Superheterodyne LO stability,power ,Mixer input impedance?

[echo47]

No, a quadrature mixer outputs one signal in complex format (sometimes called IQ format or maybe vector format).
By computing the magnitude of the complex output, you get excellent AM detection.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

But IQ format is two signals. One is Sin(wt) and other is Cos(wt).

 

[echo47]

Ok, it's just terminology, two different ways of saying the same thing.
A quadrature signal can be written as one complex signal: e^(jwt) or cos(wt)+j*sin(wt).
It is typically implemented with two wires. I think of them as two components of one signal.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

I agree with this. The problem is that any additional signal may create product(s) with other signals in receiver. This is why we need to limit number of signals inside system to minimum possible quantity. I think it is obvious. Of course if quadrature mixer provides better quality of AM detection than usual AM detector its usage may be understandable.

 

[echo47]

The I and Q waveforms have similar spectra, so there's no new potentially interfering frequencies to worry about, but there is more circuitry running at the same frequency. Quadrature designs are somewhat elaborate, so many implementations use digital processing (not to be confused with digital modulation). That gives you excellent multipliers and no DC drift.

I can't think of any reason to deliberately build an AM-only receiver using zero-IF and quadrature, but it does work fine. However, it's the only way I can think of to properly detect AM in a zero-IF system. The original author didn't say why he changed the IF to zero hertz -- maybe he has an unspoken requirement. If he can move the IF away from zero, then of course traditional AM detection techniques would be the sensible approach.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

Okay, now I understood your point. I and Q signals have the same frequency but different phase and this is the difference that may be serious sometimes. I have feeling that original question was not related to so called zero-IF. The author just experimented with simple mixer setup and with simple modulation. By the way, I worked a lot with direct conversion radios and after all these years do not like the term zero-IF. It is just a kind of personal feeling but frequency cannot be zero. The definition of frequency and word zero is in contrary. But again, it is not more than personal opinion. One more point. When somebody is asking very simple question and it is easy to see that this person is really not very involved in this issue it is probably better not to go deeply into theory. For the original author it may be confusing and definitely will distract from understanding the issue of interest. If somebody has good experience and want to discuss the question deeply it is probably better to open separate topic and inform other members about it. Who whants to discuss it will go to the new topic and those who want to talk about original question may do it without any confusions. Again, it is just my personal opinion. What do you think about it?

 

[echo47]

Yes, I sometimes confuse people with my ramblings about zero frequency and negative frequency. Some folks are comfortable with that terminology, other folks are not. I find it natural because the quadrature signal expression cos(wt)+j*sin(wt) is well behaved with positive, negative, and zero values of w. If you know a better name for the variable "w" than "frequency", I'd be happy to consider using it!

You may be correct that the author's simulation with 100 MHz RF and 100 MHz LO was simply a random experiment and not a system requirement. If that's true, then I went overboard by suggesting a complex solution (pun intended).

I too like the term "direct conversion". However it slightly backfired earlier in this discussion, so I switched to "zero IF". I agree that term won't satisfy folks who don't like negative and zero frequency.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

I think it is more for personal taste how to call direct conversion radios. I like this term more than “zero IF”, but I am okay with any other commonly used name. By the way, term “direct conversion” was introduced around 1966 when this technology was reinvented again and quickly become popular. Actually Heaviside in 1906 (if I remember right) connected generator tuned to receiving frequency to the resonant tank of TRF receiver and got big signal amplification. This was the first direct conversion radio about 10 years before Armstrong invented the super heterodyne. It is very interesting that first radios were actually UWB and even direct conversion despite it was well before CW and super heterodynes.

You are right about mathematical expression for quadrature signal, but there is one caveat. I note one very strange thing: practically all electrical engineers do not know what it is Sin(x) or Cos(x) function. They get used to all math formulas and do not afraid them but they cannot answer very simple question: “What it is Sin function". I am not kidding, it is my observation for several decades of teaching, mentoring and working together with electrical engineers. Almost nobody can answer this question right, therefore have no good understanding of these functions. This is why I prefer to explain simple things without formulas with trigonometry functions.

 

[echo47]

Heaviside - smart fellow. The first time I heard his name was while sitting in a London theatre watching the musical Cats. No kidding. Something about cat-heaven located in the ionosphere.

I've heard several definitions of sine and cosine. One uses right triangles and "rise over run", another uses an infinite series, but when I'm thinking about periodic signals I prefer the circular function definition - the projection of circular rotation onto a line. That one feels natural to me when visualizing quadrature and negative/positive frequency (clockwise/counterclockwise rotation). Some folks don't like that visualization, and perhaps see it another way.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

It is interesting, I never heard about “cat-heaven located in the ionosphere”.

You are right with projection point of view on circle functions, but believe me it is very rare case when electrical engineer understand it. Try to ask guys around you and almost all will tell about right triangles. It is strange, but true. Very often somebody from young engineers ask me why we have 2*PI coefficient in many formulas. Last time it was just a few days ago. Mathematicians know this stuff well, but unfortunately not all electrical engineers.

 

[echo47]

Hi RF-OM. That's a though-provoking observation about young engineers. I am somewhat out of touch. Nothing but semi-old-farts in our new company.

The Cats song is "The Journey To The Heaviside Layer". For more highly off-topic info, see "Cultural Impact" at this page:
https://en.wikipedia.org/wiki/Kennelly-Heaviside_layer

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

Thank you for this link. It is interesting. I knew about Heaviside prediction about ionosphere, but never heard about the musical Cats. Regarding my observation it is just an observation. After thinking about it I decided that the reason is probably in big time gap between the time when students learn trigonometry and when they study electrical network theory. When they learn trigonometry they practically concentrate on right triangle solutions and think that they know what it is Sin and other circle functions. They can calculate them and use to solve school problems. Several years later they start to use circular functions for electrical network theory and continue to use mathematical expressions for circular functions but do not understand them right. This is why I think that using trigonometry formulas for explanations of electrical phenomena is good and may help, but often is not enough for good understanding. Often when I am interviewing electrical engineer he is able to discuss all the math behind Fourier transforms, but he is absolutely sure that Sin is the ratio between leg and hypotenuse in the right triangle.