信号源测量

A VCO is a good variable frequency output device but not inherently stable. A crystal oscillator is very stable but can’t be tuned (frequency of oscillation is fixed). So we combine the best aspects of these. To have variable frequency plus stable output, we use a phased lock loop (PLL). In the PLL, we sample a portion of the VCO’s output and compare it to a crystal oscillator, which acts as the frequency reference. The difference between the two is fed as an error to the VCO’s control circuit. This tells the VCO to adjust its output accordingly. This process is called synthesis and sources that use PLLs are often referred to as synthesized sources.
In real life there are impurities and imperfections in the source that cause a less than perfect sine wave to be generated. And there are instabilities that make it difficult for the output to remain exactly where we want it to be. The objective of the source’s design is to minimize these imperfections. If they cannot be removed, then we must be able to characterize these imperfection to compensate for their effects.
The Ideal Source
Output
Frequency Amplitude
The job of a signal source is to generate a sine wave. We’d like that sine wave to be as perfect as possible. So what should a perfect sine wave look like?
In this module we will have a look at the different types of sources found in the market today, their construction and their important attributes. The attributes (features, specifications, etc.) are related to where the sources can be used. At the end of this module, you will have a broad understanding of how signal sources work, where to use them and how to tell a good one from a not so good one.
Synthesis combines the best of both worlds – the tunability of VCOs with the stability of crystal oscillators.
Output Section
ALC Modulator
From VCO
ALC Driver
We use sources predominantly to simulate a transmitter. We simulate transmitters to test receivers. We might want to check the receiver’s performance at various frequencies (to test selectivity) or at different amplitude levels (to test for sensitivity or compression). So we need a source that allows us to vary the output’s amplitude and frequency.
We also want the spectral line to be very narrow. Which means that if we want an output of 1 MHz, we want it right at 1 MHz. Not at 998 kHz, and not spread between 900kHz and 1.1 MHz. And we’d also like it to be rock steady at 1 MHz and not keep walking off that number.
Agenda
Types of sources
• CW • Swept • Signal generator (modulation)
• Analog • Digital modulation concepts
Important specifications
• Frequency and spectral purity • Amplitude
Block Diagramபைடு நூலகம்
÷R
fREF
÷S
φ
fVCO
fVCO = S/R * fREF
Now let’s take a look inside a source. At the heart of the source is the component that produces the sine waves. We need this component to be tunable because we need to be able to set the output frequency. Wider the tuning range, the better. At the same time, we want the output sine wave to be stable. Meaning, if we set the output at 100 MHz for example, we require the source output to be steady at 100 MHz and not drift around. One of the challenges with designing a source is managing stable outputs.
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SRCE 101 – Signal Source Basics
Technical data is subject to change. Copyright@2004 Agilent Technologies
But this is an ideal condition. It requires perfectly steady and resonant circuits. In reality, we would see some extra additions on the spectrum. We might see more spectral lines or a wider line. More on that later.
What the Output Looks Like
Oscilloscope
Spectrum Analyzer
Time
Frequency
Voltage Voltage
An ideal output will be a sine wave that we can see on an oscilloscope. However, there will be imperfections or impurities in the signal which may be hard to tell by looking at the oscilloscope display. A spectrum analyzer is a more useful tool here. If we look at the spectrum of an ideal signal we will see only one, very narrow spectral line – at the frequency we have set.
ALC Detector
Attenuator
Like frequency, we also desire a variable but steady output amplitude. This is achieved in a similar way. Amplitude levelling is achieved by automatic level control (ALC) circuit. Here, a portion of the output amplitude is sampled and compared with the desired level. The difference is an error voltage that is used to control the output level.