AviationBanter - View Single Post - Narco AT-150 vs. AT-155

**Ray Andraka** · November 14th 07, 07:55 PM posted to rec.aviation.owning,rec.aviation.piloting

wrote:

Which is more reliable? Which do you like better?

The AT155 is a functionally identical transponder, but constructed using
more modern circuitry than the AT150. Generally speaking, the pilot
won't notice any difference between the two other than the number
printed on the front panel (the AT155 does have a higher output power
which you may hear in your intercom if grounding is less than perfect,
and the reliability is greater for the AT155). The transmitter is a
solid state transmitter rather than the vaccuum tube (cavity) technology
in the AT150. The reciever in the AT155 uses digital logic clocked by a
quartz crystal to derive all of the timing for both the interrogation
decode and the reply generation where the AT150 uses time delays
obtained by charging and discharging capacitor-resistor networks. The
AT-150's timing needs to be calibrated at the time of manufacture, and
may need occasional calibration in the field, where the AT155 should
never need calibration (indeed, it has no calibration settings inside).
Also, the digital logic in the AT150 for the interrogation decode and
reply generation is a handful of discrete logic chips. The AT155
integrates the entire digital part of the design into one chip. Chip
failures are very often due to stresses at the input/output pins rather
than internal failures. By integrating everything into one monolithic
part, many of the failure points are eliminated. Finally, if you buy an
AT150 today, it will be a used transponder, as they are not being made
any more. An AT155 is current technology and can be bought new today.