|
I'm working on an 7B53N but I think this question applies to many/most scopes and/or time-bases.
In AUTO mode, the time-base is in a "free-run" mode which generates a ramp at the selected sweep rate and, at the end of the ramp, generates another ramp. This continues whether a trigger is present or not.
In NORM, the time-base waits for a triggering signal before executing a sweep and, at the end of the sweep, waits for another trigger signal before generating another ramp.
My question is with respect to triggering in AUTO. Since the ramp will be generated regardless of a trigger, how does the system perform triggering when in AUTO mode?
I know this is a very basic question, but I don't seem to be finding a good explanation in the manual so I thought I'd ask here.
Thanks,
Barry - N4BUQ
|
Please disregard. I now see where this is described in the manual. I had missed that!
Thanks,
Barry - N4BUQ
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----- Original Message ----- From: "n4buq" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Saturday, December 11, 2021 8:00:05 PM
Subject: [TekScopes] Timebase Question : Auto vs Norm Mode I'm working on an 7B53N but I think this question applies to many/most scopes
and/or time-bases.
In AUTO mode, the time-base is in a "free-run" mode which generates a ramp at
the selected sweep rate and, at the end of the ramp, generates another ramp.
This continues whether a trigger is present or not.
In NORM, the time-base waits for a triggering signal before executing a sweep
and, at the end of the sweep, waits for another trigger signal before
generating another ramp.
My question is with respect to triggering in AUTO. Since the ramp will be
generated regardless of a trigger, how does the system perform triggering when
in AUTO mode?
I know this is a very basic question, but I don't seem to be finding a good
explanation in the manual so I thought I'd ask here.
Thanks,
Barry - N4BUQ
|
As a practical note, you can be bitten by that on slow trigger waveforms.? AUTO will self trigger before the waveform recycles.
Harvey
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On 12/11/2021 9:04 PM, n4buq wrote: Please disregard. I now see where this is described in the manual. I had missed that!
Thanks,
Barry - N4BUQ
----- Original Message -----
From: "n4buq" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Saturday, December 11, 2021 8:00:05 PM
Subject: [TekScopes] Timebase Question : Auto vs Norm Mode
I'm working on an 7B53N but I think this question applies to many/most scopes
and/or time-bases.
In AUTO mode, the time-base is in a "free-run" mode which generates a ramp at
the selected sweep rate and, at the end of the ramp, generates another ramp.
This continues whether a trigger is present or not.
In NORM, the time-base waits for a triggering signal before executing a sweep
and, at the end of the sweep, waits for another trigger signal before
generating another ramp.
My question is with respect to triggering in AUTO. Since the ramp will be
generated regardless of a trigger, how does the system perform triggering when
in AUTO mode?
I know this is a very basic question, but I don't seem to be finding a good
explanation in the manual so I thought I'd ask here.
Thanks,
Barry - N4BUQ
|
Apparently it needs about 30Hz or better to prevent that.
Thanks,
Barry - N4BUQ
toggle quoted message
Show quoted text
As a practical note, you can be bitten by that on slow trigger
waveforms.? AUTO will self trigger before the waveform recycles.
Harvey
On 12/11/2021 9:04 PM, n4buq wrote:
Please disregard. I now see where this is described in the manual. I had
missed that!
Thanks,
Barry - N4BUQ
----- Original Message -----
From: "n4buq" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Saturday, December 11, 2021 8:00:05 PM
Subject: [TekScopes] Timebase Question : Auto vs Norm Mode
I'm working on an 7B53N but I think this question applies to many/most scopes
and/or time-bases.
In AUTO mode, the time-base is in a "free-run" mode which generates a ramp at
the selected sweep rate and, at the end of the ramp, generates another ramp.
This continues whether a trigger is present or not.
In NORM, the time-base waits for a triggering signal before executing a sweep
and, at the end of the sweep, waits for another trigger signal before
generating another ramp.
My question is with respect to triggering in AUTO. Since the ramp will be
generated regardless of a trigger, how does the system perform triggering when
in AUTO mode?
I know this is a very basic question, but I don't seem to be finding a good
explanation in the manual so I thought I'd ask here.
Thanks,
Barry - N4BUQ
|
Barry,
though you did say to disregard this question, I will answer simply because it will make the archived messages more useful to other people at a later time (and because I have been looking at this in a 7B92, so I think I understand how the free-running feature is supposed to work).
In AUTO mode the sweep will trigger when either of two things happens: 1. when the trigger condition is met (and the trigger circuit is in the ARMED state), or 2. when a timer (an RC circuit in the 7B92) finishes (and the trigger circuit is in the ARMED state). This means that you get a short pause between free-run sweeps if the trigger conditions are not met, so they will not necessarily be happening at the specified sweep frequency (in other words, if you set a sweep speed of 1 ms/div, so that the sweep takes something like 10 ms, the actual free-run rate may be something like 20 ms, or even slower depending on the auto-sweep timing).
