Rigol DS1104Z measuring 435MHz signal - Page 1
Forum section: electronics / testgear

author: frog7227
author_rank: Newbie
timestamp: January 07, 2016, 04:32:45 am
content: Hello All,I realize that thisshouldn'tbe possible, but while measuring the output of a car key less entry fob, I managed to read the output signal while looking for the modulation pin and I got the fully modulated carrier. I looked up the FCC id and it happens to broadcast at 433.92 MHz which is quite close to the measured 435 MHz. I was probing with the Rigol RP2200 probe in x10 mode, and this was done on a hacked DS1054Z BTW. I've attached the screenshots of the waveform as well. Can this be a problem that it is so close the the nyquist bandwidth that it could be under sampled? Also due to the high frequency of the signal it bugged up the FFT menu as well (20GSa/s??!).Here is the FCC page for the key-fob ("Ultra" brand 4 button remote)fcc.io/mkytxpt4gHere is a .7z of the .wfm file from the scope.https://dl.dropboxusercontent.com/u/151717184/Zip%20files/Waveform_Data_435MHz.7zThanks

author: CustomEngineerer
author_rank: Frequent Contributor
timestamp: January 07, 2016, 04:43:07 am
content: Any idea what the voltage on the pin that you probed is supposed to be?

author: frog7227
author_rank: Newbie
timestamp: January 07, 2016, 04:49:38 am
content: Quote from: CustomEngineerer on January 07, 2016, 04:43:07 amI don't have anything that can measure the true voltage, or have any idea what it is supposed to be, however it is powered by a single 12v 23a battery. This remote usually gets like ~50-60ft range to the car with the receiver so I do think it is being attenuated heavily, as the output voltage swing really isn't enough to transmit effectively IMO.

author: tautech
author_rank: Super Contributor
timestamp: January 07, 2016, 05:57:02 am
content: Quote from: frog7227 on January 07, 2016, 04:49:38 amHow could you know?The -3dB BW of these units is ~125 MHz so at 3 times that frequency the roll-off will be so horrendous and give NO possible indication of the true amplitude of the signal.

author: TheSteve
author_rank: Unknown
timestamp: January 07, 2016, 07:54:21 am
content: I can get a similar waveform if I probe a generated 435 MHz signal at a level of 10 dBm - however the signal is actually stronger with the probe in 1X mode - the ground of the probe is more like an antenna. The scope is filling in a lot of missed sample holes - check out your waveform in dot mode and experiment with sample memory.End of the day though it is giving you a rough idea that there is a signal being generated but you'll have no idea of the exact frequency, signal level or the modulation. Touching the probe to the circuit will also give it an impedance mismatch that could cause erroneous emissions, change of frequency etc.It is useful in this specific case to tell you a signal is present but can't really be relied on at all.

author: Fungus
author_rank: Super Contributor
timestamp: January 07, 2016, 11:41:19 am
content: Quote from: frog7227 on January 07, 2016, 04:32:45 amWhy not?You've got 1 GS/s sampling rate so your nyquist frequency is 500MHz. You can see signals up to that frequency.The thing youcan'tdo is measure their amplitude at that frequency. They will be very attenuated.

author: frog7227
author_rank: Newbie
timestamp: January 07, 2016, 01:06:23 pm
content: Quote from: tautech on January 07, 2016, 05:57:02 amI was going off of that there would need to be more signal swing for the modulator (I was probing the output of the PA/oscillator for the 433.92MHz; pic attached) as it’s going to have a small loss and then it would still need a few mW of power for the actual signal leaving the remote.Quote from: Fungus on January 07, 2016, 11:41:19 amI was thinking it wouldn't be possible because of the attenuation by the frontend, but it must roll off then go back up near this frequency (or much before)Also, this morning I had a little extra time so I used a 50? terminator + a bit of coax and I got the same thing with a different amplitude, hopefully with less reflections & impedance transformations, but still the amplitude will be useless. I think all the remote is doing is loading the output to adjust the amplitude to send the code. (at a quick glance at least) (I just noticed I was on DC coupling not AC this time.I can redo later if anyone wants me to.)

author: Mark_O
author_rank: Frequent Contributor
timestamp: January 07, 2016, 01:42:05 pm
content: Quote from: frog7227 on January 07, 2016, 04:49:38 amLooking at the FFT, I just think it's impressive that you managed to get the DS1104Z to sample at 20 GSa/s, and generate a 7 GHz sweep.And right down there around your 400 MHz is a tiny blip.