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Test Gear Review . . . The Digitech QM1577 is supplied with an attactive carry case, plugpack charger, probes and an optically-isolated USB interface cable. Tiny Scope Shootout By NICHOLAS VINEN Digital Storage Oscilloscopes (DSOs) have come down substantially in price while being greatly improved in terms of features. But they are still fairly bulky. What about when you are in the field, away from power and need something much more portable? We decided to have a look at two tiny alternatives: a credit card-sized scope from Seeed Studio and a combined DMM/scope from Jaycar. (1) Digitech QM-1577 ScopeMeter T HE DIGITECH QM-1577 10MHz DSO/4000 Count True RMS DMM from Jaycar is a fully-fledged DSO that you can hold in one hand. While a bit larger than a typical DMM, it does a good job of combining a single-channel scope and True RMS multimeter into a small, portable package. It has a 128 x 128 pixel monochrome liquid crystal display (LCD) with a backlight that can be switched on and off, giving decent contrast indoors and outdoors. The screen can be viewed without a backlight indoors as long as 24  Silicon Chip there is a reasonable amount of available light and this will extend the battery life. The battery is rechargeable; a plugpack is supplied. Battery life is not stated but it seems to be at least a couple of hours of continuous use in DSO mode and presumably longer in DMM mode. There is an auto-off function which shuts it down after a few minutes from any mode if no keys have been pressed, to conserve the battery. The unit measures 186 x 86 x 32mm and has a built-in desk stand. The rub- bery blue plastic surround appears to be removable but not easily so. Scope mode The critical statistics in scope mode are: 10MHz analog bandwidth, 50MHz sampling rate, sensitivity of 50mV/ div to 200V/div and 8 bits of vertical resolution (which is typical). Rising and falling edge triggers are available and input impedance is 10MΩ. A standard probe is not supplied with the unit. Instead, you simply use the standard DMM probes and since it has siliconchip.com.au Fig.1: the QM1577 operating in scope mode via an isolated USB connection to the PC. This can be used to record a series of captured waveforms and play them back later. It also has an FFT mode, which is only available when using the PC software. banana sockets, you can get plenty of accessory leads such as alligator clips and so on. The scope mode works surprisingly well for such a small instrument. While the feature list is relatively sparse, it certainly meets its specifications and the display is essentially noise-free. It gives a good solid trigger lock well beyond 10MHz (albeit with reduced signal amplitude, as you would expect). The actual analog bandwidth appears to be around 20MHz although by that point, the 50MS/s sampling rate becomes the limiting factor. The update rate is pretty good, with several screen refreshes per second at the faster timebases. The interface is responsive and the delay between the signal changing and this becoming visible on the screen is pretty low – a fraction of a second. It has a bit of a “phosphor” effect due to the time taken for pixels to “fade out” as they are turned off but that isn’t necessarily a bad thing. You can adjust the following: input coupling (AC/DC/ground), timebase (50ns/div to 20s/div in 1-2-5 steps), trigger time offset (across the width of the screen), vertical sensitivity (50mV/ div to 200V/div, in 1-2-5 steps), vertical offset, trigger type (rising/falling edge), trigger voltage and trigger mode (auto, normal or one-shot). These adjustments are made primarily using the F1-F4 soft buttons below the LCD. As you can see from the photo, the trace occupies most but not all of the display – it’s about 120 x 96 pixels siliconchip.com.au Fig.2: the QM1577 can also be used in DMM mode in conjunction with the PC software. It can then be used as a data logger in any of its measurement modes (voltage, current, etc). Measurements are shown as they are logged and can then be saved to disk. with about 12 pixels per division (10 x 8 divisions). This gives a decent view of most typical waveforms. The “HOLD” button freezes the display and you can then save it to one of 100 internal memory slots by holding this button down for a couple of seconds. Waveforms can be reviewed at any time via the MEM button and measurements can be made on them using horizontal and vertical cursors. The signal amplitude and frequency are permanently shown below the graticule with frequency in Hz/kHz/ MHz (auto-ranging) and amplitude as one of Volts (peak-to-peak) Volts (average), Volts (RMS), dB (dBu, we’re guessing). Pressing the FUNCS button cycles through these options. There is also an AUTO RANGE button which is very useful and this sets up the