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Rohde & Schwarz “Pigeon Pair”: FS300 & SM300 Mauro Grassi takes an in-depth look at Rohde & Schwarz’ new FM300 Spectrum Analyser and SM300 Signal Generator T he FS300 Spectrum Analyser can analyse the frequency domain from 9kHz to 3GHz and the matching SM300 Signal Generator can produce the same set of frequencies. In fact, the FS300 and SM300 look almost identical face on. Both housed in robust cases; the only difference in the front panels is that the signal generator has an extra LF (low frequency) output. Apart from that, both have a rotary knob, a numeric keypad, some soft buttons (meaning their use varies according to the selected sub menu) and some keys for navigating through their on-screen menus. The screen is a quarter-VGA (320x200) TFT LCD. Both the Spectrum Analyser and Signal Generator will fit snugly side-by-side in a single 19inch rack. 44  Silicon Chip At the rear of the units lie a host of connectors. From a monitor output, USB host port (for connecting a USB flash drive), USB device port (for connecting to a PC), a keyboard port (for attaching an external keyboard) and various BNC connectors for accepting an external trigger and reference signals. Menus Both the FS300 and SM300 have intuitive and easy-to-use on-screen menus. The layout of the menus is very similar in both units, making the interface rather uniform. In the user screen there is a horizontal row of menu items, each of which has a vertical set of submenus. Thus it is easy to navigate through the menus by moving across horizontally to the desired menu and then selecting the siliconchip.com.au relevant vertical item with its dedicated “soft” button. Any further submenus appear on the vertical panel and are easy to navigate. In both units, a number of automatic settings are provided which are adequate for most routine measurements, with a manual override available for custom measurements. The FS300 Spectrum Analyser Let’s begin by looking at the FS300 Spectrum Analyser in more detail and what can be done with it. The noise level is exceptional at better than -110dBm, so very weak signals – of the order of microvolts – can be detected. This is outstanding for a device in this price range. There are 16 digitally-selectable resolution bandwidths from 200Hz to 1MHz with an accuracy of 5%. The frequency menu allows start and stop frequencies to be entered via the numeric keypad. These set the range of frequencies whose amplitudes will be displayed on the screen. Alternatively, one may set the centre frequency and the span, with the range of resulting frequencies being the centre frequency minus half the span to the centre frequency plus half the span. The amplitude menu allows the amplitude axis to be set for the relevant measurement. Either a relative or an absolute scale can be selected. In relative mode, a percentage scale is used while in absolute mode, the unit can be chosen from among dBM, dBmV, dBmV, mV or mW. Fig. 1: the amplitude in the frequency domain from 88MHz108MHz (the FM radio band) obtained with a makeshift aerial (a length of wire). The peaks correspond to the carrier frequency of radio stations in the Sydney area. A Simple Radio Test We connected an antenna (simply a random length of wire) to the input of the FS300 Spectrum analyser. We set the starting frequency at 88MHz and the end frequency at 108MHz. The result we obtained is shown in Fig.1. The resulting spectrum shows the FM radio stations in Sydney with each peak corresponding to a radio station. Fig.2 shows what happens when we zoomed in on the interval between 104MHz and 107MHz, showing in more detail the peaks of RF detected, corresponding to the stations at 104.1MHz, 104.9MHz, 105.7MHz and 106.5MHz. Signal Tracking Fig. 2: the amplitude in the frequency domain from 104MHz to 107MHz (part of the upper FM radio band). The peaks (from left to right) correspond to the equally spaced FM radio stations transmitting at 104.1MHz, 104.9MHz, 105.7MHz and 106.5MHz. The FS300 Spectrum Analyser has an option called “Signal Tracking”. When enabled, the effect is that the frequency with the greatest amplitude is set to be the centre frequency. This is useful for analysing passband filters, for example, where one very narrow range of frequencies is predominant. Level Display Ranges The vertical scale range can be adjusted from a choice of five ranges. Four ranges from 8dB to 80dB and a linear scale 0% to 100% are available. In linear scale mode, 0% corresponds to a 0V or 0W reading, whereas 100% corresponds to the level offset. Moreover, up to 30000 sweeps can be averaged to reduce interference and obtain an overall impression of the amplitude of a frequency. Markers Markers can be used to measure the amplitude at a particular frequency. The reading of the marker is shown at the top of the display, and the marker can be varied by siliconchip.com.au Fig.3: the main screen of the SM300, showing the frequency and amplitude of the “carrier” signal. The horizontal row of menus can be seen, with the vertical column of menu items corresponding to sub-menus of the selected horizontal menu item. December 2007  45 the rotary knob. Up to two markers are available on the screen at any one time. Measuring the FM radio spectrum using the marker revealed that the signal at 104.1MHz (corresponding to 2DAY-FM radio station) was around 6nW (nanowatts). Two markers