Review of Fosc-21 (Focus Oscilloscopes) – a $20 USB Oscilloscope from ebay

by kacang bawang

Fosc-21 usb oscilloscope

Sometimes we make a bad spontaneous purchase. It must have happened to everyone at least once. This is the story of one such purchase.

Without doing my research I clicked ‘buy’ on a really cheap Chinese USB oscilloscope from ebay – Focus Oscilloscopes 21 (Fosc-21). The crappy-ness of this “oscilloscope” is matched only by its “software”. Instead of kicking myself, I decided to extract the maximum good that can possibly be extracted from this experience by writing a “review”. Hopefully, some poor soul about to buy this atrocity will find this post first.

What all is included?

The oscilloscope comes with just a USB cable and two BNC connectors – no probes are included, because you guessed it – probes are expensive too. I picked up a pair of cheapies:

cheap oscilloscope probes

I forgot where I read it, but someone had said that they are actually decent for the price. While I cannot confirm or deny their electrical qualities, the construction feels solid.

What about a manual?

Tough luck. The “manual” that comes with it is but a couple of pages of “stating the obvious”. On the plus side it was in English, not that it helped me any. The manufacturer does have a website, but all you can find there is a copy of the same thing that was on the CD. This is true of the software as well. To “learn” how to use it you just have to try every button/icon/menu and see what it does for yourself. Spoiler: some of it doesn’t work no matter how you try it.

I tried searching youtube for some videos explaining how to use the Fosc-21, but all I found was a 30 second clip with no sound and two comments complaining that there are no instructions and that the damn thing doesn’t work (sounds familiar?).

How does it connect to the PC?

The scope is nothing but a serial over USB device, and its drivers are that of Prolific PL2303. Neither the version that came with Win8, nor the older version that was included on the CD worked for me. I ended up downloading the latest version (installer 1.11, version from Prolific. That one ended up working, but required a reboot. I fucking hate rebooting, no I really do!

What about the software/interface?

This is where this product really shines dulls. Let me illustrate with a screenshot. Here is the fine software package called FocusOscillograph359.

FocusOscillograph359 screenshot

What you see is what you get. You cannot adjust the vertical position of the graph, and you cannot zoom in. You can adjust the horizontal scale, but it lags while doing so, and the “center” position jumps somewhere off-screen. You thought you’d just drag the spectrum with mouse button and zoom in at cursor position with wheel? Hahaha. It is able to measure horizontally, but it will take you some trying to find out how. Vertical measurement – no can do. It also comes with a helper program called “on screen ruler”, which does just that – it looks like a ruler. Is that how they originally wanted us to measure?

Oh, and by the way – triggers don’t work. The data feed freezes once you set one. Before you ask – no it’s not instantly going off, because it doesn’t matter what level it is set to. Oh, and have fun figuring out what value that is. The screen cap button captures a quarter of the window (SCRATCH HEAD) and is able to save as .bmp only.

On the plus side – it is able to draw two channels simultaneously, and even has a nifty “live” spectrum (frequency domain) mode, but unfortunately the display is too small to really be of any use.

FocusOscillograph359 screenshot

How’s the resolution?

Sampling rate is claimed at 4/8 KHz. That’s about 0.2 ms. This means that the highest frequency you will be able to capture to display on the screen is 400/800Hz. Recall that the middle C note is 440Hz, so that’s approximately the kind of signals you will be able to look at with this thing. The software has larger ambitions however, and lets you zoom in to 1 ns 🙂 Here’s what UART at 115200 baud looks like:

FocusOscillograph359 screenshot of uart 115200


Don’t throw away your money. Compared to a real oscilloscope – it is what a magnifying glass is to a microscope. It does let you look at signals, but just barely, and the software is fucking unusable. However if you really need a scope and have nothing else, it may help you in a pinch.