What is the difference between signal generator and vector signal generator ?  - Page 1
Forum section: electronics / beginners

author: electronic_guy
author_rank: Regular Contributor
timestamp: December 26, 2023, 05:38:27 pm
content: Hi,I need to know the general difference between a signal generator such as Agilent 33220A and vector signal generator like Agilent E4438C. Which applications require each of them ? I understand that I might be asking an obviously trivial question. But I'm just wondering if I need to add a vector signal generator to my lab. I already got a CMU200 with a good signal generator and also an Agilent 33220A. Still don't get the idea of "Vector" in a vector signal generator.Thanks.

author: G0HZU
author_rank: Super Contributor
timestamp: December 26, 2023, 06:38:08 pm
content: Probably the first thing to do is check the options fitted to your R&S CMU200 test set. If it has option B17 it supports vector modulation with IQ bandwidth of about 2.5MHz.If you go back 20-30 years or more, a regular RF sig gen would support AM modulation and FM (and sometimes PM) modulation. The sig gen needed to have carefully designed hardware to support each mode. i.e. a decent AM modulator and a decent FM modulator. It might also support pulse modulation via the AM modulator or it might have some other type of pulse modulator hardware fitted.A vector modulator is a bit different. It's sometimes called an I and Q modulator (in phase and quadrature) and here is why...If you can accurately generate an 'I' (in phase) signal and also a 'Q' signal (in quadrature = 90 degrees phase shifted) and feed these to a vector modulator then in theory at least, you can generate 'any' type of modulation within the bandwidth of the IQ modulator.All you have to do is change the amplitude of the I and Q inputs dynamically to create the chosen modulation. It can do AM, FM, PM, and lots of digital modes including FSK, PSK, CDMA and all the other fancy modes. It can even mimic genuine GPS signals from numerous satellites as long as you can generate the I and Q signals... ...and that's where reality kicks in... you usually require some sort of computer generated data to accurately produce your wanted I and Q waveforms.Otherwise you will just get jibberish out of the modulator. Some vector sig gens include a dual arbitrary waveform generator that can generate IQ waveforms. This allows the sig gen to generate multiple carriers (up to hundreds of carriers!), or maybe noise or even human speech. For example, it would be possible to store single sideband transmissions with a human voice and play it back as a SSB signal from the sig gen.Within reason, anything is possible within the bandwidth of the IQ modulator hardware. The spurious performance is limited by the quality of the vector modulator and how well it is calibrated. Usually, it's best to run the internal calibration routine before using it. Otherwise, there could be degraded performance in terms of carrier leakage or waveform accuracy.Vector sig gens have been around for quite a while now and they are slowly becoming the norm. That doesn't mean you need to buy one though.

author: G0HZU
author_rank: Super Contributor
timestamp: December 26, 2023, 07:01:45 pm
content: The E4438C is quite an old generator now, but it did have lots of hardware of software options available. Fully loaded, one of these would have been very expensive 15-20 years ago.It might be cheaper and more realistic to buy a sig gen (with minimal options fitted) that supports vector modulation and then use something like a Siglent dual-channel function/arbitrary waveform generator to generate the IQ data. Usually, a vector sig gen will have BNC input connectors for I and Q. In this case it would be possible to generate the IQ data using a PC and then transfer it to the Siglent as a data file. Then connect the output channels of the Siglent to the IQ inputs of the vector sig gen (making sure you don't overdrive and damage the sig gen inputs)I can only guess if this will work though as I don't own a Siglent dual-channel arbitrary waveform generator. It is tempting to buy one at some point. I have five vector sig gens here and two of them already have built in arbitrary waveform generators. However, it would be nice (and very educational) to be able to use an external IQ waveform generator with the vector sig gens at some point though...Hopefully, someone will have already done this with a vector sig gen and the Siglent and they can describe it in more detail.

