Video: Tutorial on IQ Signals and IQ modulation/demodulation - Page 1
Forum section: electronics / beginners

author: w2aew
author_rank: Super Contributor
timestamp: September 03, 2014, 01:03:57 am
content: This topic has been requested many times, so hopefully this overview and tutorial helps to shed a little light on the subject:

author: Bukurat
author_rank: Regular Contributor
timestamp: September 03, 2014, 01:33:46 am
content: Very interesting.

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2014, 02:02:12 am
content: OK, I watched the whole video, and there were two things I didn't follow:1. When you add two sine waves of the same frequency, the resulting output wave has the same frequency as the inputs. Therefore, how can you modulate the frequency of the output without changing the frequency of the inputs?2. How were you able to multiply a sine wave by a discrete 1/-1 signal using a mixer?As someone unfamiliar with the field, I was unable to fill in those gaps using existing or prior knowledge.

author: w2aew
author_rank: Super Contributor
timestamp: September 03, 2014, 02:11:31 am
content: Quote from: IanB on September 03, 2014, 02:02:12 amFrequency deviation is accomplished by sinusoidal variation of I and Q, effectively rotating the vector in the phasor diagram, similar to the second hand on an analog clock. Frequency shift is equal to the rotational speed of the I/Q vector. Another way to think about this... Continuous sinusoidal variation of I and Q results in the sum's phase being shifted by some delta amount for each carrier cycle.QuoteThe mixer is a balanced modulator, thus can multiply by positive and negative values. Check out my video on the diode ring mixer for some of the basics. Similar thing at play here.

author: Skimask
author_rank: Super Contributor
timestamp: September 03, 2014, 02:39:00 am
content: How would you keep the TX and RX L/O's synchronized? Or, at the exact same frequency?Seems to me if one drifted off the other by more than 1/2 of the smallest phase shift possible/used, everything would get out of sync and useless...

author: w2aew
author_rank: Super Contributor
timestamp: September 03, 2014, 02:43:04 am
content: Quote from: Skimask on September 03, 2014, 02:39:00 amSmall frequency differences can (and is) easily be taken into account in the IQ processing in the receiver.

author: Skimask
author_rank: Super Contributor
timestamp: September 03, 2014, 02:45:31 am
content: Quote from: w2aew on September 03, 2014, 02:43:04 amYes, makes sense. (Forgot the topic was kinda heading more towards software SDR rather than hardware only solutions)

author: coppice
author_rank: Super Contributor
timestamp: September 03, 2014, 03:46:13 am
content: Quote from: Skimask on September 03, 2014, 02:39:00 amIn the general case you don't want the LOs to be at the same frequency. As soon as one of the parties moves, Doppler means the frequency of the received signal is no longer the same as the LO at the transmitter. If the maximum frequency offset is guaranteed to be small, and the signal is differentially encoded, a simple modulation like QPSK might let you get away without doing a proper received carrier recovery - although the differential encoding doubles the bit error rate. Pretty much anything more advanced demands a proper recovery of both the frequency and phase of the carrier at the receive end.

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2014, 04:48:43 am
content: Quote from: w2aew on September 03, 2014, 02:11:31 amAh, I see. I did an experiment to see how this works out (see illustration below). And then I realized this follows immediately from trigonometry

author: coppice
author_rank: Super Contributor
timestamp: September 03, 2014, 01:33:50 pm
content: I think a lot of people will look at this video and immediately see that something is amiss with the waveforms - i.e. the kinks. I know this is an introductory video, but I think some brief mention of the need for bandwidth control in practical QPSK modulation would have helped the astute observer realise the presentation is not bogus.

author: w2aew
author_rank: Super Contributor
timestamp: September 03, 2014, 03:24:51 pm
content: Quote from: coppice on September 03, 2014, 01:33:50 pmI purposely made the phase transitions very fast so that they are obvious to see. If they were filtered as they would be in the real application, the phase change wouldn't be so obvious. I did make a text annotation in the video that talks about this. I'm assuming that the viewer is savvy enough to understand that these baseband signals would be filtered in the "real world".

author: coppice
author_rank: Super Contributor
timestamp: September 03, 2014, 04:50:21 pm
content: Quote from: w2aew on September 03, 2014, 03:24:51 pmMany people go through entire digital comms courses and still fail to grasp the idea of excess bandwidth. In fact, a lot of well know digital comms books try to avoid the issue entirely, and only cover things like TED algorithms that could never work with real world amounts of excess bandwidth.

author: MattSR
author_rank: Regular Contributor
timestamp: September 03, 2014, 10:39:36 pm
content: Thank you for the video w2aew. Complex numbers can be tricky to grasp but they are essential building blocks in signal processing.