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Old 05-15-2022, 10:46 AM   #1
Crystaleyes
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Default Calibration 50 Ohm question

So the oscilloscope needs calibrating, which needs a function generator...

The function generator also needs calibrating, and the manual is not in the clearest English.

First line in the calibration section of the manual is, "1. Push function switch to OFF position, load 50 ohms".

Is this '50 ohms' a dummy load, or is that the impedance of the cable which connects to a scope for example?


The generator is a Minipa MFG-4201 which is also known as an Escort EGC-3230 and a Kenwood FG-273

Including the first paragraph as an attachment.
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File Type: png Minipa 4201 Calib_1.png (196.9 KB, 16 views)

Last edited by Crystaleyes; 05-15-2022 at 10:50 AM..
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Old 05-15-2022, 03:08 PM   #2
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Default Re: Calibration 50 Ohm question

Quote:
Originally Posted by Crystaleyes View Post
So the oscilloscope needs calibrating, which needs a function generator...

The function generator also needs calibrating, and the manual is not in the clearest English.

First line in the calibration section of the manual is, "1. Push function switch to OFF position, load 50 ohms".

Is this '50 ohms' a dummy load, or is that the impedance of the cable which connects to a scope for example?


The generator is a Minipa MFG-4201 which is also known as an Escort EGC-3230 and a Kenwood FG-273


Including the first paragraph as an attachment.

Do you have a good image of the front of the function generator?
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Old 05-15-2022, 06:02 PM   #3
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Default Re: Calibration 50 Ohm question

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Originally Posted by keeney123 View Post
Do you have a good image of the front of the function generator?
Not the best light but all the details should be there.
Attached Images
File Type: jpg IMGA0413.JPG (134.2 KB, 11 views)
File Type: jpg IMGA0409.JPG (126.0 KB, 10 views)
File Type: jpg IMGA0410.JPG (117.7 KB, 9 views)
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File Type: jpg IMGA0412.JPG (117.3 KB, 9 views)
File Type: jpg IMGA0414.JPG (114.0 KB, 8 views)
File Type: jpg IMGA0415.JPG (118.9 KB, 8 views)
File Type: jpg IMGA0416.JPG (122.7 KB, 7 views)
File Type: jpg IMGA0417.JPG (122.1 KB, 4 views)
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Old 05-15-2022, 09:52 PM   #4
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Default Re: Calibration 50 Ohm question

Quote:
Originally Posted by Crystaleyes View Post
Not the best light but all the details should be there.
Reading the title of the calibration procedure. It would seem to me that this is an internal test of the sweep generator.
Next thing I see it is telling you to turn off the output of the 50 Ohm. The function switch I would think is the internal external switch. However, I think he is talking about the Main output switch.
One thing you can do is know how the turn knobs work. Do they pull out and turn. Do they just click on. You would have to do this to start to understand how this procedure applies to this device. You should turn the device on with nothing connected to the output and observe what each switch does.
In the next procedure to turn off the range switch. To me this is a frequency generator. I wonder does the Frequency knob pull out and push in?
What I would say is to always remember that this is an internal test so anything associated with output you will not be concern with other than turning it off. However, further reads shows that one has to take measurements at the output while adjusting a resistor.
You have input Voltage control filter that cuts off frequency by a filter. In radio one will have high band pass and low band pass frequency and also, a bell shaped band pass this is accomplished by filter made with inductors and capacitors. The other input is a frequency counter input. Your DC offset is the adjust offset knob.
I agree it is confusing but it can be figured out. My thought process is most likely also confusing.
It would also seem that the resistor to adjust the output voltage on the BNC connectors must be internal or on the back of the device. The adjusting R88 to get -5mv at the main output would have to be measured with a calibrated scope.
With all this confusion I have brought you I hope some of it will make sense.
Here is the thing about calibration. I was a calibration technician for only 6 months. What I learned is that any calibration has to be traceable. In the US it is traceable to the National Bureau of Standards. It may be called something different now. The Bureau of Standards has all the devices such as weight and all type of measuring devices all locked up. They will only take these things out once a year to see if they have changed and adjust them appropriately. All calibrations in the US have to be traceable to them. If the calibrations are not traceable then they are no good and have no meaning.

Last edited by keeney123; 05-15-2022 at 11:22 PM..
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Old 05-16-2022, 05:21 AM   #5
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Default Re: Calibration 50 Ohm question

Quote:
Originally Posted by keeney123 View Post
Reading the title of the calibration procedure. It would seem to me that this is an internal test of the sweep generator.
Ok. That already is helpful.

