Hewlett Packard (HP), now known as Keysight Technologies, is a renowned company in the field of electronic test and measurement equipment. One of their popular products is the HP 33120A function generator, which offers a wide range of waveform generation capabilities. In this article, we will focus on understanding the accuracy of the HP 33120A, specifically its internal resistance and how it affects the output voltage.
What is the internal resistance of the HP 33120A?
When using the HP 33120A function generator, it is important to consider the internal resistance of the device. While output current is not a specified parameter, it has been characterized to determine its performance. In one test, a single unit was configured for a 10Vp-p, 1 kHz sine wave output. The output current was measured using a 34401A digital multimeter (DMM) connected to the current inputs of the DMM.
The test revealed that the maximum output current of the HP 33120A was 139mA RMS. However, as the output voltage setting was decreased to 1Vp-p, the output current reading decreased linearly to 14mA RMS. This decrease in output current is due to the internal resistance of the device.
The HP 33120A is designed for a 50 Ohm load, and it includes a 50 Ohm matching resistor in series with the final output stage. This matching resistor causes a voltage drop across it, which increases as the load resistance decreases. When the output voltage of the HP 33120A is set to 10Vp-p, a significant portion of the voltage is dropped across the internal 50 Ohm resistor, resulting in a lower output voltage across the load.
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It is important to note that the internal resistance of the HP 33120A can cause impedance mismatch when connected to certain loads. In the previous test, the output current reading was measured using a DMM with a low load resistance of approximately 0.5 Ohm. This low load resistance caused a significant voltage drop across the internal resistance, resulting in a lower output current.
It is worth mentioning that the test did not measure the distortion that might be caused by the impedance mismatch. Distortion can occur when the load impedance is not properly matched to the internal resistance of the function generator. To ensure accurate waveform generation and minimize distortion, it is recommended to use the HP 33120A with a 50 Ohm load, as it is designed for.
Q: Why does the HP 33120A output only half of the programmed voltage?
A: The output voltage of the HP 33120A can be lower than the programmed voltage due to the internal 50 Ohm matching resistor. This resistor causes a voltage drop, with a significant portion of the voltage appearing across the resistor itself. As a result, the output voltage across the load is lower than the programmed voltage.
Q: Can I measure the output current directly using a digital multimeter?
A: While it is possible to measure the output current using a digital multimeter, it is important to consider the load resistance. The internal resistance of the HP 33120A can cause a voltage drop across it, resulting in a lower output current. For accurate current measurements, it is recommended to use a load with a resistance close to the specified 50 Ohm load.
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A: To minimize distortion, it is crucial to properly match the load impedance to the internal resistance of the HP 33120A. Using a 50 Ohm load, as specified by the device, will help ensure accurate waveform generation and reduce distortion. Additionally, avoiding low load resistances can also help minimize the effects of the internal resistance.
The HP 33120A function generator is a versatile device that offers a wide range of waveform generation capabilities. However, it is important to understand the internal resistance of the device and its impact on the output voltage. The internal 50 Ohm matching resistor causes a voltage drop, resulting in a lower output voltage across the load. By using the HP 33120A with a 50 Ohm load and avoiding low load resistances, users can achieve accurate waveform generation and minimize distortion.