Gas station without pumps

2015 July 1

FG085 function generator output impedance

Filed under: Circuits course — gasstationwithoutpumps @ 01:01
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The output impedance of the FG085 function generator that I assembled yesterday is supposed to be 50Ω, so I decided to measure it by plotting RMS voltage as a function of load resistance:

RMS voltages were measured with a Fluke 8060A multimeter with the FG085 set to 100Hz and 5V.

RMS voltages were measured with a Fluke 8060A multimeter with the FG085 set to 100Hz and 5V.

The RMS voltage measurements are consistent with a series resistance of 46.66Ω (slightly less than the 50Ω spec) and with a peak-to-peak voltage of 5.018V (slightly more than the 5V requested).

It would be interesting to determine whether there is a parallel capacitance or series inductance that affects the output impedance. I may have difficulty measuring that, as the values are likely to be small, and so their effects will only be visible at high frequency, which the function generator is not very good for generating.

With the Fluke 8060A meter set to AC voltage, I should be providing a 10MΩ load with at most 100pF in parallel, and with that load I see no drop in voltage until about 200kHz.  The power drops by 2 (amplitude by \sqrt{2}) at around 277kHz, after which the voltage drops as a second-order filter (that is as approximately (277kHz/f)^2).  Since the 277kHz is a lower frequency than what I saw as a cutoff with the Bitscope oscilloscope (385 kHz and a first-order rolloff), I believe it is internal to the meter.

My voltmeters have a more limited range than the FG085 function generator—only the Bitscope digital oscilloscope allows me to make measurements at a high enough frequency to see the drop in voltage due to the function generator, rather than due to the measuring device.

My voltmeters have a more limited range than the FG085 function generator—only the Bitscope digital oscilloscope allows me to make measurements at a high enough frequency to see the drop in voltage due to the function generator, rather than due to the measuring device.

I could take the function generator into the lab used for the circuits class, and characterize it a bit more precisely at the high frequency end, but I don’t think that will help much in determining whether there is extra capacitance or inductance in the output impedance of the function generator.  The problem is simply that the function generator doesn’t produce high enough frequencies for the inductance or capacitance to matter.

At 100kHz, a 33nF capacitor would have an impedance of –48j Ω, so small parallel capacitances (say, <100pF) would have negligible effect on the impedance of the series resistor. Similarly, at 100kHz, 80µH has an impedance of around 50j Ω, so small series inductances (say, < 1µH) would have negligible effect on the impedance of the function generator.

In short, I don’t see any easy way to improve the model of the function generator as a 46.66Ω source.

4 Comments »

  1. […] not surprised that the fitting had to be stopped at 10kHz, as the plot I did in FG085 function generator output impedance showed the voltmeter having a 2nd-order low-pass filter with a cutoff frequency around 9.3kHz. […]

    Pingback by Measuring voltmeter input impedance | Gas station without pumps — 2015 July 5 @ 21:30 | Reply

  2. […] previously reported on assembling the FG085 function generator kit and measuring its output impedance. I noted that the output impedance was only 46.7Ω, instead of 50Ω, and in Testing nFET body diode […]

    Pingback by FG085 function generator bugs | Gas station without pumps — 2015 July 16 @ 17:50 | Reply

  3. […] reviewed a couple of cheap function generators: the Elenco FG-500 and the JYEtech FG085 (also here and here). But I can’t remember whether I’ve reviewed the use of the function generator […]

    Pingback by Bitscope function generator review | Gas station without pumps — 2015 August 17 @ 23:11 | Reply

  4. […] outputs.  Since the FG085 has an approximately 47Ω output impedance (nominally 50Ω, but see FG085 function generator output impedance), a 470µF capacitor would give a low-pass filter with a time constant of about 22ms, or a corner […]

    Pingback by Smoother I vs V plots for LED | Gas station without pumps — 2016 January 16 @ 14:30 | Reply


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