4. Fourth - Explore
By using the basic verification tests for receive and transmit, the performance of the MX465 can
be explored by varying the frequency and level of the audio input signals. The following
examples provide additional tests that can be performed.
4.1 Measuring CTCSS Encoder response time
Use the CTCSS encoder verification on page 7 to set up to test encoder response time.
Connect one channel of a storage scope to PTT and another channel of a storage scope to
TONEOUT. Set the scope to trigger on PTT . Connect and disconnect the PTT to ground and
measure the time difference from PTT going from a logic level high to a logic low and a 90%
steady level CTCSS tone amplitude output on TONEOUT.
4.2 Measuring CTCSS Decoder response time
Use the CTCSS decoder verification on page 8 to set up to test decoder response time.
Connect one channel of a storage scope to RXIN and another channel of a storage scope to
MONIT0R . Set the scope to trigger on RXIN. Turn the CTCSS tone off and on by connecting
and disconnecting the signal generator. Measure the time difference from applying CTCSS tone
to RXIN (where the CTCSS tone level reaches 90% of a full steady state level) to MONIT0R
changing from a logic level hi to a logic level low ( were the logic level reaches 90% of steady low
state).
4.3 Measuring CTCSS Decoder band width
Use the CTCSS decoder verification on page 8 to set up to evaluate decoder bandwidth. Adjust
the frequency of the audio generator above and below the CTCSS tone’s center frequency and
measure the band edge where MONIT0R changes from a hi ( indicating no tone present) to a
low ( indicating a CTCSS tone was detected).
4.4 Measuring CTCSS response at different SINADs
A composite signal summing network can be bread boarded to simulate adverse signal
conditions. This consists of an opamp with three summing nodes. Separate signal generators
are used to simulate the 3 basic types of audio signals that may be present at RXAUDIO’s input.
One signal generator is used to simulate the CTCSS tone frequency and level. A noise generator
that is 6khz band limited gaussian noise simulates background noise. The audio sine wave
generator simulates the voice channel. A SINAD meter may be connected to the RXIN audio
path to measure SINAD of the input signal to the MX465’s CTCSS decoder.
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