2 Tips for Extending a Signal Generator’s Output Range without Sacrificing Amplitude Accuracy
2018-11-16 | 8 min read
RF signal generators are used for testing RF components, receivers, transmitters, and systems. A broad output power range is required for a wide variety of applications. In this post, you will learn what impact your measurement amplitude accuracy while a signal generator delivers high or low output power, and how to optimize amplitude accuracy. This post offers you tips for using a minimum test configuration to maintain amplitude accuracy.
A signal generator’s output power range is determined by the step attenuator, which is comprised of different combinations of attenuators, and the automatic leveling control (ALC) circuit. The step attenuator provides coarse power attenuation in 5 dB steps to achieve low power levels. The ALC is used to provide fine power level adjustment within the attenuator hold range. Figure 1 shows the ALC block diagram.
The purpose of the ALC circuit is to maintain the output power at the desired level despite drift caused by temperature variation. A direction coupler and power detector are used to measure the RF output power. The detected power level is fed back to the ALC system to adjust the ALC modulator in order to maintain a precisely controlled output level.
Figure 1: A simplified block diagram of the ALC feedback circuit
Signal generators must maintain spectral purity and level accuracy at all output power levels, including at the extreme ends, i.e., a very high-power or low-power signal.
High Output Power
It is straightforward to increase output power by adding a high-power amplifier. However, the high-power amplifier degrades spectral purity and distortion performance of a signal generator.
Adding a high-power amplifier to a signal generator will increase the broadband noise floor. While you measure the phase noise of the signal generator, the broadband noise appears at offsets greater than 1 MHz from the carrier frequency. In addition, the amplifier also affects spurious performance due to gain flatness and impedance changes. For a digital modulation signal, broadband noise degrades error vector magnitude (EVM) performance.
A power amplifier is a major contributor to non-linear distortion in RF signal generators. This includes harmonic and intermodulation distortion. These two types of distortion create in-channel, in-band, or out-of-band unwanted spectral signals. They degrade the signal generator's performance in areas such as spurious, modulation quality, and spectral regrowth.
To deliver high output power, selectable harmonic filters are used to reduce the level of harmonics, and an ALC circuit corrects the mismatches and frequency response.
Low Output Power
For low output power, signal generator’s attenuator accuracy is the most important characteristic. The lower the output power level, the poor the amplitude accuracy. This is because a combination of several attenuators is needed to achieve higher attenuation and each attenuator introduces errors.
While you output extremely low amplitude signals, the internally-generated system noise of a signal generator becomes critical. The lower the system noise floor, the higher the signal-to-noise ratio (SNR). Lower SNR results in a poor receiver sensitivity measurement.
In addition to the system noise floor, interfering signals can be a source of errors for extremely low amplitude signals. To resolve the errors, place the device under test (DUT) in a shielded environment.
Beyond the Output Range
RF signal generators are capable of outputting as high as +25 dBm and as low as -120 dBm. If you need to go beyond the specified range, you can use an amplifier to increase the output power or an attenuator to decrease it. When you extend the output range of the signal generator, there are some important factors to be aware of.
- Amplifier gain uncertainty affects the output amplitude level
- Attenuator’s flatness and accuracy performance
Tips for Optimizing Amplitude Accuracy
There are several ways to optimize amplitude accuracy while you use an external amplifier or an attenuator (or other passive accessories) with a signal generator. The common method is to use a vector network analyzer (VNA) to measure the entire signal path and enter correction values into the signal generator. Below are 2 tips to improve amplitude accuracy easily by using the built-in capabilities of new signal generators.
Tip 1: Using Flatness Correction
User flatness correction allows the digital adjustment of RF output amplitude to compensate for external losses in cables, switches, or other devices. By using power meter/sensor to calibrate the measurement system, a table of power level corrections can automatically be created.
The USB power sensor connects to the signal generator directly. The signal generator works as a power meter and measures the power at the test plane. The correction values can be saved in the signal generator’s memory and you can recall and apply the correction values the next time you use the same test configuration. Figure 2 below illustrates the flatness correction setup by using a signal generator and USB power sensor.
Figure 2: Flatness correction by using USB power sensor
Tip 2: Using External Leveling
External leveling lets you move the ALC feedback source closer to the DUT so that it accounts for most of the power uncertainties inherent to the cabling and components in a test setup.
Figure 3: Test setup for external leveling
As the RF power level at the input of the power coupler/splitter changes, the external detector returns a compensating negative voltage. The ALC circuit uses this negative voltage to level the RF output power by raising or lowering the signal generator’s power. This ensures a constant power level at the input of power coupler/splitter.
Understand, characterize, and correct RF signal paths
RF signal generators are capable of delivering a wide output range and high amplitude accuracy with step attenuator and ALC circuit. Beyond the specified range, you can use an amplifier to increase the output power or an attenuator to decrease it. If you add components between the signal generator and the DUT, the calibration plane and test plane are not on the same plane. You must correct the difference between the two planes.
Want to learn more about the key specifications of signal generators and why they matter? Download the Essential Signal Generator Guide and 9 Best Practices for Optimizing Your Signal Generator.