Topics

7A29 Weird Behavior


Stephen
 

Most of the time my scopes are DC coupled. But this time I needed this scope to be AC coupled.
This is when I realized that the signal coming out of a function generator was twice less the amplitude when this pream is AC coupled than when it’s DC coupled...
The correct amplitude is shown when DC coupled I checked It against other scopes and/or a 7A26).
But for some reason when AC coupled it’s is I switched to the next voltage up on the dial.
I’m not very experienced with this particular plugin. Is this normal, or is there something wrong with it?
The voltage was around 5V p-p when I noticed that.

Thank you


@0culus
 

Stephen,

I tried on mine and I see no amplitude difference between AC and DC coupling on a 50 kHz, 5 Vpp signal from a Type 191 constant-amplitude signal generator.

Sean

On Wed, Aug 26, 2020 at 10:58 AM, Stephen wrote:


Most of the time my scopes are DC coupled. But this time I needed this scope
to be AC coupled.
This is when I realized that the signal coming out of a function generator was
twice less the amplitude when this pream is AC coupled than when it’s DC
coupled...
The correct amplitude is shown when DC coupled I checked It against other
scopes and/or a 7A26).
But for some reason when AC coupled it’s is I switched to the next voltage
up on the dial.
I’m not very experienced with this particular plugin. Is this normal, or is
there something wrong with it?
The voltage was around 5V p-p when I noticed that.

Thank you


 

On Wed, Aug 26, 2020 at 07:58 PM, Stephen wrote:


Most of the time my scopes are DC coupled. But this time I needed this scope
to be AC coupled.
The explanation is simple:
On DC, the 7A29 presents a 50 Ohm input *resistance*. On AC, it does not. Your generator "sees" that.
You can measure by switching power on and checking with an Ohm meter.

Raymond


Stephen
 

On Wed, Aug 26, 2020 at 07:22 AM, @0culus wrote:


Stephen,

I tried on mine and I see no amplitude difference between AC and DC coupling
on a 50 kHz, 5 Vpp signal from a Type 191 constant-amplitude signal generator.


Sean
Actually someone pointed out to me (in a private message), that the 7A29 attenuates the amplitude at a low frequency signal when AC coupled. And this was actually the case. My signal was a sine wave at 500Hz.
All becomes normal again as I increase the frequency into KHz territory.
I was afraid for a moment. I guess all is good.

Thanks all.


Stephen
 

On Wed, Aug 26, 2020 at 07:30 AM, Raymond Domp Frank wrote:

The explanation is simple:
On DC, the 7A29 presents a 50 Ohm input *resistance*. On AC, it does not. Your
generator "sees" that.
You can measure by switching power on and checking with an Ohm meter.

Raymond
Yes, I understand. I should’ve checked the manual.
It was an “on the spur of the moment” panic post.


@0culus
 

No problem! That does make sense. Honestly, this scenario is why differential comparators are extremely useful to have (such as the 7A13 for 7000 scopes or Type Z plug-in for 500 series). If you have a P6201 active probe it also has a DC offset function (though it's not as flexible as the 7A13). I have a pair of these probes that I use with my 7104 when I need to probe.

Sean

On Wed, Aug 26, 2020 at 11:33 AM, Stephen wrote:


On Wed, Aug 26, 2020 at 07:22 AM, @0culus wrote:



Stephen,

I tried on mine and I see no amplitude difference between AC and DC
coupling
on a 50 kHz, 5 Vpp signal from a Type 191 constant-amplitude signal
generator.


Sean
Actually someone pointed out to me (in a private message), that the 7A29
attenuates the amplitude at a low frequency signal when AC coupled. And this
was actually the case. My signal was a sine wave at 500Hz.
All becomes normal again as I increase the frequency into KHz territory.
I was afraid for a moment. I guess all is good.

Thanks all.


 

On Wed, Aug 26, 2020 at 08:33 PM, Stephen wrote:


Actually someone pointed out to me (in a private message), that the 7A29
attenuates the amplitude at a low frequency signal when AC coupled.
That is natural behavior, because a low frequency is damped more than a high frequency; the capacitor for AC, which is put in series, has an impedance of 1/(2 x pi x freq). It's usually specified as a low-freq. -3dB point.

I thought you wrote that you saw a *lower* amplitude with DC than with AC. As I wrote, some pulse/function generators reduce output level when they see a 50 Ohm resistive load. I have a Philips instrument that does that. That's separate from the output voltage reduction to 50% because of their (50 Ohm) output impedance.

Raymond


 

On Wed, Aug 26, 2020 at 08:40 PM, @0culus wrote:


No problem! That does make sense. Honestly, this scenario is why differential
comparators are extremely useful to have (such as the 7A13 for 7000 scopes or
Type Z plug-in for 500 series). If you have a P6201 active probe it also has a
DC offset function (though it's not as flexible as the 7A13). I have a pair of
these probes that I use with my 7104 when I need to probe.
I don't understand what you're saying. What problem or behavior as discussed here is solved by compensating offset?

Raymond


Stephen
 

Interleaved.

On Wed, Aug 26, 2020 at 07:49 AM, Raymond Domp Frank wrote:

That is natural behavior, because a low frequency is damped more than a high
frequency; the capacitor for AC, which is put in series, has an impedance of
1/(2 x pi x freq). It's usually specified as a low-freq. -3dB point.
I understand. This is the cap that blocks the DC.


