7A16A high frequency compensation.


unclebanjoman
 

Hi all,

I've recently purchased a 7A16A, working, looks new, S/N B138015.

Probably the previous owner has tried to calibrate the HF response in a badly manner: risetime was 3 ns with very rounded front corners.
Suspecting a strong misadjustment of HF compensation, I did a full calibration procedure.

Problem: I'm unable to obtain a decent compromise between risetime and flat-top and square corner.

I'm using a PG506, a Type 111 and 7854 mainframe. 7A16A in the left compartment, no extender.

Following the procedure as per manual, I spent 4 hours trying to calibrate the best, but the various settings interacts with each other and I'm unable to obtain a decent rising edge.
Sometimes the rising edge is distorted in the middle of the front. Other time there is an auto-oscillation. Touch one trim and the waveform gets better. I touch another one and the waveform gets worse.
I heard that the calibration procedure was tricky, but I have never encountered a difficulty like that.
All resistive and capacitive trimmers are O.K, perfectly functional.

Does anyone know a better procedure than the (fairly simplistic) one outlined in the manual?
Moreover, in the manual there is not even a photograph of some typical waveform that can be used as a reference. Does anyone know where to find a screenshot of a typical 7A16A step response?

Cheers,
Max


Albert Otten
 

Hi Max,

My 7A16 (not A) manual shows a picture. The 7A16A performance is the same or better I guess.
I did the same setup with PG506, 7854 and "as is" 7A16A, Nice smooth step with rise time 1.55 ns, followed by just one overshoot 6.5% and one undershoot 3.5%. After this negligible aberrations from flat. With a 7A19 rise time was 1.1 ns.

Albert

On Sat, Mar 20, 2021 at 01:01 AM, unclebanjoman wrote:

Moreover, in the manual there is not even a photograph of some typical
waveform that can be used as a reference. Does anyone know where to find a
screenshot of a typical 7A16A step response?


unclebanjoman
 

Hi Albert,
thank you for the response.
After a further two hours, and comparing with the more detailed explanations given in the 7A16P manual, I was finally able to calibrate the step response in a decent manner.
I could not appreciate any variations by turning the last trim, C770. When I got the plugin the first time, it was almost completely unscrewed. I screwed in some turns to prevent the screw from falling out!

I posted some screenshots showing the final result.
Photos are in https://groups.io/g/TekScopes/album?id=262075
Album title:"A16A step response (with 7854)

What do you think about? All comments are welcome.

Furthermore, I noticed that my PCB is very recent: it shows a S/N 670-2323-08 (see photo). There's a trimmer that does not appear in the schematic.
I've marked in the photo with a red arrow. Green arrow points to the LF compensation trimmer R436 which is present in the schematic. The other one is marked LF (on the PCB) but is not present in the schematic. I presume was an additional trim point for the same purpose of the green one, so I adjusted it in the same manner as R436.

Max


Albert Otten
 

Hi Max,

How much nicer do you want it? :-).
I wonder whether the slight aberration you see after 10 ns is due to reflection in a 5 ns cable.

Albert


unclebanjoman
 

Hi Albert,
thanks!
Yeah, it's that aberration "up" in the 10 ns point I can not eliminate. I didn't think it was the cable. I generally observe the "dribble up" phenomenon. Do you think is the cable ?
I also have a 7A16 (not A) but I don't think it has that reflection. Maybe because of the smaller bandwidth?

Anyway, now the response of my 7A16A is much better than before. Initially was horrible. I think the previous owner tried to calibrate it but then gave up.
Really a tour de force: more than 4 hours fiddling the pots!

What do you think about the trim pot not shown in the schematics? May it be the very latest revision? I did not find more recent schematics.

Max


Albert Otten
 

Yeah, it's that aberration "up" in the 10 ns point I can not eliminate. I
didn't think it was the cable. I generally observe the "dribble up"
phenomenon. Do you think is the cable ?
I also have a 7A16 (not A) but I don't think it has that reflection. Maybe
because of the smaller bandwidth?
The 7A16A input causes reflection anyway. When the cable impedance doesn't match the PG506
output impedance then the reflection from there can be observed at the CRT after twice the cable travel time.
I wouldn't bother too much about not perfectly flat, in my opinion the step response is very nice.
(How did you calibrate the PG506?)

What do you think about the trim pot not shown in the schematics? May it be
the very latest revision? I did not find more recent schematics.
That must by R436 in the LF filter leg between pins 2 and 3 of U450. Has always been there according to parts list.
Is present in my 1973 unit.

Albert


unclebanjoman
 

Hi Albert,

to check my PG506 I used 3S1/3T77A pair, connected with a 20 cm GR874 airline. Measured a risetime of 750 ps and aberrations as per service manual. So I did prefer touch nothing. :-)

R436 is the one pointed by the green arrow in my photo and it's O.K.
The unknown R is the one pointed to by the red arrow. See https://groups.io/g/TekScopes/photo/262075/3

Max


Albert Otten
 

Ah yes, sorry Max, you mentioned both arrows some posts back.
I mentioned calibration of the PG506 since when calibrating the 7A16A you also try to compensate for pulse shape of the PG506.

Albert


unclebanjoman
 

Hi Albert,

you made me doubt about the PG506 now :-)
I would not like to have calibrated the 7A16A trying to compensate for the aberrations of the PG506 itself.
In the weekend I'll check the fast rise output of my PG506 using the 3S11/3T77A and also with a 2 GHz Philips PM3340 that I received a month ago (fully functional, like new).
I'll post some photos as soon as possible.

Max


Albert Otten
 

Hi Max,

It's nasty that the PG506 trigger output is not usable as pretrigger. Also (e.g. in my PG506) there is too much between-trigger jitter to view the fast edge of the next pulse. That's why a 7M11 delay line is used in the calibration section. But the 7M11 is not ideal. Hence the PG506 signal output waveform is compared with a faster TD pulser waveform. The PM3340 only does sequential sampling and requires signal delay..
BTW I found that the fast-rise rise time depends a lot on the amplitude, something like 0.75 ns for low to 1 ns for max amplitude.

Albert

On Tue, Mar 23, 2021 at 10:23 PM, unclebanjoman wrote:


Hi Albert,

you made me doubt about the PG506 now :-)
I would not like to have calibrated the 7A16A trying to compensate for the
aberrations of the PG506 itself.
In the weekend I'll check the fast rise output of my PG506 using the
3S11/3T77A and also with a 2 GHz Philips PM3340 that I received a month ago
(fully functional, like new).
I'll post some photos as soon as possible.

Max


 

On Tue, Mar 23, 2021 at 11:44 PM, Albert Otten wrote:


The PM3340 only does sequential sampling and requires signal delay..
My PM3340 has a signal delay line... Trigger pickoff right at the N-socket and a nice blue Suhner coax cable after that for each channel.
Nice but very heavy beast.

Raymond


Albert Otten
 

Thank you Raymond. I just looked briefly at internet too see whether or not it could do random sampling.
Charles can forget my inappropriate warning.
Albert


unclebanjoman
 

Hi all,
I've just uploaded two screenshots:

https://groups.io/g/TekScopes/photo/262075/4
https://groups.io/g/TekScopes/photo/262075/5

It's my PG506's (+) fast rise output directly coupled to a PM3340 2GHz Philips oscilloscope. High quality, 80 cm RG223/U with Uber-Suhner connectors (BNC to N) cable was used. 10x attenuator was used to keep the PG506's output level over 100 mV.
A nice 632 ps rise time with no aberrations. This implies that the calibration of my 7A16A is practically perfect, as Albert also said

Cheers,
Max