author: G0HZU
author_rank: Super Contributor
timestamp: December 26, 2023, 07:44:41 pm
content: Try these excellent tutorials on IQ (vector) modulation by W2AEW on youtube.Once you have watched the tutorials above, try watching this practical demonstration of an IQ modulator by W2AEW.Ultimately, few of us really need a vector sig gen. I mostly use mine to generate noise (over a chosen bandwidth) or to generate multiple carriers. It can be interesting to generate digital modulation using a vector sig gen. It's also possible to grab offair signals using a real-time spectrum analyser and then convert the data to a format that the vector sig gen can accept. It's then possible to regenerate a copy of the offair signal using the vector sig gen.The offair signal could be a radio station, or a fob transmitter or an RFID device. The vector sig gen can be used to clone the signals from these. Some vector sig gens can hold and store the IQ data in memory and play it back to the vector modulator via its arbitrary waveform generator. Alternatively, the IQ data would have to be sent to its IQ inputs externally.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 28, 2023, 12:34:17 am
content: Quote from: G0HZU on December 26, 2023, 07:44:41 pmi'm just learning this new technology... looks good in theory but my confusion is... if we receive a seemingly a single sine wave on a receiver (red trace below), how we extract the accurate sine (I) and cosine (Q) component? it can be anything! unless the "phase" is synchronized between sender and receiver... we cant rely on unit magnitude alone can we? if radio travels at a distance, the magnitude will be attenuated by much already.. i'm refering to this section of video starting at about minute 7:00 or maybe i have some misunderstanding or wrong idea about this...

author: gf
author_rank: Super Contributor
timestamp: December 28, 2023, 01:13:29 am
content: Yes, for the demodulation of QAM, etc., the receiver needs to recover the carrier.Seehttps://en.m.wikipedia.org/wiki/Carrier_recovery

author: baldurn
author_rank: Regular Contributor
timestamp: December 28, 2023, 02:01:57 am
content: Quote from: Mechatrommer on December 28, 2023, 12:34:17 amConsider the simplified case of detecting a single frequency (CW) by sampling at exactly the double frequency. For example a sine of 1 kHz and a sample rate of 2 kHz. If we use just one sampling channel (not IQ) we could be lucky to sample in phase and get a constant value. Or we could be unlucky to be sampling in the exact phase where we hit zero on each sample.Turns out that by sampling twice with 90 degree phase delay, and plot the two values in a circle, the distance for the point to (0,0) will always be the "lucky" number and the phase angel will tell how much out of phase we are currently sampling. If our sampling frequency is slightly off, then the phase angle will slowly change and the speed of change is how much we are off in our frequency.You are right that typically the I and Q will not be recovered exactly as they were. But you can do the math and correct for this. Or in some cases it doesn't matter. If we would be using the above algorithm to recover phase modulation, we wouldn't need the phase to be the same, as we are working with changes in phase. You will start by sending a start bit and when received, whatever phase that turns out to be, is your reference.

author: langwadt
author_rank: Super Contributor
timestamp: December 28, 2023, 02:36:40 am
content: Quote from: Mechatrommer on December 28, 2023, 12:34:17 amif your phase is off you coordinate system is rotated, if you frequency is off your coordinate system is rotating so they are interlinkedif you amplitude changes the "radius" of your coordinate system changesif you want to send/receive bits you always have to synchronism somehow to find the bits, like start bits, Manchester code, etc

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 28, 2023, 04:44:00 am
content: Quote from: gf on December 28, 2023, 01:13:29 ambut I and Q combined is the carrier right?Quote from: baldurn on December 28, 2023, 02:01:57 ambecause the combined signal's phase will lie between I's phase (when Q's magnitude = 0) and Q's phase (when I's magnitude = 0). without phase locked, receiver can randomly assume the position of received carrier (combined IQ) wrt I's and Q's phase... hence we'll get random magnitude of sin and cos... i see this as "under deterministic" problem...anyway, maybe not all covered in videos above, i expect some sort of "preamble" or known pattern at every beginning of comm signal to lock receiver's clock. or sort of colorburst or pilot tone described inhttps://en.wikipedia.org/wiki/Quadrature_amplitude_modulationmaybe i need to read more later, interesting subject... fwiw...

