• Compact oscilloscope, spectrum analyzer and signal generator
• Sampling rate up to 1 MHz
• Supports subsampling
• Input signal max.: 0 dBm (0.225 VRMS)
• Sensitivity: –80 dBm (22.5 µVRMS)
• Ethernet connection
• Open source
LPF
Located in the NCSA Module
INPUT Ethernet
OUTPUT
A Window
Function
Signal
Generator Synthetic
Signal Generator
FFT User
Interface ADC
Located in the PC Application
Figure 1. System block diagram.
successfully connected to the PC over the Local Area Network (LAN). The IP address that is established is also displayed here.
To the right of this is a control box labeled
‘FFT Controls’, the FFT referring to the Fast Fourier Transform. Here is where the user can vary the sampling frequency Fs, the number of points N to include in the FFT, the number of bits in the ADC cess. This allows the
PC application to be exercised without the NCSA module being attached. The synthetic signal choices provided are an Amplitude Modulated (AM) signal, a Fre-quency Modulated (FM) signal, a partially formed square wave and a test signal.
The frequency and modulation constants are all user controllable. The generated signals are ideal noiseless signals to be used for experimentation. I will discuss later how a user can add other signal options to the synthetic signal list if desired.
User Interface details
The UI is designed to give the user exten-sive flexibility to control the critical sam-pled data systems parameters. The intent is to allow the user to see how these various parameters impact the quality and performance of sampling and Fourier transforming.
Let’s take a closer look at the UI in Fig-ure 2. Starting at the lower left, you see an icon that shows if the analyzer is
Figure 2. The NCSA’s input is connected to a signal generator that’s set to produce an amplitude-modulated (AM) signal. The PC application’s UI is displaying the time plot (upper graph) of the signal and its frequency spectrum (lower graph). The 13-kHz carrier and the 1-kHz sidebands are clearly visible.
and the number of consecutive spectra to average. Clicking on the “Single FFT Real Data” button will cause the system to get N samples from the selected ADC and perform one FFT. The time domain data is displayed in the upper graphic with an oscilloscope-like format. The fre-quency spectrum is shown in the lower graph. Later in this article, the specif-ics of how these parameters interact will be discussed. The ‘ADCS’ and ‘Low Spur Fs’ items located in this box will also be explained more fully later in the article.
Also in the FFT controls box, the user can turn on the “auto FFT” button. This button causes the system to continuously sam-ple and transform the data. Additionally, the user can select to average a variable number of subsequent spectra in order to reduce the noise variance.
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1V2O RSVD
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Figure 3. The signal being analyzed here is an ideal frequency-modulated (FM) signal produced by the PC application’s synthetic signal generator. The upper time plot shows the varying frequency carrier.
The corresponding power spectrum is shown in the lower graph. The center frequency, line spacing and bandwidth are readily discernable from the graph providing a good characterization of the FM signal.
Figure 4. NCSA schematic.
tem needs an analogue interface, the fact that this dsPIC33 has a built-in amplifier is ideal. For frequency accuracy an external 8 MHz crystal is employed.
Additionally, the dsPIC33 contains several high quality peripherals needed for this application. The peripherals utilized and their functions are timers (for precise con-trol of sampling and signal generation), high-speed PWM (for signal and noise generation), the SPI module (for commu-nication with the W5500), the 12/10-bit ADC (for high-speed analogue to digital conversion) and an operational amplifier (for front-end anti-aliasing filtering and amplification).
The W5500 chip manages the commu-nication to and from the PC over Ether-net. The control of the W5500 is easily handled utilizing the dsPIC33’s SPI com-munication port and a few control lines (RDY and Reset). The electrical interface between the W5500 and the physical ernet is provided by the J1B1211CCD Eth-ernet connector. This connector incor-porates the necessary transformer ele-ments required to connect directly to the network.
You’ll note that two separate 3.3-VDC regulators are included. This isolates the analogue ADC supply and minimizes digital noise crossover. Also the various ground planes on the PCB are arranged in a star configuration and components the plotted data to be viewed in
impres-sive detail.
It is useful to note that the PC application can be loaded and run on a PC without the NCSA being connected. The user won’t have access to real data from the NCSA, but one can still play with the UI controls and the synthetic signal generator to get a feel for how things operate.
Hardware description
The NCSA module schematic is shown in Figure 4. As can be seen on the sche-matic, the main hardware components are the Microchip dsPIC33EP512MC502, the WIZnet W5500 Ethernet controller IC and the Analog Devices ADP151 low dropout low noise voltage regulators.
This particular dsPIC33 was selected for a variety of reasons. The 60 MIPS available in the dsPIC33 (utilizing the internal PLL) makes it a good fit for a high-speed ADC application. The dsPIC33 also has about 50 KB of RAM allowing large data blocks to be saved without the need for external memory. Also, since any sampling sys-The next control box to the right is the
‘Window Type’ control box. Here the user has seven choices of ‘windows’ for smoothing the data prior to performing the FFT. The details of why and how this is done will be discussed later.
The next control box is the ‘Synthetic Signal’ interface. This box allows the user to generate numer-ical arrays of numbers in the PC application corresponding AM, FM, Square Waves and a specific test signal used for system check-out. When the user
clicks inside the top item in this control box an FFT is performed on the selected signal type using the frequency and modulation constants located in the lower three boxes.
Moving to the right one more time, we see the final control box called “Signal and Noise Generator”. This is where the user selects either a sine, square, trian-gle or noise signal to be generated by the µP located in the NCSA. The generated analog signal is available at the output BNC socket on the NCSA. It can then be used in an external application, or it can be fed back into the analyzer’s input for normal NSCA System analysis.
In addition to the above controls, the UI contains powerful interactive graphics.
The yellow boxes located above each graph to the left are the cursor readouts.
They display the time and amplitude data under the cursor in the upper plot, and frequency and power levels under the cursor in the lower plot. There is also a tabulation button located in the UI upper right that allows the user to display all the time and frequency data points in tabular form. An example of this tabular display is shown in Figure 3.
Finally, the UI provides a very powerful zoom in/out capability. An example of this is shown in the upper plot of Figure 2. This multilevel zoom capability allows