Grass Valley SMS-6000 Series - Manual (Page 110)

5-6
Section 5 — Maintenance
3. Now reconnect the cables to the matrix. Ignoring the vertical spikes
which are caused by timing errors, any deviation from flatness is as a
result of an insertion gain error. The labeling on the module clearly
shows the two rows of adjustment controls.
4. Adjust the GAIN potentiometer for Output 1 for optimum flatness.
Repeat for each output in turn.
Frequency Response
For accurate frequency response measurements a sweep generator with a
sweep to 20MHz is needed to ensure that the response rolls off correctly
above 10MHz, and 40MHz on wideband systems where the bandwidth is
set to 30MHz. Also a set of differential probes (which measure at the input
and the output of the matrix) is needed to cancel out the effects of input
cabling. However not all customers will have access to such equipment so
the procedure below may be used, but while this will produce satisfactory
results it may not meet specification especially above 6MHz.
Connect a short cable from the output of the test signal generator into Input
1 of the matrix. Connect Output 1 of the matrix to the oscilloscope
terminating it at the oscilloscope in 75
Ω
.
Set the test signal generator to give a 1V p-p composite sweep or multiburst
waveform. Set the matrix so that Input 1 is routed to all outputs.
Observe the waveform on the oscilloscope. When it is satisfactory,
disconnect the leads to the matrix and join them together with a 75
Ω
barrel
or T-piece connector. Set the vertical amplifier of the oscilloscope so that
the waveform fills the screen as far as possible. The waveform should be
flat. If not you will have to make mental allowances for minor deviations.
Now reconnect the cables to the matrix. Any (new) deviation from flatness
is as a result of a frequency response error.
Adjust the HF potentiometer for Output 1 for optimum flatness. Repeat for
each output in turn as necessary. After you have finished the adjustments,
check that he timings have not been affected.
Normally if the response has been correctly set the timing or phase will also
be satisfactory. Use a vectorscope to check the phase and a color bar
generator to feed the matrix. Lock the vectorscope to an external signal (not
to the measured output) and observe the phase angle at each output and
check for deviations. If there are excessive errors then either repeat the
above tests or adjust the HF potentiometer for optimum phase.