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Turn your CRO into a spectrum analyser for just $399 Have you ever dreamed of owning your own spectrum analyser? If you're an amateur, TV technician or electronics teacher, we'll bet you have. The $20,000 plus price tag would certainly have dampened your spirits. But now there is a probe which turns your CRO into a spectrum analyser. By ALEX EADES This little gem is called the VOS-107 Spectrum Probe and at a price of $399 it is a magic accessory. When an opportunity to review the VOS-107 came my way, I jumped at the chance. Having spent years building ham radio projects - where an analyser would have been a wish come true - I needed no convincing of the value of such a device. For the uninitiated, a spectrum analyser is an essential test instrument for any equipment designE)d to process frequency information and there is plenty of that! Take for example entertainment electronics. TVs, radios, VCRs, cameras and music systems are all involved in processing the frequency components of signals. The enormous amount of this equipment in use generates a growing need for spectrum analysers. The VOS-107 Spectrum Probe looks for all the world to be just another logic probe, but it is nothing like that at all. A normal oscilloscope displays how a voltage changes with time. A spectrum analyser displays the amplitudes of signals separated on the basis of 110 SILICON CHIP their frequencies. The VOS-107 converts the latter into the former (a handheld Fourier Analyser!). And that's no mean feat, especially for $399! What is a spectrum analyser? It is a device which looks much like an ordinary CRO but with a quite different method of display. It shows the frequency components in a signal along with their strengths at very good accuracy. To get an idea of this, imagine a display like the dial of your radio with vertical lines showing all the strengths of the stations in their respective positions. Sound over the top? Not really, such devices have been around for 30 years or so. The snag has always been the cost $20,000 to $50,000! Winning Lotto was the only way the average bloke could get one in the past. Measuring frequency There are several methods commonly used to measure frequency and they are testimony to the ingenuity (desperation?) of souls needing to quantify frequency information. The first is to display the signal on the CRO and measure the time taken for the pattern to repeat. The frequency is then found by dividing 1 by the time using a calculator. The second method is to use a digital frequency meter (DFM) which provides a reading in Hertz directly, while the third technique involves using a communications receiver or absorption wavemeter. Other devices such as slotted lines and lecher wires are somewhat esoteric and will not be discussed further. The CRO method will give the fundamental frequency and an experienced operator a gut feel for what others (harmonics or otherwise) may be present. The DFM will latch onto the largest amplitude signal and ignore the rest. The receiver or wavemeter method is probably the best as they can be tuned to a certain frequency and provide a measurement on a signal strength meter. By tuning across a band, an idea of the frequencies present can be obtained. These devices have been the mainstay of frequency analysis by hams for years. The difficulty with all these methods is that they display only one frequency at a time. It's like reading a newspaper one letter at a time instead of in whole words. This makes adjusting circuits a painstaking process when several frequencies need to be monitored simultaneously. CROs & spectrum analysers Most readers are familiar with the display of an oscilloscope - a pattern of how voltage on a circuit changes with time. We have grown to rely on the bounty of information then brings up dozens of spikes on the horizontal line, representing short wave, amateur, TV and FM signals. Evaluation This is the VS-107 Spectrum Probe in its case. It also comes with an AC plugpack and a brief instruction manual. It can be used with virtually any oscilloscope with a bandwidth of 5MHz or more. these bright green traces provide. Oscillation, switching, ringing, clipping, glitching, drooping, overshoot spikes, rise time, fall time, lumps and bumps are all familiar beasties encountered on the oscilloscope screen. The CRO's main drawback is its inability to provide detailed information on the frequencies of the signals being monitored. As a great deal of circuitry is designed to manipulate signal frequencies, this limitation is restrictive. With the VOS-107 spectrum probe, these waveform lumps, bumps, lines and spikes become fundamentals, harmonics, sidebands and intermodulation products, allowing spectral purity and bandwidth to be easily seen and measured. Connecting the probe Virtually any CRO has adequate performance for use with the VOS-107. The normal vertical sensitivity setting is 50mV/div (or 50mV/cm), while the timebase setting is 0.5ms/div (or 0.5ms/cm). Connecting the probe to the CRO is simple. The probe has a figure-8 shielded output cable. One half of the cable is terminated with a BNC plug and this goes to the CRO's vertical input socket. The other half of the cable is terminated in a 3.5mm line socket and this is for the 12V AC power input from a plugpack transformer. To monitor a signal, either the probe tip is used or an adaptor for coax cable is supplied - essential for VHF work. Remember to use a terminating resistor on the cable, otherwise reflections will cause inaccurate results. The manual recommends a simple arrangement to achieve this. For high power sources such as radio transmitters, the earth lead can be clipped to the input to form a "sniffer loop" which can be placed near the transmitter output. When connected and powered up, the screen displays a waveform which