Author Topic: Help on an old "CTR Mod. SG-25" Signal Generator  (Read 695 times)

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Offline MaisterTopic starter

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Help on an old "CTR Mod. SG-25" Signal Generator
« on: March 17, 2023, 11:40:14 am »
Hi,


I purchased an old used Signal generator from eBay. It is the "CTR Mod. SG-25".
Frequency Range is 120kHz to 500MHz. Selectable in 5 Bands.

I attached the "manual" together with its schematic to this thread. (soory its only in german, but schematic everyone should be able to understand ;) )

For some reason I thought I would get a clean sine wave out of it but the signal coming out, I measured with an analog discovery (USB scope), looks like the negative half is cut off.
This can also be seen on the attached picture.

The device only features 4mm banana jacks and no BNC. So for probing I attached 2 twisted leads from 4mm banana to pin header of analog discovery. Analog discovery is high Z input btw. not 50 ohms.

The measured frequency is around 700kHz so I don't think this bad result is due to the bad probing setup.

Does anyone know if this output waveform is normal for these old kind of signal gens, or might there be something broken with the unit?

Also the amplitude varies when frequency is changed, which is annoying. Is that also something one could expect from those old units?


Thanks for any helping comments and thoughts about this.

Cheers,
Patrick
« Last Edit: March 17, 2023, 11:42:07 am by Maister »
Electronics design engineer, living in Germany.
 

Offline Specmaster

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Re: Help on an old "CTR Mod. SG-25" Signal Generator
« Reply #1 on: March 17, 2023, 12:57:05 pm »
I used to have one of those with another name, it was sold with lots of people badging it, but it is a reasonable generator for AM radios. It does however need to have capacitor in series with the output leads, this will help to improve the waveform by blocking any DC leakage.
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Offline MaisterTopic starter

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Re: Help on an old "CTR Mod. SG-25" Signal Generator
« Reply #2 on: March 17, 2023, 02:17:11 pm »
yes, you can see the DC on the attached waveform.

However, a series cap will not remove the clipping.
Electronics design engineer, living in Germany.
 


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