Oscilloscopio Tablet 4 in 1 Fnirsi DPOS350P

With this latest oscilloscope from Fnirsi, the Chinese designers have managed to pack even more functions into a single device than in their previous models. The DPOS350P is not only a dual-channel oscilloscope with 350 MHz bandwidth, but also a 50-MHz function generator, a frequency analyzer (Bode plotter) up to 50 MHz, and a spectrum analyzer up to 350 MHz. And all of this is packed in a compact case with a 7-inch touchscreen for around 270 euros. That makes you curious, doesn't it? Time to try out all those features!

Complete Package

As we've come to expect from Fnirsi, the DPOS350P comes with a fair number of accessories. Everything is packaged in a sturdy carrying case, similar to those found on some other Fnirsi instruments. Besides the oscilloscope, the case contains two oscilloscope probes, a test lead with alligator clips, a power adapter, and a USB charging cable. It also includes a bilingual manual, which is unfortunately quite small. It's easier to download and read the PDF.
 

The oscilloscope case measures approximately 19 x 13 x 4 cm and is neatly finished with aluminum side panels. The front features a 7-inch touch display that delivers a sharp image with a resolution of 1024 x 600 pixels. Operating the device is done via the screen; on the side there are only an on/off button, and a USB-C connector below it for charging the battery and connection to a PC. On the top are three BNC connectors: two for the scope and one for the generator. A foldout bracket is located on the back, and there's also an opening for a small fan to keep the internal electronics cool. Unfortunately, it's not very quiet; it blows continuously at full power.

First Impressions

After pressing the power button, the DPOS350P boots up very quickly, in 2 to 3 seconds. The display is bright and high-contrast, and easy to read even at an angle. The touchscreen is responsive and accurate; according to Fnirsi, the screen is even coated with tempered glass for extra scratch resistance. The main part of the display is reserved to show the measured signals. All controls are located on three sides. At the bottom are the main control fields for the two input channels, time base, sampling rate, triggering, and generator. Tapping any of these fields opens a panel with all the options for that function.
 

On the right are a number of frequently used buttons, such as auto-setup, sensitivity, and start/stop. At the top are some general settings, such as the System and Function menus. There's also a menu for setting measurement parameters, which can be displayed at the bottom. You can also enable measurement values for an X and Y cursor, save the displayed signal waveform as data or a screenshot, and use a browser to view or edit all saved signals.

Oscilloscope

The sampling rate of the oscilloscope section is 1 GSamples/s with an input bandwidth of 350 MHz. This is quite high for a device in this price range. I didn't attempt to check the bandwidth, as my function generator only goes up to 60 MHz (according to some testers online, it easily reaches that bandwidth). In dual-channel operation, the input channels are automatically limited to 150 MHz, suggesting the A/D converter is actually operating at 500 Msamples/s (which isn't bad at this price, by the way). The scope responds quickly to all kinds of input signals and produces a very sharp image. The auto-setup function almost always finds the correct settings quickly, so I rarely had to adjust anything myself.
 

A handy feature is the zoom option (top right of the screen). The measured signal then appears at the top of the screen, allowing you to select a portion of it, which is displayed enlarged below. This can be done for one or two channels.

Unfortunately, the trigger function is somewhat limited. You can only choose between a rising or falling edge, which could be expanded.

In the Acquisition menu, there's an option to increase the 8-bit resolution of the A/D converter to a maximum of 16 bits. This is most likely increased in software by averaging multiple samples. You can also enable an FFT display for each input, but due to the lack of a scale, it doesn't provide much information.

Signal Generator

The function generator inside the DPOS350P is a significant improvement over previous Fnirsi models with a built-in generator. It delivers clean, high-resolution signals. A total of 14 pre-programmed waveforms are available, and the signal is also perfectly symmetrical (max. +/-2.5 V), with the offset adjustable separately. It also has the ability to display waveforms saved with the Save-wave function (up to 500 waveforms, according to the manual).

DPOS350P Frequency Response Analyzer

The DPOS350P has a Bode plot function that you can use in combination with the built-in function generator. This allows you to easily determine the frequency and phase characteristics of a filter or network (up to 50 MHz). You connect the generator output to the first scope input and to the input of the circuit to be measured; the output of the circuit is connected to the second scope input. The device does the rest!
 

It sounds very easy, and plotting is done quickly, but there are a few caveats. For example, the lowest frequency is at 100 Hz, which audio enthusiasts will certainly find disappointing. Furthermore, while you can select a start and stop frequency, that range is always divided over a logarithmic scale of three decades, which sometimes results in very strange values. The vertical scales (dB/degree) are always auto-ranging, which also produces strange results. It would be nice if the user could adjust this himself.

Scanning the selected frequency range is very fast and there's no averaging, which means you sometimes see different results on the screen with each scan. All in all, a good start, but the software still needs some fine-tuning here and there.

Spectrum Analyzer

According to the manual, the built-in spectrum analyzer has a range of 200 kHz to 350 MHz, although the display shows a range of up to 500 MHz. Fortunately, here you can adjust the vertical scale yourself, both the reference value and the attenuation per division. You can also select the start and stop frequencies, as well as the center frequency and frequency width. These settings are linked: if you adjust the frequency width, different values for the start and stop frequencies are automatically chosen. It seems to me that one of these settings would be unnecessary.
 

The maximum FFT length for the spectrum analyzer is 32k. This may sound like a lot, but the analyzer always uses this length for the entire band. If you choose a narrower band, for example, 10 to 16 MHz, there are so few samples in that range that only fairly broad lobes are visible. The lower the frequency range, the worse it gets. Therefore, the analyzer is only suitable for roughly scanning the entire frequency band up to 500 MHz. Here too is room for improvement of the software.

Conclusion

The Fnirsi DPOS350P is a very nice two-channel oscilloscope with a beautiful touchscreen, a wide input bandwidth, and an excellent function generator. The frequency response analyzer and the spectrum analyzer are extras that sound nice on paper, but aren't fully developed in practice. Fun to play with, but not really useful. Nevertheless, the two basic functions of the DPOS350P - the scope and the generator - are so good that this device is definitely worth €270. And if Fnirsi were to invest a little more time in the software development of the two analyzers, this device would become a fantastic combination of four handy measurement instruments!