-- Jeff Dutky
|
Jeff,
I don't know if the 7B92 is very much different from the 7B53 series, but from what I read, when in AUTO, if there's no trigger signal, a capacitor (C786) stays charged that keeps the MILLER RUNUP section enabled which causes a continuous sweep. In other words, a sweep in AUTO mode doesn't care whether the trigger is armed.
Someone please correct me, though, if that's wrong or if it's an over-simplification.
Thanks,
Barry - N4BUQ
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----- Original Message ----- From: "Jeff Dutky" <jeff.dutky@...>
To: "tekscopes" <[email protected]>
Sent: Saturday, December 11, 2021 11:56:57 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm Mode Barry,
though you did say to disregard this question, I will answer simply because it
will make the archived messages more useful to other people at a later time
(and because I have been looking at this in a 7B92, so I think I understand how
the free-running feature is supposed to work).
In AUTO mode the sweep will trigger when either of two things happens: 1. when
the trigger condition is met (and the trigger circuit is in the ARMED state),
or 2. when a timer (an RC circuit in the 7B92) finishes (and the trigger
circuit is in the ARMED state). This means that you get a short pause between
free-run sweeps if the trigger conditions are not met, so they will not
necessarily be happening at the specified sweep frequency (in other words, if
you set a sweep speed of 1 ms/div, so that the sweep takes something like 10
ms, the actual free-run rate may be something like 20 ms, or even slower
depending on the auto-sweep timing).
-- Jeff Dutky
|
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
|
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
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Show quoted text
----- 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
|
On Sun, Dec 12, 2021 at 04:33 PM, n4buq wrote: 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?
C535 is discharged even with auto-generated sweeps. Ozan
|
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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Show quoted text
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
|
Ah, okay. I didn't know that U520 did that without a trigger so Jeff's correct. That makes a difference and I don't think I saw that in the circuit description.
Thanks,
Barry - N4BUQ
toggle quoted message
Show quoted text
----- Original Message ----- From: "Ozan" <ozan_g@...>
To: "tekscopes" <[email protected]>
Sent: Sunday, December 12, 2021 7:47:02 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm Mode On Sun, Dec 12, 2021 at 04:33 PM, n4buq wrote:
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?
C535 is discharged even with auto-generated sweeps.
Ozan
|
Hi Barry,
Internal circuit diagram and block diagram of U520 is on TekWiki.
Ozan
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Show quoted text
On Sun, Dec 12, 2021 at 07:23 PM, n4buq wrote:
Ah, okay. I didn't know that U520 did that without a trigger so Jeff's
correct. That makes a difference and I don't think I saw that in the circuit
description.
Thanks,
Barry - N4BUQ
----- Original Message -----
From: "Ozan" <ozan_g@...>
To: "tekscopes" <[email protected]>
Sent: Sunday, December 12, 2021 7:47:02 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm Mode On Sun, Dec 12, 2021 at 04:33 PM, n4buq wrote:
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?
C535 is discharged even with auto-generated sweeps.
Ozan
|
Hi Barry,
Now that you mention it, you may be right, and I may have misunderstood: maybe the AUTO generated sweep does not drain the capacitor. That would result in a much faster AUTO sweep than the way I assumed it worked (and it would explain what I actually observe with a scope in AUTO trigger mode. I'm certainly not seeing a 10 Hz sweep rep rate).
I may also have misread your explanation. I apologize if I was curt or otherwise rude.
-- Jeff Dutky
|
That's what "triggered" my original question. I know that AUTO still triggers when a trigger is present but I wasn't understanding how triggering occurred when the time-base is basically in free-run when a trigger is absent. The manual seemed to indicate (at least the way I was understanding it) that C535 is discharged when an incoming trigger signal is present which implies that is stays charged without a trigger and, thus, "continuous" sweeps; however, Ozan points out that that's not the case and the IC resets that during auto-generated sweeps.
I think I'm good with this now.
Thanks guys,
Barry - N4BUQ
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Show quoted text
----- Original Message ----- From: "Tom Lee" <tomlee@...>
To: "tekscopes" <[email protected]>
Sent: Sunday, December 12, 2021 7:54:32 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm Mode 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
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
|
Jeff,
Nope - not curt or rude at all. I'm learning and appreciate any/all help with this.
Thanks,
Barry - N4BUQ
toggle quoted message
Show quoted text
----- Original Message ----- From: "Jeff Dutky" <jeff.dutky@...>
To: "tekscopes" <[email protected]>
Sent: Sunday, December 12, 2021 9:51:57 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm Mode Hi Barry,
Now that you mention it, you may be right, and I may have misunderstood: maybe
the AUTO generated sweep does not drain the capacitor. That would result in a
much faster AUTO sweep than the way I assumed it worked (and it would explain
what I actually observe with a scope in AUTO trigger mode. I'm certainly not
seeing a 10 Hz sweep rep rate).
I may also have misread your explanation. I apologize if I was curt or otherwise
rude.
-- Jeff Dutky
|
Hi Ozan,
I was tempted to ask for that but thought it wasn't available. That should answer several questions!
