Computing stuff tied to the physical world

Noise levels

In Hardware on Mar 25, 2012 at 00:01

Triggered by the recent signal generator checks, and those FM radio stations creeping into the signal yesterday, I wanted to do another test to see how and when this happens, using a series of scope FFT snapshots.

Here’s a 50 Ω coax cable of 2m length hooked up, with 50 Ω termination on both sides but no signal. The scale is 5 dBm per division, and I’ve zoomed into this very low range with a baseline of about -105 dB. The following 200 MHz wide FFT measurements were all done with the scope input set to max sensitivity, i.e. 1 mV/div:

SCR35

Note the slight FM radio station RF signal pickup, even with a fully terminated coax cable!

Same thing, but disconnected on one end, i.e. only one 50 Ω terminator inside the scope:

SCR36

I don’t know why there’s a peak at 25 MHz – the signal generator was completely powered down.

Here’s the spectrum with no coax at all, i.e. nothing connected to the scope, but its 50 Ω shunt still enabled:

SCR37

When also adding an external 50 Ω terminator, the lower frequencies drop ever so slightly further:

SCR38

And here’s what happens when the 2m 50 Ω coax cable is attached back on, without the 50 Ω termination:

SCR39

As you can see, the coax cable now acts as antenna, picking up a few more signals at 38.45 MHz and 46.38 MHz. And FM reception shooting up to 20 dB above the noise floor. Even though it’s shielded!

The slight drop in noise across the screen from 0 to 200 MHz is probably nothing more or less than the scope’s bandwidth: a 200 MHz scope is specified as having a 3 dB drop at 200 MHz, which fits amazingly well with what all the above screen shots are showing.

These tests confirm the superb signal processing specs of the Hameg oscilloscope front end: a -105 dB noise floor @ 1 mV/div maximum sensitivity. For even lower noise levels (and a higher frequency range, as would be needed for 868 MHz and 2.4 GHz RF measurements), probably only a “real” spectrum analyzer will do better.

Whee… with a multi-meter probe wire attached as antenna, I can easily pick up all major AM radio stations:

SCR45

Tomorrow, I’ll close off with one more post about signal processing: accurate frequency measurements.

Update – As with yesterday’s post, these FFT’s were produced with the Rectangle window function. As a bonus, here’s the frequency spectrum produced by the noise generator in my new AWG:

SCR60

Down by 30 dB at 50 MHz, but a pretty good source of white noise at lower frequencies. The AWG can add an adjustable amount of this noise to the generated waveforms – can be useful to see how well a filter, demodulator, or other detector behaves, for example.

  1. Ah – the wonderful world of RF !

    These results show several interesting effects – firstly, the contrast between ideal and practical. An ideal coaxial transmission line would show no pickup when terminated correctly by Z0. However, if you look at how a typical coaxial cable is made, the outer conductive cylinder is only approximated by a mesh of fine copper wires. The gaps and finite resistance of the copper allow some energy to “leak” through. Higher-priced cable will use double woven shielding and/or a metal foil to improve on this.

    Note that this effect is symmetrical – external RF fields will leak in and/or RF energy passing through the cable will leak out. The latter is put to good use in long road tunnels. These are good shields against external RF transmissions – effectively blocking FM station reception. A simple broadband amplifier collects these transmissions at a tunnel opening and drives a long coaxial cable running inside the tunnel. This creates a distributed aerial that “leaks” a replacement signal the entire length of the tunnel – important for traffic safety announcements (TA).

    The second effect is that any length of transmission line acts as a frequency-dependent impedance transformer, even at extreme values. For example, when this ‘2m’ cable was left “open” – effectively an infinite termination – when viewed from the ‘scope end, its impedance actual sweeps through ∞ to 0 to -∞ as the frequency changes. Making some assumptions about the type of co-ax used, at ~ 6.5 Mhz the open circuit is transformed into a pure resistive 50Ω, matching the characteristic line impedance exactly. This effect repeats at each additional lambda that “fits” the cable length/characteristics, so in this case the open circuit transforms to an exact match every ~ 13Mhz, viz. 6.5, 19.5, 32.5 …. Mhz.

    Why is that relevant? Well, the open-ended coax is certainly acting as an aerial for RF fields in the vicinity, but the clarity of the screen shots fools you into thinking this is a true snapshot of the environment and that you can make accurate relative measurements of the strength of various signals.

    Unfortunately not – the frequency dependent behaviour of the coax cable is acting as a hidden, variable attenuator with gain lobes repeating every ~ 13Mhz. A simple check of this effect is to sample the same environment with a different (non-integral multiple) length of the same cable – the absolute levels of detected interferers will change.

            At RF, what you see is often not what you get

Comments are closed.