How to improve the function of RF signal generator?

RF signal generator is a common electronic test and measurement instrument. Different from traditional function signal generator, it is mainly used to generate RF continuous wave signal or modulated signal with RF signal as carrier. RF signal generator can be used as excitation source and is widely used in test and measurement of electronic products and circuit experiments in various electronic laboratories.

There are many factors that determine the performance of an RF signal generator. In summary, there are three key aspects:

1) Design experience of R&D engineers

2) Hardware cost consumed

3) Calibration test platform used in production and time spent

The technical parameters of many economical RF signal generators can basically meet the basic application requirements. However, for some higher professional fields, there is still some gap in key indicators. So, how can we effectively improve the performance of RF signal generators and expand its application scope while saving budget? This article will share some practical tips.

1. Improve the amplitude accuracy of the RF signal generator by connecting an external power meter.

Due to the limitations of the output amplitude accuracy of the signal generator itself and the frequency response characteristics of the connector between the signal generator and the DUT, the signal amplitude reaching the DUT may have a larger error than expected.

2. By connecting an external high-precision constant temperature crystal oscillator, the stability of the RF signal generator can be improved and the near-end phase noise level can be improved.

We know that the only factor that determines the frequency accuracy and stability of a signal generator is the reference crystal oscillator used in the machine. For cost considerations, economical signal generators generally do not use expensive constant temperature crystal oscillators. However, in some special test scenarios, we will pay close attention to the absolute frequency accuracy of the signal generator, or the stability caused by drift over time and temperature. At this time, we only need to purchase a high-precision constant temperature crystal oscillator option as an external reference for the signal generator to effectively solve this problem.

In addition, when the frequency offset is less than 1kHz, the stability and phase noise of the signal emitted by the signal generator will be mainly determined by the reference clock. The use of a high-precision constant-temperature crystal oscillator will help improve the near-end phase noise level of the signal.

3. Reduce the harmonics and spurious of the RF signal generator through an external low-pass filter

When the signal generator outputs the set fundamental signal, it will be accompanied by harmonics, subharmonics and noise signals of a certain amplitude. In most cases, we do not need the harmonic components, or the existence of these harmonic components will cause certain interference to normal testing.

According to the actual test situation, we select a suitable low-pass filter to connect to the output of the signal generator, which can effectively filter out the high-frequency harmonic components and improve the spectral purity of the signal.

Connecting a low-pass filter will inevitably have a certain impact on the amplitude and flatness of the output signal. We can compensate for the flatness and accuracy by increasing the output signal level and connecting an external USB power meter.

4. Increase the maximum output level of the RF signal generator through an external power amplifier

Generally speaking, the maximum output level of an economical signal generator is usually not greater than 20dBm. If you want to get a higher signal power, an external power amplifier is a good choice. It should be noted here that the harmonic level of a large signal is usually relatively high. After the external power amplifier is connected, the harmonic level will increase more. If you have certain requirements for the spectral purity of the signal, you can refer to method 3 and add a suitable low-pass filter after the power amplifier to reduce the impact of the harmonic component on the test results.

We can increase the maximum output level of the signal generator by connecting an external power amplifier, so can we extend the minimum output level of the signal generator by connecting an external attenuator? The answer is no. Usually the minimum output level of an RF signal generator is determined by the hardware structure of the product. The design determines its noise level, and the minimum signal it can output is generally about 3~5dBm higher than the noise level. When the signal generator outputs a small signal, an external attenuator is connected. Since the attenuator only attenuates the effective signal and has no effect on the background noise of the instrument, the small signal will be lower than the background noise of the machine and cannot be detected.

5. Improve the matching of the RF signal generator through an external attenuator

The measurement uncertainty of a signal generator refers to the fact that the output impedance of the signal generator is not the ideal 50Ω, so after the output signal enters the ideal load, part of it will be reflected back, and the signal generator cannot completely absorb this part of the reflected signal. Part of the reflected signal will be output to the load again. This back and forth reflection will change the effective input level of the device under test, thus causing measurement uncertainty. Every signal generator has a key indicator VSWR, which is specifically used to measure its measurement uncertainty.

A simple and effective way to improve the measurement uncertainty of an RF signal generator is to connect a well-matched fixed attenuator to the output of the signal generator.

When we have high requirements for a certain indicator, but the signal generator at hand cannot meet it, we can use the above methods as temporary solutions. However, these methods require some additional costs and make the entire test system more complicated, so when using them, we need to make comprehensive considerations to ensure that we can achieve our test purpose without introducing new errors.