Understanding FFT (Fast Fourier Transform)

If your old oscilloscope broke down and deemed beyond repair, you probably want to replace it. You will find out that modern DSO’s (Digital Storage Oscilloscopes) can do much more than classic, cathode-ray models. One of the many new and exciting functions is FFT, a feature I immediately checked out when I purchased my Atten ADS-1102CAL. A review of my model can be found here.

FFT adds an extra dimension to the standard time domain: the frequency domain. All oscilloscope users are familiar with the time domain; just feed a waveform into your scope and the display will show shape and voltage over time:

ADS00003The frequency domain on the other hand shows what voltage is present at each frequency and will produce a screen we know from the spectrum analyzer:

GT-3-UHF2Some measurements which are very hard in the time domain are very easy in the frequency domain. Consider the measurement of harmonic distortion. It’s hard to quantify the distortion of a sine wave by looking at the signal on an oscilloscope. When the same signal is displayed on a spectrum analyzer, the harmonic frequencies and amplitudes are displayed with pinpoint precision.

Another example is noise analysis. Looking at an amplifier’s output noise on an oscilloscope basically measures just the total noise amplitude. On a spectrum analyzer, the noise as a function of frequency is displayed. It may be that the amplifier has a problem only over certain frequency ranges. In the time domain it would be impossible to tell.

Most modern DSO’s, even the affordable models made by Atten and Rigol, are capable displaying both the time- and the frequency domain, and can do so at the same time. The time domain will be displayed at the upper half of the screen, the frequency domain at the lower half of the screen.

Atten-FFTWhile FFT on a DSO is not as accurate as a real spectrum analyzer (especially when reaching the upper limit of the scope’s bandwidth), it will give you a reasonably good idea of what’s happening.

How FFT works
If you’re new to FFT, your head might explode while trying to get a grip on it. I still struggle with it.

Fourier’s theorem: any waveform in the time domain can be represented by the weighted sum of sines and cosines. The FFT software samples the input signal, computes the magnitude of its sine and cosine components, and displays the spectrum of these measured frequency components.

Got it in one go? I didn’t when I tried it for the first time. Yesterday I ran into this website, which will be of some help.

fourierYou can see this in a simulation when you click here: http://hascanvas.com/fftvisualize2

The guys from Tektronix made a nice YouTube video which explains a lot too.


Note
: do not connect your FFT capable DSO to whatever transceiver without proper external attenuation. Check the manual for maximum input level (often referred to as ‘damage level’) before measuring.

Feed the DSO with as little power as you can get away with. This will prevent overloading the DSO’s circuits and will result in more accurate measurements.

Have fun!

Google+ community

While I’m finishing up the Puxing/Wouxun review I started a Google+ communityGoogle+ to discuss Chinese ham-related products. These could be (but not limited to) radios, test equipment, accessories, kits, spare parts or components.

I also plan to invite manufacturers and sellers to join, which will give members the opportunity to give feedback and ask questions.

There’s a fine line between information and SPAM; this will defined in more detail later in time.

 

Fixing the “Wouxun Memory Loss” bug

Remember the various posts about Wouxun handhelds losing memory contents? Although the chance of encountering this problem seems to be remote, it does happen. A small component (a 24C64 serial memory IC) can give up in time.

The good news: it can be repaired/replaced at almost no cost (about 60 cents). The bad news: you need to be comfortable with working on small components and must have the tools to do it. If you don’t, have it done by someone with more experience.

Wouxun Memory

The complete repair guide can be read in detail on Mike Mercury’s, which is the website of Tim N8NQH.

Link to a Chinese eBay seller (10 pieces for $2.38): http://www.ebay.nl/itm/290951221455

 

Building a legacy PC

Sony playerIt’s amazing to find out how many (ham-related) applications still rely on legacy interfaces (parallel, serial) and legacy operating systems such as DOS, Windows 98 or XP. Windows XP has been a reliable partner here in the home for programming radios, transferring songs to certain older audio devices I still own and use.

I have two Sony Net MD players for example, and their exotic Vaio Pocket Music Player VGF-AP1 is still in use here also. I still use a HP Laserjet 4+ printer too, which requires a parallel port.

All fine, until your hardware starts to die. My ancient IBM NetVista desktop, tucked away somewhere in a dark corner, decided it was time to retire. First the RTC quit working (no, it was not the battery). Then one USB bridge gave way. I quickly made a backup of all my data and considered my options.

  • Option 1: use my Windows Vista notebook. Vista is somewhat unpredictable at times though, and there are no parallel or serial ports.
  • Option 2: use my Windows 7 notebook. Unfortunately Sony never bothered to support Windows 7, and the lack of legacy ports is the same.
  • Option 3: build a legacy PC with all the ancient bells & whistles I need and install a fresh Windows XP on it.

I decided to go with option 3, and made it as cheap as possible by purchasing ‘New Old Stock’ hardware.

Hardware
  • Mini ATX case,
  • Biostar N68S motherboard, AMD Sempron 3200+, 1 GB RAM,
  • Hitachi DeskStar HD 1 TB,
  • 2x serial ports, 2x parallel ports (one of which was on board already),
  • DVD writer and floppy drive (salvaged from the IBM),
  • My last, never installed ‘IT journalist give away’ version of Windows XP Professional,
  • A dirt cheap 15.6″ Asus LED wide-screen monitor instead of the old CRT,
  • New multimedia keyboard ($5) and optical mouse (free).

OLYMPUS DIGITAL CAMERAAs usual assembling a PC is easy and straightforward, but getting all the software back to work can be more challenging. Windows XP refused to install updates, and it took a few nerd/geek interventions to get it to work. After that it was a matter of patience, a lot of patience. I lost count of the number of updates and the time it took to install all of them.

Sony Stuff
Another problem I ran into is that I initially couldn’t locate my original Sony software CD. Normally that doesn’t bother me much, most companies will support their products for a long time. Sony however removed the software I needed last year. In the end I remembered where I stored the CDs.

But nothing worked. The PC didn’t recognize the Net MD, nor the VGF-AP1. After a lot of hair pulling experiments, all in vain, I tried another USB cable. Bingo. With a little bit of luck the LPT bracket and an extra PCI card with 1x LPT 2x RS232 on board will arrive today, after which this PC can be tucked away in that dark corner once proudly owned by IBM.

Video: Baofeng UV-82 charger repair

My review of the Baofeng UV-82 seems ages ago. At first I thought that the erratic / defective chargers were just bad luck, but six months later complaints are still rolling in. The fix (link) was easy — no technical knowledge required.

John Reinhart made a video of the process and posted it on Facebook, which I %$@&#* couldn’t watch without having an account. John immediately sent me the original, which I then posted on Youtube. Have fun.