Aircraft transponder teardown

[Richard Head]

A friend of mine replaced his faulty ancient aircraft transponder and gave me the dud unit.
A repair attempt had been made by an AMO already and they didn't reassemble it when I recieved it.
I took a few pictures when I had it on the bench.
I hope you find them interesting.
Spot the little paper sticker that the manufacturer placed on the inside chassis!
The assembly date is about mid 1978 when Iran/USA relations were not at their best.
The little aluminium object with the coaxial cables going into it is a klystron, I think.
The rectangular flat block is an interdigital filter for the reciever.

Dick

[nixfu]

Wow.. look at all those VINTAGE Mototrola MC830P NAND Gates.

BTW, a device around that era can actually be worth some money due to the gold used.  Check to see if the used gold much on the parts/contacts/leads etc.

[N2IXK]

I don't think that is a klystron.  Probably a triode oscillator in a tuned cavity. RCA sold lots of those things for various frequencies.

[Richard Head]

Has anybody yet spotted the little paper decal inside?
I had a good chuckle when I saw it.
N2IXK, you may well be right about it being a tuned triode oscillator. There is a small high voltage inverter on the PC board that generates about 1500V for the anode. Of course, it's a pulsed device so the duty cycle is very small and average power pretty low.
I've started dismantling the things that I intend to keep such as the silvered mica RF caps.
There's not much else to salvage.

Dick

[pickle9000]

Nice f**kin sticker.

[djacobow]

Yup, it's a cavity tube. They're in pretty much every general aviation transponder up to a few years ago when solid state became feasible. They need regular replacement. Some info here: http://www.mbtelectronics.com/test-equipment.php

Transponders typically have four settings: off, standby, on, a on + mode c (altitude reporting). The standby setting keeps the tube warm, but otherwise the device is inactive. The new solid state transponders have standby, too, I suppose for old time's sake?

I don't think that is a klystron.  Probably a triode oscillator in a tuned cavity. RCA sold lots of those things for various frequencies.

[Wuerstchenhund]

The new solid state transponders have standby, too, I suppose for old time's sake?

Nothing to do with old times. Stby is used when changing the transponder code in flight to avoid squaking emergency or distress codes on transponders with rotary encoders (that's less of a problem with devices that have a keypad), even if just for a second.

[Alexei.Polkhanov]

Yup, it's a cavity tube. They're in pretty much every general aviation transponder up to a few years ago when solid state became feasible. They need regular replacement. Some info here: http://www.mbtelectronics.com/test-equipment.php

I am curious - sounds like these cavity tubes are not very reliable and they need replacement, yet you say that they were recently used in every ADBS transponder. Why? By "recently" did you mean like 1950s? or 70s? or like 2000s? Wouldn't it be easier to use OCXO or YIG oscillator?

[WarSim]

OCXO was not used very often, only a few in panel models.  Yes YIGs became more common 1990ish - 2000ish.  After 2000ish there was a push for more reliable solid state devices.  I left the field before the RF amps made it to the solid state era. 


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[djacobow]

I am curious - sounds like these cavity tubes are not very reliable and they need replacement, yet you say that they were recently used in every ADBS transponder. Why? By "recently" did you mean like 1950s? or 70s? or like 2000s? Wouldn't it be easier to use OCXO or YIG oscillator?

I'm speaking not as a transponder expert, but a pilot. They seemed to start putting solid state transponders in new GA aircraft 10 years ago, and I'd say it's only been a few years now that people seem more likely to put in a drop-in solid state unit where a vacuum unit had been.

Regarding ADS-B, I don't know what the technology that started with, but I suspect that the 1090-ES type units were cavity just like the regular transponders, and the 978MHz UAT are all solid state. That's a guess.

[WarSim]

the term solid state was a marketing buzzword for the industry.  Yes the front end was all solid state buy the back end was still a tube.  They just gave new solid state sounding names to the amplifier tubes for years. 


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[Richard Head]

The RF side of things is pretty crude, but obviously works and is adequate.
The front end consists of a four resonator interdigital filter (presumably to remove the image).
This then goes to a single diode mixer where it's mixed down to some lower IF and amplified.
The long narrow pc board in the tin can is the mixer/IF amp etc.
The discrete logic takes up most of the board space. How things have changed!
I did a sweep of the inter digital filter with my spec analyser and tracking gen and the response looked pretty good still. I can't remember the TX and RX frequencies anymore but I think it's 1030Mhz RX and 1090Mhz TX, or the other way around.

[miguelvp]

I did a sweep of the inter digital filter with my spec analyser and tracking gen and the response looked pretty good still. I can't remember the TX and RX frequencies anymore but I think it's 1030Mhz RX and 1090Mhz TX, or the other way around.

According wikipedia
http://en.wikipedia.org/wiki/Air_traffic_control_radar_beacon_system

you are spot on:

Theory of operation
The steps involved in performing an ATCRBS interrogation are as follows: First, the ATCRBS interrogator periodically interrogates aircraft on a frequency of 1030 MHz. This is done through a rotating or scanning antenna at the radar's assigned Pulse Repetition Frequency (PRF). Interrogations are typically performed at 450 - 500 interrogations/second. Once an interrogation has been transmitted, it travels through space in the direction the antenna is pointing at the speed of light until an aircraft is reached. When the aircraft receives the interrogation, the aircraft transponder will send a reply on 1090 MHz after a 3.0 ?s delay indicating the requested information. The interrogator's processor will then decode the reply and identify the aircraft. The range of the aircraft is determined from the delay between the reply and the interrogation. The azimuth of the aircraft is determined from the direction the antenna is pointing when the first reply was received, until the last reply is received. This window of azimuth values is then divided by two to give the calculated "centroid" azimuth. The errors in this algorithm cause the aircraft to jitter across the controllers scope, and is referred to as "track jitter." The jitter problem makes software tracking algorithms problematic, and is the reason why monopulse was implemented.