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A Cheap and Dirty Transponder Tool
A friend had a problem with his transponder. It didn't seat all the way in
the rack and would not connect with the coax(AT150). I started thinking.....(I know) Does anyone out there know of a way to make a simple transponder field unit that will transmit the necessary pulses( 3, I think) at theone gig and change and then a simple receiver to see if it responded(one shot and led). Don't need squawk code or altitude..Any ideas....Jim! This is probably an illegal device but at very low power, hmm? John Prince Pitts N3DR |
#2
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You've already got most of something equivalent built into your transponder.
The reply light in the AT-150 is triggered by the pulses in the reply pulse train picked off just before they go to the transmit cavity. If the reply light is blinking without pushing the ident or holding the switch in the test position, then the transponder is receiving proper interrogations and generating replies. If ATC doesn't see you but your reply light is blinking, then either your cavity is shot, the high voltage power supply for the cavity is broken, or your reply timing is so far off that atc doesn't recognize the reply. All 3 cases warrant a visit to the avionics shop. On the other hand, if the reply light is not blinking , then operating the transponder in the test position may be helpful in diagnosing the problem. The test mode in the AT-150 injects a stream of pulses into the receiver that mimics pulses normally recieved. If the test pulses are properly recognized by the pulse logic, then replies are generated and the reply lamp lights. If the reply light lights for test mode, then the pulse decode and reply generation logic in the transponder are probably OK. Using these facts plus being somewhere where ATC interogations reach your antenna should be enough to determine the health of the system. YOu can also use one of the transponder monitors or passive traffic detectors to monitor the reply from the transponder. Note also that you can't legally remove the transponder and put it back in its tray without a sign-off. It is specifically called out as a not-allowed owner maintenance action in the FARs. That said, a unit that exercises the transponder and monitors it for a reply could be put together with a small number of parts, however the design time would not be economical because there would be no market for such a device seeing that only an avionics shop can legally remove/reinstall the transponder and those shops already have transponder test sets that do far more. The transponder replies to a 2 pulse sequence. The third pulse (which is between the other two), if present, is supposed to suppress the transponder's reply. Each pulse is 1030MHz (or is it 1090?...I always get the transmit and receive frequencies mixed up), lasts nominally for 0.8 usec. For a mode C interrogation, the leading edges of the pulses are 21 usec apart, for mode 3/A, they are 8 usec apart. A pulse 2 usec after the first pulse will cause the reply to be suppressed. If you kept the transmit power to a few milliwatts, while not strictly legal, it would probably not raise anyone's hair. As stated before though, this unit would tell you little more than what you can already determine using the reply light and being either at an airport or in the air where you can recieve ATC interrogations. A simple interrogator circuit could be built using a 1030MHz oscillator gated by a 0.8 usec wide pulse that happens every 8 us (mode 3/A). That circuit could take the place of the ATC interrogations for testing on the ground at an airport where you don't normally recieve ATC interrogations on the ground. You could use that in conjuction with the reply light to test the transponder the way you are intending. A suitable oscillator circuit could be found in the ARRL handbook. The gate circuit could be done with a 10 bit digital shift register clocked by a clock with a 0.8 usec period and arranged in a ring shifting a single '1' through the 10 bits. John P wrote: A friend had a problem with his transponder. It didn't seat all the way in the rack and would not connect with the coax(AT150). I started thinking.....(I know) Does anyone out there know of a way to make a simple transponder field unit that will transmit the necessary pulses( 3, I think) at theone gig and change and then a simple receiver to see if it responded(one shot and led). Don't need squawk code or altitude..Any ideas....Jim! This is probably an illegal device but at very low power, hmm? John Prince Pitts N3DR -- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759 |
#3
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Ray,
