into the LA101. This is essential if some of the less common test segments are to be used
as they are not all provided in the default sequences. In particular, the slow, 20s,
frequency sweep (segment S), the MOL test (segment H) and the user level segment (K)
are extremely useful. Sequence editing is very simple and is explained fully in chapter 4.
3.10 Interpreting Sequence Results Interpreting sequence results Relative measurements
A slight caution is needed with regard to the 5 second sweep used in the default
sequences. This gives a quick accurate result for most purposes, but because it is quite
fast it is subject to the normal limitations found on any sweep system or spectrum
analyser with regard to very steep filters. Low pass anti-aliasing filters for digital
systems, for example, cannot be expected to show their true rate of roll-off if the rectifier
cannot follow the change quickly enough. Ringing and sidebands can also lead to small
errors, and low frequency overshoot can sometimes be observed as the result of a
complicated interaction between the output of a high pass filter and its own ringing; even
though this is not present in static tests. None of these effects need be feared on normal
tape machines etc. but if in doubt always confirm with a manual test or use the 20 second
sweep segment (segment S) instead.
In very extreme cases the LA102 may fail to take a valid speed or phase measurement and
will indicate this by showing -999° for an invalid phase measurement or -99.9% for an
invalid speed measurement. This is very unlikely even on very poor quality cassette
machines.
Tape speed
Some clarification of working levels may be appropriate. Distortion measurements are
truly relative, (ie harmonic separation) based on the measured level of signal received, but
the specified level of the distortion test is of course relative to test level. If TL OUT reads
+2dB and the distortion segment operates at +8dB then the measurement was actually
done with an output level of +10dB. Crosstalk and noise measurements are relative to the
measured test level on the measured channel.
Reference level
As with any test system, very low level measurements will also be limited by the
performance of the oscillator and measuring set (see specification in appendix J),
although generally this is unlikely to be a problem. If in doubt, run the sequence with the
LA101 connected directly to the LA102.
3.11 Single Channel Operation Single channel operation Sequence channel Mono sequence
Sequences normally measure the left channel of a system and then the right channel by
repeating every segment. It is therefore much quicker, when testing a mono circuit, to run
the sequence only once, on the channel to be tested. This is readily achieved by pressing
the
key on the LA101 when in SEQ mode. The display will cycle through L, R and
L+R. When a sequence number is entered the sequence will run on the channels
indicated.
50
3. Sequence Testing
To Next Page
Loading...