author: gf
author_rank: Super Contributor
timestamp: December 28, 2023, 04:06:07 pm
content: Quote from: Mechatrommer on December 28, 2023, 04:44:00 amNo. The "carrier" is a CW signal (i.e. constant frequency, amplitude and phase) which is then modulated by a modulating signal in order to obtain the modulated signal.With quadrature modulation, the carrier (at 0° phase) iscos(2*pi*fc*t), I(t) and Q(t) are the two components of the modulating signal, and the modulated signal isI(t)*cos(2*pi*fc*t) - Q(t)*sin(2*pi*fc*t), where fc is the carrier frequency and t is time. IOW, the I(t) & Q(t) signals are the instantaneous amplitudes of the cosine and sine components of the modulated signal.See here for more details:https://wiki.gnuradio.org/index.php?title=IQ_Complex_TutorialThis tutorial also considers the complex-valued interpretation of IQ signals.QuoteFor example.Also search for "Costas Loop".

author: baldurn
author_rank: Regular Contributor
timestamp: December 28, 2023, 06:39:29 pm
content: Quote from: Mechatrommer on December 28, 2023, 04:44:00 amQAM and other modern digital modulation will work like that. But IQ can be used to demodulate any modulation including FM, AM and SSB. Usually you can use a FIR filter to do the work of recovering the voice signal from the IQ signal.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 28, 2023, 08:14:16 pm
content: i watched a video but i have a suspicion that in order to lock phase, known pattern must be sent first, esp in case of analog IQ... because analog IQ can be anywhere in the quadrants.. yes?

author: ejeffrey
author_rank: Super Contributor
timestamp: December 28, 2023, 08:41:38 pm
content: Quote from: Mechatrommer on December 28, 2023, 04:44:00 amNo. In the simplest case, where the LO is equal to the carrier, I is the carrier basically by definition, and Q is a phase shifted version of the carrier. In other situations, you might set the synthesizer LO frequency outside the signal band entirely, and use the IQ inputs to generate one or more carriers and their sidebands.Yes, you need a phase reference for it to be meaningful. Normally this isn't a problem because normally we don't communicate with pure sine waves, since they carry no information. A realistic signal has a carrier plus sidebands. The sidebands contain the information, and their phase and possibly amplitude are defined relative to the carrier which acts as a reference. If the carrier is present in the signal, as it is for traditional AM and FM, as well as many other modulation techniques, that's easily accomplished. For carrier suppressed or spread spectrum signalling it's necessary to find some other way to reconstruct the carrier from the signal. This does usually put a constraint on the signal, or require it to contain redundant information. A simple example is carrier suppressed double sideband AM signalling. The upper and lower sidebands are redundant, and their relative phase can be used to infer the carrier. A more sophisticated way is to require that the long term average of the signal has zero offset from the carrier with a specified time constant. This can be achieved by scrambling the signal before sending it to the modulator.QuoteThat's another way to do carrier recovery.

author: gf
author_rank: Super Contributor
timestamp: December 29, 2023, 07:43:09 am
content: Quote from: Mechatrommer on December 28, 2023, 08:14:16 pmThe quadrant/sector is still ambiguous. But you can do the PSK with differential encoding, then you don't need an absolute phase reference (i.e. for each symbol, you transmit the phase difference to the previous symbol).

author: G0HZU
author_rank: Super Contributor
timestamp: December 29, 2023, 04:34:25 pm
content: If it helps, it should be possible to send several different modulations at the same time. This could be analogue or digital or both. You could transmit several radio station signals spaced apart by however much you choose. eg simulataneously send a USB voice transmission at 9.005MHz and also an AM voice signal at 9.07MHz and an FM voice signal at 9.12MHz and an FSK signal at 9.17MHz. They could be spaced apart much further than this, but it would require lots of memory in the vector sig gen or you would have to send the IQ signal to the vector sig gen externally in real time.A multimode receiver could then be tuned across this frequency range to listen to each transmission using the appropriate mode. To the receiver it would look like four different radio stations but in reality it is just one vector sig gen producing the four different signals in parallel.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 31, 2023, 03:41:10 pm
content: The incoming vector modulated signal is demodulated with a pair of mixers operating with LOs 90' apart. That gives you the original I and Q components.Check this link for an explantion of how the orginal data is derived from the IQ components:www.ece.unb.ca/cgi-bin/tervo/qam.plAn application specific algorithm will validate the recieved data stream in firmware or code.