Now that's what I wanted to know!!! Thanks for the explanation. I didn't think a traffic detector would work in this case. Your right, the ARRL handbook would take care of the RF portion and the shiftreg the other part of the solution. The shift register, lets see, would have to be a one looped back on itself (on that bit) and then shifted out, I think. Anyway, that's easy to "fix" until I get it right. I needed the "numbers"(.8us). The only thing that the test switch on the AT150 circuit does not do is check for the actual transmission. Right? In my case, with the friend, the bulb on the AT150 was burned out, (as it turned out) so the unit appeared dead. I didn't think it actually sent a string and then checks the receive functions. The friend called a avionics tech and came out to check for proper codes, then found the bad bulb, then the RF connector was not seated. Multiple problems. Even if I had the circuit I've been talking about, it would not have solved anything. But, if the antenna was connected the circuit would have closed the loop on transmissions from the transponder. It would still have to be checked for codes and altitude but the interrogation lite would have been lit. It would be a fairly simple set of circuits for a simple check of limited value...... Thanks Again.. John "Ray Andraka" wrote in message ... You've already got most of something equivalent built into your transponder. The reply light in the AT-150 is triggered by the pulses in the reply pulse train picked off just before they go to the transmit cavity. If the reply light is blinking without pushing the ident or holding the switch in the test position, then the transponder is receiving proper interrogations and generating replies. If ATC doesn't see you but your reply light is blinking, then either your cavity is shot, the high voltage power supply for the cavity is broken, or your reply timing is so far off that atc doesn't recognize the reply. All 3 cases warrant a visit to the avionics shop. On the other hand, if the reply light is not blinking , then operating the transponder in the test position may be helpful in diagnosing the problem. The test mode in the AT-150 injects a stream of pulses into the receiver that mimics pulses normally recieved. If the test pulses are properly recognized by the pulse logic, then replies are generated and the reply lamp lights. If the reply light lights for test mode, then the pulse decode and reply generation logic in the transponder are probably OK. Using these facts plus being somewhere where ATC interogations reach your antenna should be enough to determine the health of the system. YOu can also use one of the transponder monitors or passive traffic detectors to monitor the reply from the transponder. Note also that you can't legally remove the transponder and put it back in its tray without a sign-off. It is specifically called out as a not-allowed owner maintenance action in the FARs. That said, a unit that exercises the transponder and monitors it for a reply could be put together with a small number of parts, however the design time would not be economical because there would be no market for such a device seeing that only an avionics shop can legally remove/reinstall the transponder and those shops already have transponder test sets that do far more. The transponder replies to a 2 pulse sequence. The third pulse (which is between the other two), if present, is supposed to suppress the transponder's reply. Each pulse is 1030MHz (or is it 1090?...I always get the transmit and receive frequencies mixed up), lasts nominally for 0.8 usec. For a mode C interrogation, the leading edges of the pulses are 21 usec apart, for mode 3/A, they are 8 usec apart. A pulse 2 usec after the first pulse will cause the reply to be suppressed. If you kept the transmit power to a few milliwatts, while not strictly legal, it would probably not raise anyone's hair. As stated before though, this unit would tell you little more than what you can already determine using the reply light and being either at an airport or in the air where you can recieve ATC interrogations. A simple interrogator circuit could be built using a 1030MHz oscillator gated by a 0.8 usec wide pulse that happens every 8 us (mode 3/A). That circuit could take the place of the ATC interrogations for testing on the ground at an airport where you don't normally recieve ATC interrogations on the ground. You could use that in conjuction with the reply light to test the transponder the way you are intending. A suitable oscillator circuit could be found in the ARRL handbook. The gate circuit could be done with a 10 bit digital shift register clocked by a clock with a 0.8 usec period and arranged in a ring shifting a single '1' through the 10 bits. John P wrote: A friend had a problem with his transponder. It didn't seat all the way in the rack and would not connect with the coax(AT150). I started thinking.....(I know) Does anyone out there know of a way to make a simple transponder field unit that will transmit the necessary pulses( 3, I think) at theone gig and change and then a simple receiver to see if it responded(one shot and led). Don't need squawk code or altitude..Any ideas....Jim! This is probably an illegal device but at very low power, hmm? John Prince Pitts N3DR -- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759 |
#4
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It doesn't check the actual transmission, nor the receiver RF strip. It injects