I thought you wrote that you saw a *lower* amplitude with DC than with AC.
Nope, the opposite.

As I wrote, some pulse/function generators reduce output level when they see a 50
Ohm resistive load. I have a Philips instrument that does that.
That's separate from the output voltage reduction to 50% because of their (50 Ohm)
output impedance.
I see...


 

On Wed, Aug 26, 2020 at 08:49 PM, Raymond Domp Frank wrote:


That is natural behavior, because a low frequency is damped more than a high
frequency; the capacitor for AC, which is put in series, has an impedance of
1/(2 x pi x freq). It's usually specified as a low-freq. -3dB point.
The Service Manual of the 7A29 suggests a -3dB point of 1 kHz from a 50 Ohm source when AC-coupled. Since the 7A29 is meant specifically for high frequency use, the -3dB low frequency cutoff with AC coupling generally isn't too interesting.

Raymond


 

On Wed, Aug 26, 2020 at 08:53 PM, Raymond Domp Frank wrote:


I don't understand what you're saying. What problem or behavior as discussed
here is solved by compensating offset?
I think I see now what you were getting at. But in a 50 Ohm system, a 7A13 or a P6201 create an impedance mismatch. The 7A13 cannot be used at the high frequencies that a 7A29 generally is used at and the max. allowable offset without attenuator for a P6201 is limited to less than 6V. With attenuator, it is far less sensitive. The 7A29 may be used at full sensitivity and several tens of offset voltage Volts *if* the input is precharged before connecting. At lower frequencies and especially in HiZ circuits the 7A13 is a godsend.

Raymond


@0culus
 

I agree with you, and that is what I was getting at. The OP was talking about using his 7A29, AC coupled, at *very* low frequencies anyway, which to mean implies that he might be able to better use a differential comparator if he is dealing with a large DC offset. I would think the 7A13 (or a P6201 with a coaxial adapter) would work just fine with a good quality 50 ohm feed-through termination so the transmission line is properly terminated before the input. Not ideal for higher frequencies (in fact, the P6201 manual recommends using the scope's internal termination when possible, iirc), but the 7A13, as you point out, wouldn't be used where that would be a problem.

I've noted that using feed-through terminations is a common trope in Tek manuals for calibrating Hi-Z-only plugins using tools like the 184 time-mark generator and 191 constant amplitude signal generator that expect to be terminated in 50 ohms.

Sean

On Wed, Aug 26, 2020 at 12:24 PM, Raymond Domp Frank wrote:


On Wed, Aug 26, 2020 at 08:53 PM, Raymond Domp Frank wrote:



I don't understand what you're saying. What problem or behavior as
discussed
here is solved by compensating offset?
I think I see now what you were getting at. But in a 50 Ohm system, a 7A13 or
a P6201 create an impedance mismatch. The 7A13 cannot be used at the high
frequencies that a 7A29 generally is used at and the max. allowable offset
without attenuator for a P6201 is limited to less than 6V. With attenuator, it
is far less sensitive. The 7A29 may be used at full sensitivity and several
tens of offset voltage Volts *if* the input is precharged before connecting.
At lower frequencies and especially in HiZ circuits the 7A13 is a godsend.

Raymond


 

On Thu, Aug 27, 2020 at 02:38 AM, @0culus wrote:

Hi Sean,


I agree with you, and that is what I was getting at. The OP was talking about
using his 7A29, AC coupled, at *very* low frequencies anyway, which to mean
implies that he might be able to better use a differential comparator if he is
dealing with a large DC offset. I would think the 7A13 (or a P6201 with a
coaxial adapter) would work just fine with a good quality 50 ohm feed-through
termination so the transmission line is properly terminated before the input.
I didn't realize that the OP was looking at a 500 Hz signal. The use of a 7A29 suggests use at a much higher frequency. In his situation, it seems that an ordinary 7A26 would have been a better choice (-3dB below 20 Hz with AC coupling).

A P6230 (P6231) is useful at low impedances where a limited variable DC offset suffices.

Not ideal for higher frequencies (in fact, the P6201 manual recommends using
the scope's internal termination when possible, iirc), but the 7A13, as you
point out, wouldn't be used where that would be a problem.

I've noted that using feed-through terminations is a common trope in Tek
manuals for calibrating Hi-Z-only plugins using tools like the 184 time-mark
generator and 191 constant amplitude signal generator that expect to be
terminated in 50 ohms.
Somewhere in the vertical calibration of the 465 portable 'scope, the "high-voltage" output of a PG506 is fed via feed-through attenuators to reduce the signal to a nicely-shaped square wave of less than 1V @ 1 kHz.

Raymond


Stephen
 

On Wed, Aug 26, 2020 at 02:34 PM, Raymond Domp Frank wrote:

I didn't realize that the OP was looking at a 500 Hz signal. The use of a 7A29
suggests use at a much higher frequency. In his situation, it seems that an
ordinary 7A26 would have been a better choice (-3dB below 20 Hz with AC
coupling).
I agree with you Raymond, and with all of you, but I’m gonna try to be more specific. When I turned on this scope, and this particular function gen, it was incidentally set to 500Hz; which I hadn’t realized or even checked when I turned it on. I wasn’t particularly trying to look at a 500Hz signal. It just happened to be that way. And this is when I realized that AC coupling the scope (which is something I needed to do), reduced the amplitude by quite a bit, and I freaked out for a min or two. 🤷‍♂️🤷‍♂️🤷‍♂️