timebase, vertical sensitivity and so on, to suit the waveform you are probing. Of course, you can always do this yourself but most of the time AUTO RANGE will do a reasonable job so you might as well let it. DMM mode In multimeter mode, the QM-1577 has all the basic functions you would expect for a True RMS multimeter. That includes voltage (DC/AC), current (DC/AC) with both 400mA and 20A ranges, resistance, diode test, continuity (with buzzer), capacitance, frequency, duty cycle, and temperature/hFE/clamp meter modes which require probe adaptors to function. It’s an auto-ranging Cat III meter rated for up to 1000V. Accuracy and precision are typical for a 4000-count meter, however a few modes available in other meters are not present such as low current (microamps) – resolution in mA mode is 10µA. Rather than change modes with a rotary switch, you cycle through them by pressing the FUNCS button. In most of the modes, you can make relative measurements (ie, zero the reading) and it can also do peak-hold for both minimum and maximum measurements while displaying the currently measured value. These extra functions are accessed via the F1-F4 soft buttons. The drawback of DMM mode is that it’s only a 4000-count meter. Dedicated DMMs of this size and cost usually have an extra digit of resolution. One big advantage though is that with that large dot-matrix display, the read-out is neat and it can show you a lot of information simultaneously. For example, for voltage measurements in peak-hold mode, you can simultaneously see the reading, the mode (DC/ AC), the range and the current minimum and maximum values. In fact, in every mode and range, the maximum possible reading is shown in the upper-left corner of the screen and this provides you with a quick indication of whether it’s likely to change ranges automatically, depending on what you are measuring. You also get some extra information in some modes – for example, in resistance mode, you can also get the August 2013  25 The Seeed Studios DSO Nano V3 is much smaller than the Digitech ScopeMeter and fits easily into a shirt pocket. It’s shown here only slightly smaller than actual size. (2) Seeed Studios DSO Nano V3 T HE DSO NANO is a much smaller single-channel portable DSO. It fits in a shirt pocket (92 x 60 x 13mm) and doesn’t have any DMM features other than frequency and amplitude measurement of the input signal but it has a square wave signal generator output. The Nano has a 320x240 pixel colour TFT screen which measures 7cm diagonally. It’s housed in a black anodised aluminium case and has just a few controls: five pushbuttons on the front, two on the top and a power switch on the right. On the left side are the probe input and signal generator output sockets (both 3.5mm phono types) and a USB socket for charging the internal Li-Po battery. Two types of probes are supplied, both about 55cm long. One has black and white mini hooks, which are handy for clipping onto component leads (probably more so than the traditional type of oscilloscope probe) and the other is terminated in two small pins which can be pushed into a breadboard. Depending on what you are trying to monitor, the probe leads are a bit short and you need to tether the Nano so that it is not pulled around, making it awkard to view. It does come with a small aluminium desk stand and we would be inclined to attach that to a small piece of timber, to give it some more mass. Also supplied is a vinyl Digitech QM-577 ScopeMeter – continued conductance (the inverse of resistance, measured in Siemens) read-out below the resistance display. What you are reading (voltage, current, etc) is indicated at the top of the screen so there’s never any doubt. Windows software The QM-1577 has a USB port and can be connected to a computer, either for data logging or to use the features of the scope/DMM remotely. The supplied cable is optically isolated for safety however it still plugs into a socket on the unit, rather than using a purely optical interface via a transparent window like many other meters. That means you need to keep the protective cover closed when not 26  Silicon Chip using the USB interface, to prevent dirt and moisture from getting in. Also, being isolated, the internal battery is not charged when plugged into a computer so you are limited to a few hours of such use, by which time you will need to unplug the computer and plug in the charger instead. The supplied software is Windowsonly. It works in Windows 7 but you need to enable XP compatibility mode and download the latest CP2102 USB-to-serial adaptor driver from the SiLabs website. This software allows for real-time operation of the scope (Fig.1) and/or multimeter (Fig.2) via on-screen displays which more or less mimic those on the unit itself, plus scope waveform recording, spectrum protective case and a couple