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causeitso
Joined: 12 Aug 2006
Posts: 11
Location: egypt
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 30 Jun 2008 7:36 Superheterodyne LO stability,power ,Mixer input impedance? |
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Hi , I have exactly three questions about superheterodyne receivers .
They may seem at first like simple ordinary questions , but I searched left and right and didn't find any satisfactory answer.
Question (1)- In a superheterodyne receiver I know that to convert the RF to IF we mix the RF with an LO of a predetermined frequency , that's ok.
But during my simple idealistic simulation of an RF modulated signal of say (110 MHz) multiplied by an LO of (100 MHz) the result (information) is at (10 MHz).
The simulation ran so smoothly and I could recover the information exactly as it was transmitted (whether it is ASK , FSK or PSK).
But to test the non-ideal behavior of the receiver I changed the LO frequency very slightly (100.001 MHz) , this very slight change caused a disastrous effect on the demodulated (information) signal , which manifests it self as if the information signal is now riding (or being multiplied by) a very low frequency , and this low frequency periodically Nulls the information signal.
The more accurate the LO frequency say(100.0001 MHz) the lower the frequency of this interfering signal .
The problem is that getting so incredibly unrealistically close to a LO frequency of (100 MHz) doesn't even begin to solve the problem , as this creates a so incredibly low frequency signal that will Null the information sooner or later.
This Nulling happens when the RF and LO are out of phase by 90 degrees.
Do you see what I am talking about?
So how could we avoid this problem (knowing that I didn't see any tutorial on superheterodyne receiver address this problem as if it doesn't exist).
Plz , Correct my understanding if I am wrong.
Question (2)- In mixer design they talk about LO input power and IF input power which affect the 1-dB compression and TOI.
How could the input LO power be calculated , I mean the LO has an output resistance and the mixer have a nonlinear input resistance , so is the LO input power is the power dissipated on the output resistance of the LO?
Or is it the power that get dissipated on the input resistance of the mixer or what?
Same goes for IF input power.
Last Question- If I am set to design a Double-balanced mixer (4 diodes) , then what BALUN to use (1:1 , 1:4 etc..).
I know that the choice depends upon the required impedance matching , but the problem is that the diodes have nonlinear impedance , so how could I know the input impedance of the diodes?
Is this DBM balun design has a rule of thumb or something , because I figured that different diodes are not really that different .
So is there a general Balun ratio used for DBM that use silicon diodes (0.7 volt forward bias voltage).
Sorry for these long questions , but I wouldn't ask if I had of any other possible option.
Thank you.
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LvW
Joined: 07 May 2008
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Location: Germany
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| 30 Jun 2008 9:44 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| causeitso wrote: |
The more accurate the LO frequency say(100.0001 MHz) the lower the frequency of this interfering signal .
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Question: Is this effect - resp. the frequency of the interfering signal - dependent on the chosen baseband frequency ?
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echo47
Joined: 07 Apr 2002
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| 30 Jun 2008 10:18 Superheterodyne LO stability,power ,Mixer input impedance? |
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Is your receiver continuous-time (analog) or discrete time-sampled (digital)?
When you multiply 110 MHz by 100 MHz, you get both 10 MHz and 210 MHz. Be sure to filter out the 210 MHz. If your system (or simulation) is time-sampled, the 210 MHz product could alias down and cause great confusion with your 10 MHz IF. The confusion usually appears as an ugly beat frequency. If your mixing product frequencies, phases, and sample rate are just right, the ugly beat frequency becomes zero hertz and may seem to disappear. That scenario seems to match your symptoms.
I don't know your other two questions.
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causeitso
Joined: 12 Aug 2006
Posts: 11
Location: egypt
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| 01 Jul 2008 10:16 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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thank you.
LvM , No the frequency of the interfering signal is dependent upon the difference between local oscillator frequency and the RF carrier frequency.
eg (100.001 MHz-100 MHz=.001 MHz).
My receiver is an analog receiver , i attached a picture of the simulation.
You notice when the LO frequency is (100.1 MHz) the demodulated signal experiences nulls every now and then.
The real problem is that even if the LO frequency got arbitrarily close to (100 MHz) the nulls would still occur but with a greater distance from null to null.
So how could practical real receivers avoid this fundamental problem?
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LvW
Joined: 07 May 2008
Posts: 618
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Location: Germany
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| 01 Jul 2008 10:32 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| As I can see from your picture the input carrier frequency is 100 MHz (AM modulated with 1 MHz). Is this correct ? And the LO also runs at 100 MHz ?
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causeitso
Joined: 12 Aug 2006
Posts: 11
Location: egypt
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| 01 Jul 2008 10:46 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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Yes this is correct,
AM Carrier frequency=100 MHz
AM modulating frequency=1 MHz
Modulation index=1
Local oscillator frequency=100 MHz
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echo47
Joined: 07 Apr 2002
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| 01 Jul 2008 11:00 Superheterodyne LO stability,power ,Mixer input impedance? |
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By changing the IF to zero hertz, you now have a direct-conversion receiver. That requires a quadrature LO and quadrature mixer to preserve the modulation information which is now centered around zero hertz. Also, you need to properly interpret the mixer's complex output.