looks like a video signal. There is a negative-going sync pulse on the far left, closely followed by a vertical zero reference line, a 'noisy' horizontal line and, on the far right, the beginnings of the next sync pulse. The zero line represents minimum frequency and its height a level of 50dB a hove the noise floor. The vertical trace position and horizontal timebase controls are adjusted so that the sync pulses are off screen. Now the horizontal line represents frequencies from 1MHz to 100MHz. Touching the probe tip The essential specifications for the probe are: frequency range 1-lO0MHz; dynamic range 50dB; vertical output 5mV/dB; IF bandwidth 180kHz; and horizontal linearity ± 10%. These performance specs are moderate when compared to a laboratory grade spectrum analyser but still quite useful. To check these specs, I connected the probe to a CRO, RF signal generator and a spectrum analyser so that comparisons could be made. The overall frequency range of the sample probe was 1MHz to 103MHz which is slightly greater than the specifications. The horizontal frequency scale is linear within the specification, covering about 10MHz per division on a CRO with 10 divisions. Vertical scale accuracy was tested at 10, 50 and 100MHz with excellent results throughout, each vertical division measuring l0dB. You'll have more error just reading the CRO than from the probe! The easiest vertical set-up is to adjust the zero reference line for 5 divisions, which automatically gives lOdB per division. Comments The probe does not offer a direct reading in say dBm, however the vertical scale is useful for relative measurements which after all are the most common type. (For example, harmonics on a radio transmission are specified relative to the carrier level). A reliable lOdB/div scale is all you need. To simplify use, I attached a horizontal scale to the screen of the CRO using masking tape and a ballpoint pen. It's a bit rough I guess, but certainly effective, making frequency measurements a breeze. Minimum sensitivity of below 40/lV is plenty good enough to sniff out the majority of signals. The most serious limitation in my opinion is the IF bandwidth as this NOVEMBER 1990 111 Turn your CRO into a spectrum analyser ..• This is the normal oscilloscope display at 0.5ms/div and 50mV/div, here showing harmonics produced by an RF oscillator. determines the resolution of the display. The 180kHz bandwidth means that the spe.c trum monitor is unable to resolve the sidebands of a typical narrowband voice modulated signal from a CB or amateur transceiver. Sidebands determine the width of the signal in the band and contain the information in the transmission, so the monitor's inability to display them is an unfortunate limitation. The manual specifies the probe's resolution as 0.5MHz, a figure my measurements support. Amateur radio uses To monitor the output of my SSB transceiver, I placed the probe's sniffer loop near the coax to the dummy load. Only 5 or 10 watts were needed to produce a useful display - a very good result. The object of an oscillator is to have a pure single frequency at the output. Viewing this output on a CRO will give some qualitative idea of the purity. Is it a good looking sine wave or one containing lumps and bumps? With the probe, the main signal peak (the frequency we are trying to generate) is visible along with any other frequencies (usually harmonics) - a greatly improved display. The effect of your adjustments to the circuit can be easily seen. 112 SILICON CHIP This CRO photograph shows the video format of the Spectrum Probe's output waveform. Notice the negative sync pulses. A "mixer" is a circuit designed to change a signal of one frequency to another and in the early days of radio was called a "frequency changer". The tuner in a TV set and the "front end" of a radio are usually mixers. Mixers require careful adjustment to obtain the best results input levels, balance and output tuning need to be spot on. The adjustment can only be performed accurately in the frequency domain, hence the need for a spectrum analyser. The aim is to maximise the desired output and minimise the unwanted signals - difficult with a CRO alone but a breeze with the VOS-107 probe. TV technicians could also find the VOS-107 probe a great tool in their efforts to track down and nail circuit gremlins. TV IF performance, especially bandwidth and linearity, is clearly displayed. How about viewing the frequency spectrum of the signal from video heads or actually seeing the modulator working? Teaching Fourier Analysis Fourier analysis is a mathematical process of taking a time dependent signal (as on a CRO displayj and turning it into its frequency components. For example a square wave consists of the fun- damental (or clock frequency), 3rd, 5th and all odd harmonics with strengths inversely proportional to the harmonics number. The 3rd harmonic is 1/3 the strength of the fundamental, the 5th, 1/5 the fundamental, and so on. If all these signals could be generated and added together in the correct phase relationship, the original signal would be regenerated. If you think this sounds all very dry, you're not alone - thousands of students would agree. Enter the VOS-107. The device is of such low cost that institutions could afford to teach students "hands on". You simply feed various signals into the analyser and note what comes out and compare it to the theory. I wish these devices were around when I did Fourier analysis! Conclusion All in all, I am impressed with the performance of the VOS-107 and am sure that it will become common in the near future. (I'll have to ring Leo and tell him it'll take me a few months to finish evaluating the probe - should give me enough time to complete a few ham projects!) Our review sample came from David Reid Electronics, 127 York Street, Sydney 2000. Phone (02) 267 1385. ~