Thanks,
Barry - N4BUQ
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Show quoted text
----- Original Message ----- From: "Ozan" <ozan_g@...>
To: "tekscopes" <[email protected]>
Sent: Sunday, December 12, 2021 9:49:14 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm Mode Hi Barry,
Internal circuit diagram and block diagram of U520 is on TekWiki.
Ozan
On Sun, Dec 12, 2021 at 07:23 PM, n4buq wrote:
Ah, okay. I didn't know that U520 did that without a trigger so Jeff's
correct. That makes a difference and I don't think I saw that in the circuit
description.
Thanks,
Barry - N4BUQ
----- Original Message -----
From: "Ozan" <ozan_g@...>
To: "tekscopes" <[email protected]>
Sent: Sunday, December 12, 2021 7:47:02 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm Mode On Sun, Dec 12, 2021 at 04:33 PM, n4buq wrote:
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?
C535 is discharged even with auto-generated sweeps.
Ozan
|
On Sun, Dec 12, 2021 at 07:51 PM, Jeff Dutky wrote: Now that you mention it, you may be right, and I may have misunderstood: maybe
the AUTO generated sweep does not drain the capacitor. That would result in a
much faster AUTO sweep than the way I assumed it worked (and it would explain
what I actually observe with a scope in AUTO trigger mode. I'm certainly not
seeing a 10 Hz sweep rep rate). The cap is discharged. If the trip point at pin 6 is 5V it will take 0.4 time constants. That would make ~ 20Hz rate if it could fire back to back. Datasheet says trip point is 4.2V +/- 0.3V and the cap doesn't reset to zero volts (at least Vce,min); therefore, in reality it is probably even a little faster. As far as I can tell C is +/-20% and R is +/-5% so time constant can vary by about +/- 25% on top of this. Ozan
|
On Sat, Dec 11, 2021 at 08:00 PM, n4buq wrote: In AUTO mode, the time-base is in a "free-run" mode which generates a ramp at
the selected sweep rate and, at the end of the ramp, generates another ramp.
This continues whether a trigger is present or not. In AUTO mode, any sweep ramp is followed by some HOLDOFF time, during which the sweep circuit watches to see if it gets a triggerable signal from the input. If a triggerable signal arrives during the HOLDOFF time, a new ramp is generated. If no triggerable signal arrives during the HOLDOFF time, an auto-generated ramp is started. I'm not familiar with this plugin, but, in general, the HOLDOFF time is adjustable. If I'm reading the manual for this plugin correctly, the center knob of the sweep rate assembly can be set to control the HOLDOFF time if some internal jumper is set. In NORM, the time-base waits for a triggering signal before executing a sweep
and, at the end of the sweep, waits for another trigger signal before
generating another ramp. In general use, the difference between AUTO and NORM is that, with no input signal, AUTO gives you a flat line, while NORM gives you a blank screen.
|
Hi all.
I am trying to follow along this discussion, but not sure if I have the correct Block diagram and Schematic for U520.
Is it this one on Tek Wiki .... 155-0109-01 ... ???
If not, web address would be appreciated.
--
Cheers,
Ian,
Melbourne, Australia
|
Hi Barry and Jeff,
I had a little more time to look at the block diagram and I realized I swapped the Auto timing and Hold timing caps. Block diagram has the caps oriented different than the schematic and I was looking at the wrong cap.
In summary, Auto timing only looks at trigger input as you identified. C535 indeed would be charged up to ~ 5V if no trigger is coming. It doesn¡¯t go to +15V because the U520 is powered to +5V and ESD structures/parasitic diodes would prevent it going much above 5V.
Holdoff cap C533/C101/C103 is the one that is reset every sweep.
At the end of the sweep Holdoff timer is started. When holdoff timer expires sweep waits until either arrival of trigger signal or auto timer expiration, whichever is earlier.
No trigger case:
Auto cap charges up to its trip point if there is no trigger in ~ 40ms +/- 25% (assuming 4.2V trip point). It continues up to ~5V.
When hold-off timer expires if it sees Auto cap charged, a new sweep starts automatically. Still there is a gap of holdoff time between end of a sweep and start of a new one.
Trigger case:
To answer Barry¡¯s original question why a new sweep doesn¡¯t immediately start in AUTO mode when there is a valid trigger: As long as there is trigger happening auto timer is reset, so after holdoff time sweep still waits for earlier of trigger or auto timer expiration. Unless trigger is very slow trigger will come before auto timer expiration so sweep is synchronized to the trigger.
Length of holdoff time changes with A sweep knob (switched elements C101, C103, R105). Faster sweeps has shorter holdoff times.
Sorry for the mixup earlier.
Ozan
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On Sun, Dec 12, 2021 at 07:51 PM, Jeff Dutky wrote:
Now that you mention it, you may be right, and I may have misunderstood: maybe
the AUTO generated sweep does not drain the capacitor. That would result in a
much faster AUTO sweep than the way I assumed it worked (and it would explain
what I actually observe with a scope in AUTO trigger mode. I'm certainly not
seeing a 10 Hz sweep rep rate).
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