Racal 1992

[inoxidabile]

Hi everybody!
After some search for the better bargain, I bought a Racal 1992 having options 04A / OXCO and GPIB.
I have had to repair just a couple of things but, at the time being, it seems ok .

Anyway, there is something I don't understand: looking at instruction manual found on the Web, the AUTO TRIG selections shouldn't have any effects on C channels but only on A or B related inputs.

But when I measure a signal on input C, if I select AUTO TRIG as on then the Gate led turns on in a slowly rate and also the indications on display change less frequently... it's like to add a sort of little integration.
Moreover, it seems that the Gate led turns on at a slow but fixed frequency, regardless of the resolution setup.

Of course, without the AUTO TRIG selection on, the Gate led lamps at a different frequency related to the resolution setup.

The software release shown at turn on is 8572  04.04

OK, it's not a problem on itself , the reading seems fine but it's different than expected behaviour and I don't know what is the meaning of change I see.

Thanks in advance for any explanation about !

[Andy Watson]

Mine does that too - it's normal.
The mechanism for finding the "auto" trigger level is to raise the level until the trigger stops, then lower the level until until it stops - i.e. it tracks the maximum and minimum signal levels. I assume the auto level is then set mid-way between the upper and lower level, and then the frequency is measured. So for every measurement displayed, the processor makes three attempts - three gates - so it slows down. You can monitor the trigger levels on the rear panel - there to two DAC outputs for each channel. Internally, the input frequency is measured by reciprocal counting, i.e. the input frequency is used to gate the reference clock. The auto trig level tracking will therefore cause the gate to operate.
I assume the reason that it still tracks the trigger level when using the C channel is that there is an option of measuring the ratio of C/B.

[inoxidabile]

Hi Andy, many thanks for Your kind reply and report, I posted here because I fear it was something wrong and to repair... 
Thanks for explanation, it confirms (more or less) my hypotesis but something still doesn't sound clear for me.
Looking at schematics I can't see a complete confirmation because the DAC are not present in C channel but I agree with You, it should be a sort of "weighted measure" enabled after the trigger level fixed and then I see its effect in terms of slow gate.
Moreover sometimes the trig leds for A and/or B lit and looking in various threads I found someone saying that there could be some little stray coupling or noise at open input.
Anyway, coming back to the topic, the main point has been solved: it's a normal behaviour then it's a lack of explanations in manuals