of small stick-on rubber feet. No USB cable is supplied but all you need is a standard mini USB (not micro) cable and it can be charged from a computer or mains/vehicle power with an appropriate adaptor. Specifications The specifications for the DSO Nano are modest, with an analog bandwidth of 200kHz and a sampling rate of 1MS/s, although vertical resolution is good at 12 bits. Maximum input voltage is 80V peak-to-peak and input impedance is around 0.5MΩ. The square-wave generator has a frequency range of 10Hz to 1MHz in 1-2-5 steps analysis and multimeter data logging features. Conclusion The QM-1577 is supplied with a carry case (quite a nice one), mains charger (plugpack) with USB plug, optically isolated USB interface cable, driver/software CD and a pair of standard DMM probes. The only documentation on the CD is for the PC software but the unit is pretty easy to figure out. It is a good all-round portable instrument that is easy to carry and use and provides a range of very useful functions. Its recommended retail price is $399. The unit is available from Jaycar stores or by mail order via their website – see www.jaycar.com.au or www. SC jaycar.co.nz siliconchip.com.au and a fixed amplitude of about 3.3V peak-to-peak. If you’re thinking that those features sound a lot like what’s available on a microcontroller, you’d be right as the DSO Nano is based on an ARM Cortex M3 processor with its 1MS/s ADC forming the crux of the input system. The colour display is clear and crisp when viewed front-on or at moderate vertical offsets. The software features cursor support, adjustable trigger sensitivity, a single measurement display (frequency, RMS voltage, peak-to-peak voltage, duty cycle or average voltage) and multiple trigger modes including one which automatically adjusts the timebase and vertical sensitivity to suit the signal. We found that with noisy signals, the triggering was rather fussy, with the signal jumping around even when the trigger sensitivity was set at minimum. This possibly indicates a lack of dedicated trigger hardware, ie, the software likely monitors the ADC to determine when to start sampling rather than using a dedicated comparator circuit. The DSO Nano has a vertical sensitivity range of 10mV/div to 10V/ div and a timebase adjustment range of 1μs to 10s in 1-2-5 steps, although the faster timebases are not very useful given that 1MS/s equates to one sample per division at the fastest setting (1μs/div). There is no option for AC input coupling; you would need to connect a capacitor in series with the probe to achieve this. The Nano we bought has a microSD card slot for storing waveforms but we couldn’t get it to work with the 16GB cards we had on hand; perhaps the capacity was too high. The latest version, though, has 2MB of built-in flash memory for this purpose instead and we presume screen grabs stored in this memory can then be downloaded later via the USB port. The internal battery is also charged over USB. Its capacity is 500mAh <at> 3.7V (nominal) which gives a life of about one hour of continuous use. If and when a new version of the firmware becomes available, you can update it via USB. We noted a few minor bugs in the version that came with our unit (stray pixels left behind on screen under some circumstances) so this should come in handy. Note that there are also third-party firmware images available which have certain advantages over the official firmware (which is supplied with the unit), so this gives further upgrade options. But we also noticed that the PDF manual refers to a different set of features than is available on the unit we purchased, presumably because it was written for a different firmware version. Overall, this is a very basic scope in a convenient package. It’s pretty easy to use although it takes a few minutes to figure out the rather terse on-screen descriptions. The big advantage though, besides portability, is the price. It’s available from the Seeed Studios website for US$90 including airmail postage from Hong Kong. Delivery time is typically 1-2 weeks. The unit is also available locally from www.soundlabsgroup.com.au We should point out that Seeed Studios also sell the DSO Quad for $169$199 which is much more capable and slightly larger. It’s 98 x 60 x 14.5mm but has two analog channels with 10MHz bandwidth which run at 36/72MSps (8bit vertical resolution) plus two digital channels. The $169 version comes in a plastic case while the $199 version SC has an aluminium case. electronics design & assembly expo Australia’s largest dedicated electronics industry event Design, Develop, Manufacture with the Latest Solutions! In association with Register Online Now www.electronex.com.au +61 3 9676 2133 electronics design & assembly expo Melbourne Park Function Centre 11-12 September 2013 siliconchip.com.au August 2013  27