a series of pulses into the pulse width/spacing discriminator logic, which in turn triggers the generation of replies, whcih modulate the cavity. The reply light circuit is driven by the cavity modulator. John P wrote: Ray, Now that's what I wanted to know!!! Thanks for the explanation. I didn't think a traffic detector would work in this case. Your right, the ARRL handbook would take care of the RF portion and the shiftreg the other part of the solution. The shift register, lets see, would have to be a one looped back on itself (on that bit) and then shifted out, I think. Anyway, that's easy to "fix" until I get it right. I needed the "numbers"(.8us). The only thing that the test switch on the AT150 circuit does not do is check for the actual transmission. Right? In my case, with the friend, the bulb on the AT150 was burned out, (as it turned out) so the unit appeared dead. I didn't think it actually sent a string and then checks the receive functions. The friend called a avionics tech and came out to check for proper codes, then found the bad bulb, then the RF connector was not seated. Multiple problems. Even if I had the circuit I've been talking about, it would not have solved anything. But, if the antenna was connected the circuit would have closed the loop on transmissions from the transponder. It would still have to be checked for codes and altitude but the interrogation lite would have been lit. It would be a fairly simple set of circuits for a simple check of limited value...... Thanks Again.. John "Ray Andraka" wrote in message ... You've already got most of something equivalent built into your transponder. The reply light in the AT-150 is triggered by the pulses in the reply pulse train picked off just before they go to the transmit cavity. If the reply light is blinking without pushing the ident or holding the switch in the test position, then the transponder is receiving proper interrogations and generating replies. If ATC doesn't see you but your reply light is blinking, then either your cavity is shot, the high voltage power supply for the cavity is broken, or your reply timing is so far off that atc doesn't recognize the reply. All 3 cases warrant a visit to the avionics shop. On the other hand, if the reply light is not blinking , then operating the transponder in the test position may be helpful in diagnosing the problem. The test mode in the AT-150 injects a stream of pulses into the receiver that mimics pulses normally recieved. If the test pulses are properly recognized by the pulse logic, then replies are generated and the reply lamp lights. If the reply light lights for test mode, then the pulse decode and reply generation logic in the transponder are probably OK. Using these facts plus being somewhere where ATC interogations reach your antenna should be enough to determine the health of the system. YOu can also use one of the transponder monitors or passive traffic detectors to monitor the reply from the transponder. Note also that you can't legally remove the transponder and put it back in its tray without a sign-off. It is specifically called out as a not-allowed owner maintenance action in the FARs. That said, a unit that exercises the transponder and monitors it for a reply could be put together with a small number of parts, however the design time would not be economical because there would be no market for such a device seeing that only an avionics shop can legally remove/reinstall the transponder and those shops already have transponder test sets that do far more. The transponder replies to a 2 pulse sequence. The third pulse (which is between the other two), if present, is supposed to suppress the transponder's reply. Each pulse is 1030MHz (or is it 1090?...I always get the transmit and receive frequencies mixed up), lasts nominally for 0.8 usec. For a mode C interrogation, the leading edges of the pulses are 21 usec apart, for mode 3/A, they are 8 usec apart. A pulse 2 usec after the first pulse will cause the reply to be suppressed. If you kept the transmit power to a few milliwatts, while not strictly legal, it would probably not raise anyone's hair. As stated before though, this unit would tell you little more than what you can already determine using the reply light and being either at an airport or in the air where you can recieve ATC interrogations. A simple interrogator circuit could be built using a 1030MHz oscillator gated by a 0.8 usec wide pulse that happens every 8 us (mode 3/A). That circuit could take the place of the ATC interrogations for testing on the ground at an airport where you don't normally recieve ATC interrogations on the ground. You could use that in conjuction with the reply light to test the transponder the way you are intending. A suitable oscillator circuit could be found in the ARRL handbook. The gate circuit could be done with a 10 bit digital shift register clocked by a clock with a 0.8 usec period and arranged in a ring shifting a single '1' through the 10 bits. John P wrote: A friend had a problem with his transponder. It didn't seat all the way in the rack and would not connect with the coax(AT150). I started thinking.....(I know) Does anyone out there know of a way to make a simple transponder field unit that will transmit the necessary pulses( 3, I think) at theone gig and change and then a simple receiver to see if it responded(one shot and led). Don't need squawk code or altitude..Any ideas....Jim! This is probably an illegal device but at very low power, hmm? John Prince Pitts N3DR -- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759 -- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759 |
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