How to use FG-100 Function Generator

(This guide was written by Jos Verstraten on 17-11-2018)

The FG-100 is supplied in a sturdy plastic housing. The front is not printed on this, but on a solid
self-adhesive foil that is glued to the housing. We ordered two specimens for this test, with one this
foil was loosened in a corner, which unfortunately caused an ugly fold in the front. On some models
the manufacturer ‘EZM Electronics Studio’ is mentioned, other suppliers however deliver a housing
without mentioning the manufacturer.

The specifications

• Waveforms: sine, rectangle, triangle, positive and negative sawtooth
• Sine wave frequency range: 1 Hz ~ 500 kHz
• Rectangle wave frequency range: 1 Hz ~ 20 kHz
• Triangle wave frequency range: 1 Hz ~ 20 kHz
• Sawtooth wave frequency range: 1 Hz ~ 20 kHz
• Output voltage: 0 V ~ ±10 Vpeak-to-peak
• Output impedance: 50 Ω
• Sine wave distortion: less than 1 % (1 kHz)
• Frequency setting resolution: 1 Hz
• DC offset: ±10 V max.
• Display: LCD1602, 2 x 16 characters, 58 mm x 9.4 mm
• Microcontroller: ATMega48, 8 bit
• Supply voltage: 3.5 Vdc ~ 10 Vdc
• Supply current: 300 mA typical
• Dimensions: 140 mm x 80 mm x 35 mm
• Weight: 184 g
  The power supply
  To save money, the FG-100 is delivered without power supply. You must power the unit from a
  stabilized DC voltage between 3.5 V and 10 V. On the left side is a standard 3.5 mm power
  connector. The device is delivered with a relatively short power cable that ends in a USB-A
  connector. For the tests we have used a stabilized mains power supply set to 5.0 V.
  Operating the FG-100
  The low price obviously means that the designers have removed all expensive components such
  as rotary switches and accurate potentiometers. You need to operate the device with eight push
  buttons and two low-cost potentiometers.

Working with the FG-100
Operating the FG-100 is not as convenient as you would expect from a handy function generator.
When the device is switched on, the software sets the FG-100 to a 100 kHz sine wave with the
device in STOP mode. Be careful! A DC voltage of -10 V appears on the output when the
amplitude potentiometer is fully open. After pressing the RUN/STOP button, the generator delivers
the promised sine wave with a frequency of 100 kHz and a peak-to-peak value of ±10 V. A strange
choice of the designers, more convenient would be a start-up frequency of 1 kHz, because 100
kHz is completely outside the specified range for rectangle, triangle and sawtooth. These are only
specified up to 20 kHz.

Setting the frequency
To set the output frequency to another value, follow these steps:

• Press the RUN/STOP button, the output voltage goes to 0 V.
• Use the CURSOR button to select the digit you want to adjust, this digit will flash.
• Use the + and – buttons to set the digit to the desired value.
• Finally, press RUN/STOP again, the output delivers a sine wave at the selected frequency.
  Setting the waveform
• Press the RUN/STOP button, the output voltage goes to 0 V.
• Press the MODE button until the desired waveform appears on the display.
• Finally, press RUN/STOP again, the output provides the selected waveform.
  Adjusting the amplitude
  One potentiometer is available on the right side of the housing. At this point you will have to deal
  with the limitations of such a cheap device. In addition to an amplitude potentiometer, professional
  function generators also have a rotary switch with which you can select the decade of the output
  voltage:
• 0 mV to ±10 mV.
• 0 mV to ±100 mV.
• 0 V to ±1 V.
• 0 V to ±10 V.
  With such a setting, it is no problem to set the output voltage to 5 mV to control a sensitive
  microphone amplifier. However, with the FG-100 with its one potentiometer setting between 0 V
  and ±10 V, this does not work. Even if you open the potentiometer slightly, the FG-100 already
  delivers a voltage of ±100 mV, with a lot of noise and quantisation effects. To get useful mV signals
  from the FG-100, you need to connect an external voltage divider to the output.

Setting the offset
This option allows you to superimpose an adjustable DC voltage on the output signal. This option
is enabled by pressing the DC OFFSET ON/OFF button and turning the potentiometer on the front
panel. Please note that the output voltage is limited to +10 V and -10 V and that the usability of this
option is very limited.
Switching on a LPF-filter
The FILTER push button is supposed to switch on a low-pass filter to minimize the quantification
noise on the output signal. However, we have not been able to determine the function of this filter
for either of the two test specimens. Even if a rectangle of 20 kHz is generated, no difference can
be seen on the scope with or without filter.
The printed circuit board of the FG-100
The double-sided PCB looks very professional. Because the power supply is at least +3.5 Vdc and
the output can be ±10 V, a lot of electronics must be present in the power supply. The core of this
part of the circuit is an MC34063A switched voltage multiplier, followed by two diode rectifiers for
the positive and negative supply voltages and an AMS117 stabilizer for 5 V. Three electrolytic
capacitors smooth out the three supply voltages +13.0 V, -13.0 V and +5.0 V.
The analog output stage is built around two operational amplifiers TL072C. With their bandwidth of
3 MHz and slew-rate of 13 V/µs, they determine the maximum frequency range for the waveforms
rectangle, sawtooth and triangle. These waveforms contain many high harmonics that are
attenuated by the limited bandwidth of these operational amplifiers.

The function generator in practice
All tests were performed with a load of 1 kΩ. Terminate the device with this resistor hardly affects
the amplitude of the output signal. From the slight voltage drop, we could calculate that the output
impedance is equal to 45.4 Ω, almost equal to the specified 50 Ω. All oscillograms are made with a
Hantek DSO5102P digital oscilloscope.
The sine waveform
We have tested the sine waveform for frequencies of 1 kHz, 10 kHz, 100 kHz and 500 kHz. The
amplitude of the output signal remains constant until 100 kHz and then decreases gradually. The
signal shape remains good at first sight, unfortunately we do not have a distortion meter available
to measure the harmonic distortion. The accuracy of the frequencies is excellent, as can be
expected from a digital system.

The rectangle waveform
We have tested the rectangular waveform for frequencies of 1 kHz, 10 kHz, 20 kHz and 100 kHz.
The amplitude of the output signal remains constant. As expected, the rise and fall times take up
an increasing part of the total signal period. The rise time of the pulse is 1.36 µs, independent of
the frequency setting. This corresponds to a slew rate of 7.2 V/µs, much less than the used opamps can handle. Probably the parasitic capacities in the R-2R network cause the deterioration of
the slew-rate of the device.

The triangle and sawtooth waveforms
These signals are not very interesting because you will rarely use them in practice. At 1 kHz the
waveform is of course excellent, you can see what it looks like at 20 kHz and 100 kHz in the
oscillograms below.

What happens at 999,999 Hz?
The software allows to set the display to 999,999 Hz. Of course, we were curious about the output
voltage at this frequency, which is completely outside the specified range. To our surprise, the
device delivers a usable sine wave, although considerably attenuated compared to the reference
sine wave at 1 kHz. As the left oscillogram shows, there is a slight modulation on the signal.
Moreover, the output signal is not symmetrical relative to the ground. The other waveforms result in
completely chaotic output voltages like the ‘rectangle’ in the right oscillogram.