Quote:
The function switch I would think is the internal external switch. However, I think he is talking about the Main output switch.
Reading further through the instructions, it turns out that the FUNCTION switch is the waveform selector.


Quote:
One thing you can do is know how the turn knobs work. Do they pull out and turn. Do they just click on. You would have to do this to start to understand how this procedure applies to this device. You should turn the device on with nothing connected to the output and observe what each switch does.
Apart from the FREQUENCY and AMPLITUDE buttons, the rest of the knobs are dual purpose and have a separate field of control when pulled out.

Quote:
I agree it is confusing but it can be figured out.
Agreed


Quote:
My thought process is most likely also confusing.
You said it... LOL


Basically, I bought this super cheap as 'not working' and had to resolve a missing power selector socket and five missing transistors, but it seems to be fairly close to giving out clear waveforms.
It obviously doesn't help that I have no calibrated scope here, however, that said, it did give nice waveforms on a scope yesterday.

What I had wanted to know, was whether the 50 ohm 'load', was a cable or termination.
Looking further online, it appears most likely that it is a termination. Couldn't find one in the shops so shall maked one out of eight 100Ω carbon-film resistors in a series/parallel arrangement.
Attached Images
File Type: jpg IMGA0404.JPG (125.9 KB, 9 views)
File Type: jpg IMGA0405.JPG (123.0 KB, 10 views)
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Old 05-16-2022, 12:28 PM   #6
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Default Re: Calibration 50 Ohm question

Quote:
Originally Posted by Crystaleyes View Post
Ok. That already is helpful.



Reading further through the instructions, it turns out that the FUNCTION switch is the waveform selector.




Apart from the FREQUENCY and AMPLITUDE buttons, the rest of the knobs are dual purpose and have a separate field of control when pulled out.



Agreed




You said it... LOL


Basically, I bought this super cheap as 'not working' and had to resolve a missing power selector socket and five missing transistors, but it seems to be fairly close to giving out clear waveforms.
It obviously doesn't help that I have no calibrated scope here, however, that said, it did give nice waveforms on a scope yesterday.

What I had wanted to know, was whether the 50 ohm 'load', was a cable or termination.
Looking further online, it appears most likely that it is a termination. Couldn't find one in the shops so shall maked one out of eight 100Ω carbon-film resistors in a series/parallel arrangement.
If you look at the meter it tells you the BNC connecter that has a 50 ohm load output. I believe this is what he is talking about. I do not think he is asking for a terminator or a cable of 50 ohms. He is referencing what to turn off to calibrate the device for the triangle wave. I believe he is setting the minimum voltage on the 50 ohm main output to -5mV. This is kind of zeroing the output before he goes into the triangle wave adjustment.

Last edited by keeney123; 05-16-2022 at 12:37 PM..
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Old 05-16-2022, 06:24 AM   #7
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Default Re: Calibration 50 Ohm question

50ohm BNC passthrough - very easy to find

btw, imo calibration isnt important - you will just be looking at the waveform if it doesnt have "readout" capability

the voltage selector issue is common with old scopes - so is some scopes using custom connectors for the mains lead.
on one of mine i removed the voltage selector and telequipment/tectronix cable inlet and fitted a plate with a standard 3pin inlet on it.
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Old 05-16-2022, 12:22 PM   #8
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Default Re: Calibration 50 Ohm question

Quote:
Originally Posted by stj View Post
50ohm BNC passthrough - very easy to find

btw, imo calibration isnt important - you will just be looking at the waveform if it doesnt have "readout" capability

the voltage selector issue is common with old scopes - so is some scopes using custom connectors for the mains lead.
on one of mine i removed the voltage selector and telequipment/tectronix cable inlet and fitted a plate with a standard 3pin inlet on it.
So would you help crystaleyes with this so he can have a more direct understanding?
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Old 05-16-2022, 12:46 PM   #9
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Default Re: Calibration 50 Ohm question

if the generator has a 50ohm output inpedence then we are good.
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Old 05-16-2022, 03:23 PM   #10
Crystaleyes
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Default Re: Calibration 50 Ohm question

Hi again fella's.

I na nutshell, I have almost no experience in calibration, but would like to think I'm capable of following the instructions in the manuals (assuming having the required test items).
As for the 50 ohm question, I'm still in Brazil and none of the local electronics shops have 50 ohm BNC's, passtthrough or termination, but never mind, if it is not so necessary.

As you can see in the photo's, the function generator is working to a fair degree. I wanted to calibrate that, mainly to properly calibrate the Kenwood scopes which are now repaired, in order to sell them and make space for the next project or two.

But thank you
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