Communications Engineering Laboratory Manual
Figure 1.1 shows the type of signals generated in the Hewlett Packard (HP
33120A) function generator.
Figure 1.1: Waveforms generated by a function generator
This unit also generates other waveforms such as Sinc (sinx/x), exponential and cardiac functions. Each waveform may be adjusted on the front panel controls, or remotely for frequency, amplitude and DC offset voltage. Consider a sine function:
A sin(2
) +
OFF
Here,
is the frequency,
A
, the amplitude, and,
shown in Figure 1.2. For a sine wave, the RMS value is V
OFF
, the offset voltage as
RMS
= V
A
/1.41.
Figure 1.2: Sine wave with amplitude
A
, frequency
, and offset
OFF
The generator can be remotely programmed and read by computer via a General
Purpose Interface Board (GPIB).
The function generator uses a digital signal processing technique called direct digital synthesis to generate complex functions as shown in Figure 1.3.
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Communications Engineering Laboratory Manual
Figure 1.3: Block diagram of the HP33120A waveform generator.
The random-access-memory (RAM) stores the function (e.g. sine) in digital form, and may be addressed sequentially through an increment register. The frequency of the voltage waveform is proportional to the speed with which the RAM is addressed. The output data from the memory is a digital bit stream, which is converted to analogue wave-shape using a Digital-to-Analogue Converter (DAC). A low pass filter at the output ensures a smooth waveform. The amplitude and offset are controlled by changing the signal gain of the amplifier at the output of the DAC. Any linear circuit may be represented by a Thévenin equivalent circuit. This is shown in Figure 1.4a.
gen
represents the waveform (sine, pulse, etc.) and
T
is the Thévenin resistance (output resistance).
Figure 1.4: (a) Thévenin's equivalent circuit; (b) voltage divider between the output and load resistors. The
50 Ω . This implies that the actual output voltage measured over the load will vary with the load resistance because of the voltage divider, as shown in
Figure 1.4b. The output amplitude is calibrated for a 50 Ω load resistance, which means that the voltage shown on the function generator's display panel corresponds to the actual voltage V
LOAD
over the load only
50 Ω . In other words, the value of V selected) by the function generator. gen
is double the value displayed (or
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If the function generator's output is measured with no load connected
(open circuit or infinite resistance), the output voltage will be
the displayed amplitude. So be careful when applying the output voltage of the function generator to a circuit whose input resistance is different from 50 Ω . In general, it is a good practice to measure the amplitude of the waveform using a
Digital Multi-meter (DMM), or an oscilloscope instead of relying on the function generator display reading. You may set the output impedance to high.
The front panel of the HP 33120A is shown in Figure 1.5. There are two outputs on the right hand side of the front-panel. One is the output BNC type connector
OUTPUT . The other output is the SYNC that provides a digital signal (TTL levels) that is "high" when the waveform's output is positive, relative to the zero level (or the DC offset value). The SYNC signal is "low" when the output is negative. The SYNC signal is often used as a trigger signal for the scope.
Figure 1.5: HP 33120A front panel function generator
We may enter the amplitude, frequency and offset voltage values for a waveform in three ways:
(i) Use the circular knob and < keys. One of the digits will be blinking on the display. When you turn the large circular knob you can increase or decrease the number that is blinking.
(ii) Use the arrow keys <, up or down
(iii) Use the "Enter Number" mode to enter the number with the appropriate units shown on the right side of the panel. Press the "ENTER NUMBER" key, followed by the number (see green numbers next to the keys on the front panel) and the units. To finish, push the ENTER key to enter the number
(see Figure 1.6). To cancel the number mode, press SHIFT CANCEL keys.
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Figure 1.6: Entering numbers using the "Enter Number" mode.
At power-on, the function generator outputs a sine wave of 1 kHz and amplitude of 100 mV peak-to-peak (for a 50 Ω load resistance). You can select another function or change the frequency, amplitude and offset voltage using the
MODIFY keys.
To select one of the four standard waveforms (see Figure 1.1), push the key with the icon of the desired waveform (sine, pulse, triangle, or ramp) as shown in
Figure 1.5. You can modify the amplitude, frequency (and duty cycle in case of a pulse), using the modify keys on the front panel.
Press the FREQ key (Figures 1.5 and 1.7) to enter a frequency value. After entering the number, press one of the arrow keys to set the units (MHz, kHz,
Hz).
Figure 1.7: Using the Modify Keys to select the frequency, amplitude, offset or duty cycle
Select the Amplitude Mode with the AMPL key (Figure 1.7) and enter the number. You may enter the amplitude in mV peak-to-peak (VPP), in mV RMS, or dB using the arrow keys on the right of the front panel. As explained above, the function generator is calibrated for a 50 Ω load, which implies that the output voltage will be different from the one selected when the load resistance is different from 50 Ω ! The minimum amplitude is 50 mV
PP
and the maximum 10 V
PP for a 50 Ω load (and 100mV
PP
and 20 VPP, respectively, for an open circuit).
