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:
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:
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:
When also adding an external 50 Ω terminator, the lower frequencies drop ever so slightly further:
And here’s what happens when the 2m 50 Ω coax cable is attached back on, without the 50 Ω termination:
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:
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:
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.