Here's how you know that you've misinterpreted how the trigger mode works. In your description, there's no way for a stable signal to be displayed in AUTO mode. In every scope I'm familiar with, AUTO merely generates a baseline if no valid triggering event is seen after some interval (and in most scope, that interval is a function of the timebase setting). But once a valid trigger event happens, synchronization happens, too. If it didn't, AUTO mode would be sort of useless, and one couldn't properly call it a trigger mode.
-- Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
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On 12/12/2021 16:33, n4buq wrote:
Hi Jeff,
I'm not trying to argue and I know I'm probably the least likely one in this group to fully understand the circuit, but I way I understand it, as long as C535 is charged, the sweep starts immediately after the previous sweep finishes and, C535 is discharged only when a trigger is present; therefore, with no trigger, there's virtually no delay from end-of-sweep to beginning-of-sweep.
Is that incorrect?
Thanks,
Barry - N4BUQ
----- Original Message -----
From: "Jeff Dutky" <jeff.dutky@...>
To: "tekscopes" <[email protected]>
Sent: Sunday, December 12, 2021 2:02:39 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm Mode
Barry,
I'm pretty sure that you have misinterpreted how AUTO mode works, the 7B53A
service manual (May 1988 revision) clearly says on page 3-9 "When the AUTO
button is pressed, a weep is produced as in NORM except that a free-running
trace is displayed when a trigger pulse is not present." Then on page 3-11 the
Auto Trigger Mode section reads:
"Operation of the Main Sweep Generator circuit in the AUTO position of the MAIN
TRIGGERING MODE switch is the same as for the NORM position just described when
a trigger pulse is applied. However, when a trigger pulse is not present, a
free running reference trace is produced in the AUTO position. This occurs as
follows:
"The Auto Triggering circuit consists of pins 1, 3, 6, and 19 of U520. When the
AUTO button of the MAIN TRIGGERING MODE switch is pressed, a low at pin 19 of
U520 enables the Auto Circuit. When a repetitive trigger signal above 30 Hz,
and of adequate amplitude, is applied to the Main Sweep Start Comparator and
pin 1 of U520, the internal Auto Multi at pin 6 of U520 charges towards five
volts through C535 and R535, but is discharged by each incoming trigger pulse.
"In the absense of a trigger pulse, C535 charges toward +5 V, switching pin 6 to
its high state and pin 3 to its low state. Q547 turns off, its collector rises
and a high is coupled through emitter follower Q551 to pin 1 of U580, causing
the sweep to run."
This basically identical to the Auto Sweep circuit in the 7B92, which also uses
a sweep process IC, and an RC etwork off of pin 6 (the AUTO TIMING pin). The
sweep process IC in both the 7B53A and the 7B92 is the same Tek custom IC,
155-0049 (-00 in the 7B92 and -01 in the 7B53A).
The upshot is, as I said, that the sweep occurs either a) when you get an
appropriate trigger condition, or b) after some short delay determined by the
RC network. The RC network off of pin 6 in the 7B53A is 120k ¦¸ and 1 uF, giving
a TC of 120 ms, but the voltage charging C535 is +15 V, so it will get to +5 V
after only a couple of TCs, meaning that the actual repetition rate of the
sweep in AUTO mode will only be in the range of ten Hertz, independent of the
selected horizontal sweep speed (maybe I did the math wrong, because the sweep
rep rate I observe in AUTO mode seems much faster than the TC of the RC network
would imply. I'd guess that the sweep rep rate around at least 30 Hz, if not 60
Hz. Obviously I could observe the sawtooth directly to confirm my observation,
but I haven't done that because I'm lazy).
-- Jeff Dutky