The waveforms have an offset voltage of 0V, which means that the waveform will vary between a positive and negative value. To offset the waveform, add an offset voltage as defined in Figure 1.2. To set the offset voltage, select the
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Communications Engineering Laboratory Manual
Offset Modify Mode and enter the number for the offset voltage. The HP waveform generator requires that the offset voltage meets the following restrictions:
|V
OFF
| <= 2 x V pp
and |V
OFF
| + V pp
/2 <= V max in which V
PP
is the peak-to-peak voltage and V max a 50 Ω load and 10V for a high impedance load).
is the maximum voltage (=5V for
This applies only to square waves. The default duty cycle is 50% and can be changed as follows. Select the Square Wave function by pushing the key with the square wave icon. Next, enable the duty cycle modify mode by pushing the
SHIFT and %DUTY keys (see Figure 1.7). Enter the number in % and press the
ENTER key.
There are five built-in arbitrary waveforms stored in non-volatile memory. These include Sinc, exponential rise and fall, negative ramp and a cardiac function
(which simulates the human heartbeat), as shown in Figure 1.8. In addition to these five functions, one can have four user-defined waveforms.
Figure 1.8: The five built-in arbitrary functions
To see the list of arbitrary functions, push the SHIFT and the ARB LIST keys as shown in Figure 1.9.
Figure 1.9: Selecting the Arbitrary Functions by pressing the SHIFT and ARB
LIST keys
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The first choice is the SINC function displayed for about 10 seconds. Use the keys or the large circular knob, to move across to the other functions (EXP-
RISE, EXP-FALL, NEG-RAMP, and CARDIAC) and press the enter key. You may change the amplitude, frequency and offset voltage as described above. Once you have selected an arbitrary waveform, this function will be assigned to the
ARB key. Pressing the ARB key displays the waveform. You can now set the parameters of these functions using the MENU functions of the waveform generator as explained below.
One can modulate the waveform using amplitude or frequency (AM and
FM) modulation. You may also generate a Frequency Shift Keying (FSK) signal, frequency sweep, or a burst waveform. These are selected through the frontpanel menu. The menu is organised in a top down tree structure as shown in
Figure 1.10. You move down the menu tree using the UP or DOWN keys, and move within a level using the ‘and’ < keys. The top level lets you select the main functions as shown in Figure 1.10. The second level is the command level , and the third level is the parameter level .
Figure 1.10: Menu organisation
To turn the menu on, press the SHIFT and MENU On/Off keys. Use the, <, UP and DOWN keys to navigate through the menu. To enter a command press the
ENTER key. To recall the last menu command that was executed press the
SHIFT RECALL_MENU keys.
Select the
(choice between 50 Ω output resistance, or an infinite output resistance, High Z). The default output resistance of the function generator is 50 Ω . To select the output impedance, turn the menu on by pressing the SHIFT and Menu On/Off keys. Move across the system menu using the
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Communications Engineering Laboratory Manual arrows until the following message is displayed: D : SYS MENU. Move down the
system menu to the OUT TERM command (see Figure 1.10). The function
generator will display 1 : OUT TERM. Move down one level and then across until
HIGH Z is displayed. Press ENTER to save this setting. Turn off the menu.
The standard waveforms may be modified to create more complex signals. For example, one can modulate the amplitude (AM) or frequency (FM) of the waveforms as shown in Figure 1.11 for an AM modulated sinusoid. The blue one is called the carrier frequency whose amplitude is being modulated by the lower frequency (red) sinusoid, called the modulating signal. The modulating signal does not have to be a sinusoid but can be an y signal (e.g. a speech signal or music, as is the case for AM radio transmission).
Figure 1.11: AM modulated sinusoid
To modulate a sinusoidal signal, select the sine function by pressing the key with the sine icon; adjust the frequency and amplitude using the modify keys. Select
AM mode by pressing the SHIFT AM keys (see Figures 1.5 and 1.12). You will notice that the AM annunciator is on.
Figure 1.12: Selecting the AM mode
Use the Menu to select the shape of the modulating waveform. Press the SHIFT and RECALL_MENU buttons (this brings you directly to the AM menu). You will notice the message 1 : AM SHAPE being displayed. Move down one level until
SINE is selected. Press the ENTER key to select the sine waveform. Set the modulating frequency by pressing the SHIFT FREQ key (Figure 1.12). An AM indicator will flash. You can now set the frequency of the modulating signal using any of the three methods to enter numbers as described above. Next press the
SHIFT LEVEL key to set the modulation depth. The modulation depth is
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Communications Engineering Laboratory Manual expressed in % and has to be between 0 and 120%) (see Figure 1.10 for definitions).
You can store up to 4 waveforms. Once you have set up your arbitrary waveform such as the AM modulated waveform above for instance, you can store them for later use. Once a waveform is stored you can recall it any time. To store a waveform, press the SHIFT STORE keys and the number (from 1 to 4) of the register in which you want to store the function. To recall the waveform press the RECALL key followed by the number of the register. Note: This signal generator treatment was based on material from the Net.
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