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DO-253D Change 1
the frequency range of 108.000 to 117.975 MHz for any valid VDB channel
frequency. If one or both of these conditions exist, repeat steps 2 through 4 with
the undesired VDB Message/Signal Generator tuned to the image or half-IF
frequency for the affected VDB channel frequency or frequencies.
1. The term “half-IF frequency” refers to an undesired signal response that is due to
mixing of the second harmonic of a local oscillator with the second harmonic of
an undesired signal. The undesired signal frequency in this case is located one-
half of the Intermediate Frequency (IF) away from the desired signal on the same
side as the local oscillator frequency, i.e., half-way between the desired signal
frequency and the local oscillator. Example: Selected VDB frequency = 117.000
MHz. IF = 10.700 MHz. With the Local Oscillator on the low side of the desired
signal at 106.300 MHz, the half-IF response of the VDB receiver may produce an
undesired output from an undesired signal at 111.650 MHz. Mixing of second
harmonics yields 2x111.650 - 2x106.300 = 10.700 MHz, which is an undesired
mixer output that the IF filter cannot reject. Half-IF rejection is generally
dependent on the IF frequency chosen, the mixer design, and filtering prior to the
2. The VDB receiver design should avoid frequency responses, including image and
half-IF responses, that would allow decoding of undesired VDL broadcasts that
use similar modulation in the VHF COM frequency band, 118 to 137 MHz.
Receiver Recovery from Short Term Excess Undesired Signal Power
The equipment list and connections used for the receiver recovery from short term excess
undesired signal power test are the same as those corresponding to Section 22.214.171.124.7.1.
Detailled Test Procedure
1) Connect the equipment as shown in Figure 2-20.
2) Adjust the VDB signal power at the VDB receiver input from one of the VDB
Signal Generators to Smax + 9 dB as specified for the maximum adjacent channel
interference signal power level specified in Section 126.96.36.199 and adjust the VDB
signal power at the VDB receiver input from the other VDB Signal Generator to
Smin as specified in Section 2.2.5.
3) Synchronize the two VDB Signal Generators such that the generator set at the
lower power level follows in the slot(s) immediately after the slot(s) occupied by
the generator at the high power level and verify that the generators are
synchronized within 95.2 microseconds.
4) Tune both of the VDB Message/Signal Generators and the VDB Receiver to
5) Using the message format specified in Section 188.8.131.52.2, command the VDB
Message/Signal Generators to transmit the number of test messages specified in
Section 184.108.40.206.3. The two generators must output different messages.
6) Using the VDB Receiver and Data Collection Computer, determine the MFR of
the low signal level slot(s) that are space by the recovery time from the high power
slot using the methodology specified in Section 220.127.116.11.3. The VDB Receiver does
not need to meet MFR requirements for the high signal level slot(s) or low power
slots within the recovery time of 3 slots maximum. The test can however be
conducted with a shorter recovery time to reduce the total test time.
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