HP 8920A, User Manual

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Chapter 8, Signaling Decoder Screen
Dual-Tone Multi-Frequency (DTMF) Decoder
Input Level
This field specifies the signal level that you input (after de-emphasis if used). The
higher the level of signal expected by the analyzer, the higher the trigger level is
set.
Operating Considerations
The unit-of-measure is determined by the
AF Anl In
setting.
The input level should be set high enough to prevent false triggering, but low
enough to allow triggering for fluctuating signal levels.
De-emphasis Effects
De-emphasis is a single-pole, low-pass filter with a 212.2 Hz corner frequency. It
is enabled/disabled using the
De-Emphasis
field on the AF ANALYZER screen.
(Refer to
Figure 15, "AF ANALYZER Functional Block Diagram," on page 100
.)
The Input Level is the expected level at the output of the de-emphasis network.
Assuming a mean DTMF frequency of approximately 1 kHz, decoding with de-
emphasis on (set to
750 µ
s) requires the input level to be set to 212/1000 = 0.212
times the peak deviation, or about 1/5 the incoming level of the tone.
Examples of Input Level Settings
Example 1
Peak deviation 3 kHz, De-emphasis off. Set the input level to 3 kHz.
Example 2
Peak deviation 3 kHz, De-emphasis 750 ms. Set the input level to 3
×
0.212 =
636 Hz