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Signals arriving at the intermediate frequency shall be suppressed at least 80 dB. All other spurious response or signals within
the 960 MHz to 1 215 MHz band and image frequencies shall be suppressed at least 75 dB.
3.5.4.2.7 Recovery time. Within 8 microseconds of the reception of a signal between 0 dB and 60 dB above minimum
sensitivity level, the minimum sensitivity level of the transponder to a desired signal shall be within 3 dB of the value
obtained in the absence of signals. This requirement shall be met with echo suppression circuits, if any, rendered inoperative.
The 8 microseconds are to be measured between the half voltage points on the leading edges of the two signals, both of
which conform in shape, with the specifications in 3.5.5.1.3.
3.5.4.2.8 Spurious radiations. Radiation from any part of the receiver or allied circuits shall meet the requirements
stated in 3.5.4.1.6.
3.5.4.2.9 CW and echo suppression
Recommendation.— CW and echo suppression should be adequate for the sites at which the transponders will be used.
Note.— In this connection, echoes mean undesired signals caused by multipath transmission (reflections, etc.).
3.5.4.2.10 Protection against interference
Recommendation.— Protection against interference outside the DME frequency band should be adequate for the sites
at which the transponders will be used.
3.5.4.3 Decoding
3.5.4.3.1 The transponder shall include a decoding circuit such that the transponder can be triggered only by pairs of
received pulses having pulse duration and pulse spacings appropriate to interrogator signals as described in 3.5.5.1.3 and
3.5.5.1.4.
3.5.4.3.2 The decoding circuit performance shall not be affected by signals arriving before, between, or after, the
constituent pulses of a pair of the correct spacing.
‡3.5.4.3.3 DME/N — Decoder rejection. An interrogation pulse pair with a spacing of plus or minus 2 microseconds,
or more, from the nominal value and with any signal level up to the value specified in 3.5.4.2.3.3 shall be rejected such that
the transmission rate does not exceed the value obtained when interrogations are absent.
Chapter 3 Annex 10 — Aeronautical Communications
3-49 23/11/06
3.5.4.3.4 DME/P — Decoder rejection. An interrogation pulse pair with a spacing of plus or minus 2 microseconds, or
more, from the nominal value and with any signal level up to the value specified in 3.5.4.2.3.4 shall be rejected such that the
transmission rate does not exceed the value obtained when interrogations are absent.
3.5.4.4 Time delay
3.5.4.4.1 When a DME is associated only with a VHF facility, the time delay shall be the interval from the half voltage
point on the leading edge of the second constituent pulse of the interrogation pair and the half voltage point on the leading
edge of the second constituent pulse of the reply transmission. This delay shall be consistent with the following table, when it
is desired that aircraft interrogators are to indicate distance from the transponder site.
Pulse pair
spacing (μs) Time delay (μs)
Channel
suffix Operating mode Interrogation Reply
1st pulse
timing
2nd pulse
timing
X DME/N 12 12 50 50
DME/P IA M 12 12 50 –
DME/P FA M 18 12 56 –
Y DME/N 36 30 56 50
DME/P IA M 36 30 56 –
DME/P FA M 42 30 62 –
W DME/N – – – –
DME/P IA M 24 24 50 –
DME/P FA M 30 24 56 –
Z DME/N – – – –
DME/P IA M 21 15 56 –
DME/P FA M 27 15 62 –
Note 1.— W and X are multiplexed on the same frequency.
Note 2.— Z and Y are multiplexed on the same frequency.
3.5.4.4.2 When a DME is associated with an MLS angle facility, the time delay shall be the interval from the half
voltage point on the leading edge of the first constituent pulse of the interrogation pair and the half voltage point on the
leading edge of the first constituent pulse of the reply transmission. This delay shall be 50 microseconds for mode X channels
and 56 microseconds for mode Y channels, when it is desired that aircraft interrogators are to indicate distance from the
transponder site.
3.5.4.4.2.1 For DME/P transponders, no time delay adjustment shall be permitted.
3.5.4.4.3 Recommendation.— For the DME/N the transponder time delay should be capable of being set to an
appropriate value between the nominal value of the time delay minus 15 microseconds and the nominal value of the time
delay, to permit aircraft interrogators to indicate zero distance at a specific point remote from the transponder site.
Annex 10 — Aeronautical Communications Volume I
23/11/06 3-50
Note.— Modes not allowing for the full 15 microseconds range of adjustment in transponder time delay may only be
　
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