can operate in “delta” mode, where the value measured is the difference in level between the two markers. A very useful feature allows the reference level (amplitude) and centre frequency to be set according to the current value of a marker. Noise power density and signal bandwidth measurements Fig.4: this shows the output of the SM300 signal generator in FM mode (blue). The carrier frequency is 200kHz. Notice the signal frequency measured by the oscilloscope is not exactly 200kHz, as expected. The green trace is the square wave modulating source of around 20kHz. Using an on-screen marker and moving the rotary knob allows a particular frequency to be singled out. The FS300 can then measure the Noise Power density for that frequency. Moreover, by setting two markers the FS300 can measure the signal bandwidth. That is particularly useful when working with filters. Display Lines Activating a display line on the FS300 allows you to detect whether a particular frequency exceeds a certain level of amplitude. You simply change the display line with the rotary knob. The reading is displayed above the line. This feature can be considered a vertical cursor. Frequency counter A built-in 6-digit frequency counter allows exceptionally precise frequency measurements to be made, with resolution from 1kHz down to 1Hz. All in all the FS300 is an impressive Spectrum Analyser with quite a fast response. The SM300 Signal Generator Fig.5: by contrast, the output in AM mode (blue). The carrier frequency is 200kHz. Notice the signal frequency measured by the oscilloscope is exactly 200kHz, as expected. The green trace is the square wave modulating source of around 20kHz. Let’s now turn to the SM300 Signal generator. This is a robust and exceptionally precise signal generator with a range of modulation options. It can produce digitally modulated signals in the continuous spectrum between 9kHz and 3GHz. Frequency, amplitude, phase, pulse and IQ modulations are possible. For IQ (in-phase/quadrature) modulation, the SM300 accepts the I and Q components from external BNC connectors on the back of the unit, meaning virtually any kind of modulation is possible. The SM300 can produce a stable source of test signals applicable to diverse areas such as mobile radio and video equipment. And its high IQ bandwidth allows it to be used for testing wifi and other wireless digital streams. A typical screen shot of the signal generator is shown in Fig.3. Modulation Fig.6: the signal output in IQ modulation mode. The blue trace is the signal output, which is IQ modulated with a carrier frequency around 200kHz. The green trace is the I component, a square wave at around 160kHz. 46  Silicon Chip Either an internal LF (low frequency) generator or an external signal can be used as the modulation source. We connected a 20kHz square wave as the modulation source and set a carrier frequency of around 200kHz. We then tried in turn amplitude, frequency and IQ modulation and the resulting waveforms were captured using an oscilloscope. Fig.4 shows the result of frequency modulation, Fig.5 amplitude modulation and Fig.6 IQ modulation. We should clarify that the I component was a square wave at around 160kHz while the Q component was shorted out siliconchip.com.au This shows the screen shot of the FS300 Spectrum Analyser displaying harmonics of a 145kHz square wave. On the right is the data recorded with the screen shot by the PC software – very handy for future reference. with a 1kW resistor. Frequency Sweeps A powerful feature of the SM300 signal generator is its frequency sweep. The frequency can be varied gradually from a start to a stop frequency in user-selectable steps. The dwell time (ie, the time between steps) can be set from micro seconds to seconds. This feature is useful for measuring the frequency response of a piece of equipment, including (for example) a filter. The SM300 signal generator is exceptional and well suited to a vast range of testing applications. PC connections Both the FS300 and SM300 have standard USB device ports (rev 1.1) for connection to a PC. This allows screen shots and data to be acquired directly from the device. The device can also be controlled by the software. As well as this, an external keyboard and monitor can be connected. Conclusion The FS300 and SM300 are a compatible, complete frequency domain solution for testing and d iagnostics. For more information on pricing and availability, contact the Australian distributors: Rohde & Schwarz Australia Unit 2, 75 Epping Rd, North Ryde NSW 2113. Tel (02) 8874 5100 Website: www.rohde-schwarz.com.au SC Contribute T he Best And Choose Your Prize . . . As you can see, we pay good money for each of the “Circuit Notebook” items published in SILICON CHIP. But now there are four more reasons to send in your circuit idea. Each month, the best contri- siliconchip.com.au bution published will entitle the author to choose their prize: an LCR40 LCR meter, a DCA55 Semiconductor Component Analyser, an ESR60 Equivalent Series Resistance Analyser or an SCR100 Thyristor & Triac Analyser, with the compliments of Peak Electronic Design Ltd (find them at www.peakelec.co.uk). So now you have even more reasons to send that brilliant circuit in. Send it to SILICON CHIP and you could be a winner. You can either email your idea to silchip<at>siliconchip.com.au or post it to PO Box 139, Collaroy, NSW 2097. December 2007  47