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Electronics => Beginners => Topic started by: Andy-In_over_my_head on December 30, 2016, 04:50:35 pm

Title: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: Andy-In_over_my_head on December 30, 2016, 04:50:35 pm
I have this Oscilloscope with Function Wave Gen included. I was provided 2 probes. I have 2 channels, Gen out & of course the Triggers.
I don't necessarily plan on Ghetto rigging something together because I can order more probes and wait.
However, is it possible to Mod something so you could use both channels with Probes provided and still utilize the Wave Generator if necessary?
Or is it just not worth damaging the oscilloscope?
Thanks in advance for input.
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: tggzzz on December 30, 2016, 05:02:30 pm
It would help if you would indicate what scope, what probes, and what function generator (inc frequency and output impedance).

I would expect that scope probes could not be used "backwards" to connect the function generator output to a circuit. Exception: if they are merely bits of wire with a BNC on one end and croc clips on the other.

FFI on probes and how to use a scope safely (for the scope, the UUT, and you), have a look at the references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: Andy-In_over_my_head on December 31, 2016, 03:10:12 am
It would help if you would indicate what scope, what probes, and what function generator (inc frequency and output impedance).
Hantek DSO4102C - 100MHz/1GSa/s
Arbitrary/Function Waveform Generator - 25MHz/200MSa/s

I didn't know if the scope was necessary. I figured you either could or couldn't mod something.
But it's helpful knowing that the probes don't work as output.. I feel like an asshat.
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: tggzzz on December 31, 2016, 08:26:50 am
I feel like an asshat.

No need. You asked a reasonable question and have listened to and thought about the answer.

"Let's make new mistakes" - as I taught my daughter.
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: Andy-In_over_my_head on January 13, 2017, 06:52:03 am
so what do I use for the gen. output on the oscilloscope?
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: JPortici on January 13, 2017, 07:05:22 am
a coax cable / a coax cable with clips at the other end / a coax to banana adapter + 2 banana jacks / a coax to 3.5mm adapter / ... depending on what you have to do
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: tautech on January 13, 2017, 07:35:22 am
so what do I use for the gen. output on the oscilloscope?
You can just use it like a separate Sig gen or you might use it to inject a signal into a project under development or into a device for troubleshooting by following the signal path with the scope to find the faulty part of the circuit.

In short; standard Sig gen use.  ;)
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: Cupcakus on January 13, 2017, 08:25:04 am
If it's a generic 1x scope probe I don't see why you couldn't use it for function generator output unless I'm missing something?
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: tggzzz on January 13, 2017, 09:01:47 am
If it's a generic 1x scope probe I don't see why you couldn't use it for function generator output unless I'm missing something?

You are missing something: in *1/*10 probes the internal wire has a resistance of ~200ohms.

"High" impedance probes aren't simply bits of wire, nor are they bits of coax: they are lossy transmission lines to damp down internal reflections.

Neither are they high impedance , except at audio frequencies. At 100MHz the input impedance of a typical *10 probe is <100ohms, due to the 15pF tip capacitance.
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: macboy on January 13, 2017, 04:11:35 pm
The signal generator has a 50 Ohm output with BNC connector. Ideally, you connect this with a BNC-to-BNC cable to another piece of equipment that has a 50 ohm terminated input. Proper cabling and termination are only actually important for higher frequency (>MHz for sine waves, or any square wave or pulse output). As mentioned by others, if you just need to inject a signal into something, use a BNC-to-whatever cable (BNC-to-croc clips is handy), but know that this works well only at sub-MHz frequencies. Keep in mind that if not terminated into 50 ohms, the output voltage may be 2x as high as expected. This is because the 50 ohm input impedance effectively forms a voltage divider with the 50 ohm output impedance of the signal generator. If the impedance is higher, like the 10 to 100 kohm input impedance of an audio amplifier, then the voltage will be higher.  Many signal generators have an output impedance switch to select 50 ohm or high-Z - this doesn't change the output impedance of the signal generator (it's always 50 ohms) but adjusts the output voltage to compensate for the expected loading so that you get the waveform amplitude that you have selected.
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: Cupcakus on January 14, 2017, 11:46:10 pm
If it's a generic 1x scope probe I don't see why you couldn't use it for function generator output unless I'm missing something?

You are missing something: in *1/*10 probes the internal wire has a resistance of ~200ohms.

I honestly didn't know that, I thought in 1x mode probes were basically just a shielded coax, but I tested it myself and sure enough my probes are ~300Ohm and my function generator cables are much lower, 1-2Ohm.  Probably should have known that.
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: tggzzz on January 15, 2017, 12:50:16 am
If it's a generic 1x scope probe I don't see why you couldn't use it for function generator output unless I'm missing something?

You are missing something: in *1/*10 probes the internal wire has a resistance of ~200ohms.

I honestly didn't know that, I thought in 1x mode probes were basically just a shielded coax, but I tested it myself and sure enough my probes are ~300Ohm and my function generator cables are much lower, 1-2Ohm.  Probably should have known that.

Probes and probing technique are vital topics. FFI, see https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: djnz on January 15, 2017, 03:49:17 am
You should be careful about matching impedances. For example, when the exact same signal is fed into different loads we see very different results. 



Signal generator (which expects 50 ohm load) + 1m of 50ohm coax + 50 ohm termination (actual signal):

(http://i.imgur.com/J3ILOO4.png)


Signal generator (which expects 50 ohm load) + 1m of 50ohm coax + 75 ohm termination (some reflections happening):

(http://i.imgur.com/IfNVHQS.png)


Signal generator (which expects 50 ohm load) + 1m of 50ohm coax + 1M ohm termination (lots of reflections, crazy stuff  happening):

(http://i.imgur.com/HFXeIoL.png)
Title: Re: Oscilloscope/Function Gen. Combo (Can you mod a Gen. output Probe?)
Post by: tautech on January 15, 2017, 07:39:05 am
You should be careful about matching impedances. For example, when the exact same signal is fed into different loads we see very different results. 
Exactly.

Nice screenshots.   :-+

As seen in graphic detail in this thread too:
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/ (https://www.eevblog.com/forum/testgear/show-us-your-square-wave/)