PIC would have a timer at the microsecond level? – Cassio Sep 24 '11 at 11:56
@Cassio - It is often not obvious what is and isn't obvious. Your question "So, how do we identify if it is one kind of interrogation or another?" sounded very much like you didn't understand the material on the page, so I explained it. If it had been obvious (to me) that the question did not mean that you were asking HOW the methods are identified then I wouldn't have wasted my time explaining. Hardware - next comment. – Russell McMahon Sep 24 '11 at 13:49
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A counter is clocked at an appropriate rate - 1 MHz or faster. Incoming pulses are steered alternately between a reset and a latch input on the counter. This can be done with a flip flop. The first pulse resets the counter so that it starts to count when the pulse is removed. the second pulse stops the counter. The counter contents are proportional to the time between pulses. With a 1 MHz clock the counter advances 1 count per microsecond. A faster clock is better so the granularity is not so coarse. – Russell McMahon Sep 24 '11 at 13:53
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