B4.4
T855 Functional Testing
M850-00
31/09/98 Copyright TEL
4.4 Audio Distortion
The level of distortion measured at the line output (refer to Figure 1.3 in Part F) gives an
indication of the accuracy of the IF alignment.
Apply an accurate on-channel signal from the RF generator at a level of -70dBm
with ±3kHz deviation (±2.4kHz) [±1.5kHz] at 1kHz.
Adjust RV210 (front panel line level) to give +10dBm into 600 ohms.
Check that the distortion is approximately 1% THD.
Note:
For a flat response, the distortion should always be better than 2% for wide
bandwidth sets or 4% for mid and narrow bandwidth sets.
Adjust RV205 (front panel monitor volume) to give 2V rms into a 4 ohm resistive
load.
Check that the distortion at the monitor output is better than 2% THD.
Reset the controls before proceeding to the next set of tests.
4.5 Ultimate Signal-To-Noise Ratio
Apply a signal from the RF generator at a level of -57dBm with ±3kHz deviation
(±2.4kHz) [±1.5kHz] at 1kHz.
Select de-emphasis on the links provided in the audio processor (refer to Section
3.5), and link pins 2 & 3 of PL240 to include the 300Hz filter.
Adjust RV210 (front panel line level) to provide +10dBm output.
Switch off the modulation, checking that the residual noise is lower than -45dBm
(-43dBm) [-39dBm] at the line output (this corresponds to S/N of 55dB (53dB)
[49dB] and is in accordance with EIA measurement conditions).
Note:
You can make the measurement without the 300Hz high pass filter, but the
result will be 10dB worse.
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