Author Topic: Basic usage of a Function generator  (Read 1682 times)

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Offline sämäx

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Basic usage of a Function generator
« on: October 09, 2016, 09:42:29 am »
Hi I bought a function gen and now I want to check couple of things so I won't break it.

1 What kind of short circuit protection does a basic function gen have a? Example what happens if I have a capacitor between my voltage rail and ground.  My theory is that when I increase the frequency the reactance of the capacitor will go down and act like a short circuit. Will this break my function gen?

2 How do you normally fit the impedance of the circuit to match the 50 ohms? In my understanding the impedance will change according to the frequency that’s fed in to the circuit.

3 Should I keep something else important in my mid while I am experimenting with my new function gen so I won’t blow it up?
« Last Edit: October 09, 2016, 10:12:37 am by sämäx »
 

Offline danadak

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Re: Basic usage of a Function generator
« Reply #1 on: October 09, 2016, 10:16:52 am »
1 What kind of short circuit protection does a basic function gen have a? Example what happens if I have a capacitor between my voltage rail and ground.  My theory is that when I increase the frequency the reluctance of the capacitor will go down and act like a short circuit. Will this break my function gen?

To answer this you have to look at generator manual. Most lab grade generators have protection.
As far as C goes depends on the C. Most C's have a rising Z characteristic past a certain point
due to internal parasitic L.

2 How do you normally fit the impedance of the circuit to match the 50 ohms? In my understanding the impedance will change according to the frequency that’s fed in to the circuit.

The generator sources 50 ohms via circuit design, same is true for load.
There are many methods to Z match load to source, http://electronicdesign.com/communications/back-basics-impedance-matching-part-1

If you are working with LF types of applications, audio for example, Z matching not considered
an issue if load is high Z. That will impact generator Vout.

3 Should I keep something else important in my mid while I am experimenting with my new function gen so I won’t blow it up?

Have on hand attenuator pads and start with a 20 db pad when applying fgen to circuit,
that will protect you in almost all applications where voltages and power levels are small
signal based. fgens and scopes on 50 ohms get toasted often because power is exceeded.
If you have a load that can source power use a pad. Note the manual usually discusses
operator safety for the fgen, methods and approaches.

http://cp.literature.agilent.com/litweb/pdf/5989-8790EN.pdf


Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Offline rstofer

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Re: Basic usage of a Function generator
« Reply #2 on: October 09, 2016, 04:29:07 pm »
Interesting and timely subject.  I'm just about to buy a Rigol DS4160 waveform generator.  So, I read the manual...

Quote

Short-circuit protection, automatically disable waveform output when overload occurs

 

Offline evb149

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Re: Basic usage of a Function generator
« Reply #3 on: October 09, 2016, 04:39:32 pm »
I suggest
(a) reading the FG manual carefully, then read it again.  Understand how all the controls work, particularly ones that affect output levels.
(b) Perform experiments and tests with a DMM or oscilloscope as the load for the FG to be sure you can see that you can make the controls put out the expected level, frequency, duty cycle, waveform before applying it to a more delicate load circuit.
(c) Unless it is a high quality very robust unit I would not suggest operating it into a load where it will deliver high power or current such as near short circuit cases.  It may be well protected but maybe not.
(d) If possible measure the actual peak to peak output voltage with a DMM or oscilliscope before applying the FG to a delicate load that could be damaged by an applied overvoltage.  It can be easy to accidentally have the FG put out too much voltage which could be enough to damage the input of an IC / transistor.
(e) Be mindful of the risks of applying any input signal voltage to a circuit when that circuit itself does not have its power supply running.  Even turning off some equipment temporarily while leaving the FG applying a signal to the input of that circuit could damage some circuits that are not well protected.

Specifically
(f) Most FGs can put out something like +/- 10VAC or +/-15VAC  peak signals.  Many ICs and transistor inputs could be damaged by a level of only +0.7V (transistors) or +3.3VDC (typical ICs when they are powered on).  So there can be a large risk that the FG could accidentally be set to output enough voltage that will destroy the delicate circuit it is connected to. 
(g) The FG can typically supply negative voltages relative to ground level.  Many ICs cannot tolerate applied negative voltages of any amount less than maybe -0.4V relative to ground.  Obviously circuits designed for AC input are not in such a category.  But be careful that you DC bias the output of the FG so the peak minimum value will be acceptable for the IC you apply the FG to (e.g. equal to or greater than ground for some cases).

Also be careful of what happens while you're adjusting buttons / switches on the FG.  It is not impossible that while you are adjusting a "touchy" control or one involving a switch position that there could be some glitch generated on the output that might exceed the expected voltage or something temporarily.

And of course be careful of ESD when connecting the FG to delicate equipment like directly to ICs or development boards that lack much protection since you can harm the load if not as easily the FG itself.

You might, for instance, make some protective adapters like this:
(a) Schottky diode in reverse parallel with a 3.3V zener diode.  One end goes to FG ground.  The other end ties through some series resistor like 1000 Ohms and series capacitor like 47uF ceramic or whatever to the FG input.  The non-grounded end of the diodes then is connected to your output to external equipment.  That would tend to somewhat protect many types of 3.3V powered logic / analog circuits from excessive FG voltage application to them.  The diodes and resistor would help dissipate any excessive FG output voltage beyond +3.3VDC and below -0.3VDC.  Of course clipping to -0.3VDC would only be applicable for normally small AC signal levels.

(b) Pi or similar resistive attenuators that reduces the MAXIMUM possible output voltage of the FG to some particular value e.g. +/- 1.5VAC, +/- 0.25VAC, whatever.  That way you can be sure if you fit a certain attenuator in series that under no condition will you generate more than a certain peak output voltage and you could be free to just set the FG output level to maximum, or whatever.

And be careful of the DC output coupling option if there is one.  Even if the AC output level is under peak to peak control, if the external circuit is DC coupled to the FG it may not like the DC output level of the FG if there is one configured.  For many cases you may prefer to use AC coupling into an output that is biased by the external circuit if that is known to be suitable.

 

Offline evb149

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Re: Basic usage of a Function generator
« Reply #4 on: October 09, 2016, 04:42:17 pm »
You might also want to consider making / using an power amplifier for the FG output if for some reasons you need output voltage / current / power levels near or beyond the limit of the FG.  That will protect the FG's output as well as enhance your load driving capability.

 

Offline rstofer

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Re: Basic usage of a Function generator
« Reply #5 on: October 09, 2016, 07:52:06 pm »
If your load is high impedance, you can put a 50 ohm resistor across the FG leads, right at the input.  You can also put a coax 50 ohm thru-terminator right at the end of the coax before the pigtails (whatever) hooks into the circuit.

If your load is low impedance, you can put a 50 ohm resistor in series with the FG leads.

The FG metering may depend on the output being terminated at 50 ohms or you may be able to select between assuming Hi-Z and 50 ohms for metering.

No link to the FG was provided so no totally correct answer can be given.  But the guidelines from others (above) will provide for a safe operating condition.
 


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