Computing stuff tied to the physical world

RF Shielding

In Hardware on Mar 24, 2012 at 00:01

As shown in yesterday’s post, once you’re looking at signals in the high MHz range, it’s easy to make mistakes. Looking at that screen shot again, you can see a whole bunch of 90..100 MHz spikes:


These are in fact local FM radio stations – being picked up by my clunky scope probe hookup to the AWG. In other words it’s an antenna: radiated RF signals are accidentally being received by the probe and mixed in with the conducted 25 MHz signal. The way to get rid of these is to use “shielding” to keep those radio waves out.

Here’s the same signal, using a 50 Ω coax cable all the way between signal generator and oscilloscope:


No more weird stuff, just the 25 MHz multiples – indicating that there’s a slight distortion in the generated 25 MHz sine wave. This is normal for any signal generated by a direct digital synthesizer, because the waveform is created through through a digital-to-analog converter, fed at 125 million samples per second in this case. IOW, it’s an approximated sine wave (20 datapoints per sine wave @ 14-bit resolution).

So the big change is that the FM radio stations are gone, and that the signal’s noise level is now in fact a few dB cleaner than before – as you can see from the slightly lower tail towards 200 MHz.

I did have to use a small trick to make these graphs comparable: the second one has the scope’s 50 Ω internal terminator enabled, so that the path from signal source to signal destination is now done by the book: a 50 Ω source (in the AWG), feeding a 50 Ω coax cable, terminated by 50 Ω at the destination (in the scope). This does “attenuate” (i.e. reduce) the signal level by half, so I had to raise the baseline by 3 dB on the second FFT screen shot to make the height of the 25 MHz peak identical in both screens.

One other minor difference is that the second graph is smoothed over 256 samples i.s.o. 64 – cleaning up the resulting line slightly more.

So you see… it’s possible to do RF-type stuff without understanding all the details – which I certainly don’t, yet – and get decent results. The 25 MHz wave coming into the scope is very clean: the first harmonic is some 50 dB below the signal itself, which means that the first harmonic has 100,000 times less energy than the main signal.

Tomorrow, another post about this topic: cables, termination, and noise…

Update – As John Beale pointed out in the comments below, the FFT baseline is caused by the choice of FFT windowing function. Here’s the 50 Ω coax example again, using a Hanning window:


Much better for comparing relative dB differences between peaks.

(Tomorrow’s post will also use the default Rectangle window, sorry about that…)

  1. *sigh*

    Missing ‘lopik’…

  2. I don’t know if your analyzer (o-scope?) gives you a choice, but the overall shape of the spectrum display (especially the “shoulders” around a peak) is affected by the choice of windowing function used before taking the FFT, for example there is Hamming, Gaussian, Lanczos etc. See also

    • Aha, thank you! I don’t know anything about FFT windowing, so I left it at the scope’s default (Rectangle). I’ve updated the post with an example of a Hanning window – much nicer in this context, indeed!).

  3. Correct me if I’m wrong, but even when your signal is measured in dBm all deltas will be in dB. So “the first harmonic is some 50 dB below”.

  4. The Hanning window (or confusingly also the Hamming window) cleans up the display shown in the last graphic well with little impact from reduction in sensitivity to the higher harmonics.

    A small DC offset in the test signal can be implied (probably some slight mis-alignment of the output driver) – switching the ‘scope to AC coupling should show a reduction in the even harmonics displayed, without disturbing the validity of the observations.

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