I did a quick Google search and found lots of examples.
Here's one. Look for "Zero IF (Direct Conversion)" towards the bottom:
http://www.avren.com/Courses/TX_RX_Architectures_plain.htm
If you are only doing simple AM detection, there may be a shortcut technique that I haven't seen.
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causeitso
Joined: 12 Aug 2006
Posts: 11
Location: egypt
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| 01 Jul 2008 12:41 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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Yes , this is a direct conversion receiver , but the problem I am talking about here would still occur with double or triple conversion receivers.
So my question is: can I get away with an LO frequency that doesn't exactly (theoretically) match the Rf frequency (for direct conversion) or match the IF frequency (for double conversion)?
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echo47
Joined: 07 Apr 2002
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| 01 Jul 2008 13:24 Superheterodyne LO stability,power ,Mixer input impedance? |
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If your IF bandwidth includes zero hertz, then you need a quadrature LO and mixer at that stage. It doesn't matter how many other IF stages the receiver has.
Yes you can have slightly unequal LO and RF. Quadrature techniques preserve the IF spectrum even when it's around zero hertz. A simple LO and mixer would be inappropriate because it mirrors the IF spectrum around zero hertz, causing the undesirable interference effects (nulls) you see in the simulation.
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RF-OM
Joined: 03 Aug 2007
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| 04 Jul 2008 7:44 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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1. You have simple beating between carrier frequency and unequal LO signal. You need to use IF filter to extract your required frequency. Mixers are not good for AM demodulation, they are better for DSB and SSB.
2. It is rare case in RF, but you can use input voltage from LO across mixer input resistance to determine mixer's input power. Usually power is determined through power budget calculations. You need consider matching also. If you want to determine mixer input impedance it will depends on mixer circuit. Usually there are two points: when mixing element is on and when it is off. You may use the geometric means from these two numbers. For balanced mixers this can be done for each pair for corresponding half of period.
3. See step 2.
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RFDave
Joined: 18 Feb 2004
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| 05 Jul 2008 15:39 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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Lot's of dancing around this answer. In short, as the LO frequency is offset from the ideal frequency, your BER will degrade. Cellular standards have transmit frequency accuracy standards that are less then 1 PPM (1 Hz error per 1 MHz frequency) to address this.
Dave
www.keystoneradio.com
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RF-OM
Joined: 03 Aug 2007
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| 05 Jul 2008 23:56 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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To RFDave,
Talking about the first question it is very simple case. AM have nothing to do with BER and its degradation. LO instability consideration is also not for this case. Anybody who try to use SSB receiver to listen AM station knows this effect.
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causeitso
Joined: 12 Aug 2006
Posts: 11
Location: egypt
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| 07 Jul 2008 19:18 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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Thank you very much for your helpful answers.
Although i didn't fully understand the problem but i think i am on my way to something.
thank you all especially RF-OM.
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echo47
Joined: 07 Apr 2002
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| 08 Jul 2008 3:12 Superheterodyne LO stability,power ,Mixer input impedance? |
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| It may help you to visualize the signal's spectrum during mixing. A 100 MHz signal with 1 MHz AM has components at 99 MHz, 100 MHz and 101 MHz. If you mix that down to zero hertz, the components move to -1 MHz, 0 MHz, and +1 MHz. When using an ordinary (non-quadrature) mixer, the -1 MHz and +1 MHz components both become 1 MHz and interfere with each other, causing the ugly beat frequency you saw when you slightly detuned the system. If you use a quadrature mixer, the -1 MHz and +1 MHz components remain distinct, and don't interfere with each other.
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RF-OM
Joined: 03 Aug 2007
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| 08 Jul 2008 3:54 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| But why it is necessary to use quadrature or image reject mixers with AM? Why you guys always try to use digital modulation concepts to this simple analog case? AM is AM and that is it. Your advises may confuse person who ask help. Let him to understand what is going on with his experiments and then he will be able to go ahead. If he ask about digital modulation then you may explain it.
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echo47
Joined: 07 Apr 2002
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| 08 Jul 2008 6:37 Superheterodyne LO stability,power ,Mixer input impedance? |
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I suggested it because I think it's a good way of understanding why the malfunction occurred in the given zero-IF receiver, from a spectrum point of view. Zero-IF AM detection can work fine. I agree that other AM receiver architectures may be simpler or more economical.
I don't see how "digital modulation" pertains to this discussion.
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biff44
Joined: 24 Dec 2004
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| 08 Jul 2008 12:00 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| You need to add a lowpass filter, either analog or digital, at the mixer output. You are probably having aliasing problems with the 210 MHz and any non-linear intermod outputs.
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RF-OM
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| 09 Jul 2008 18:37 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| Just because quadrature mixers are used for digital modulation, not for AM.
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echo47
Joined: 07 Apr 2002
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| 10 Jul 2008 1:14 Superheterodyne LO stability,power ,Mixer input impedance? |
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| A quadrature mixer simply shifts the frequeny band -- the type of modulation doesn't matter. I've used them successfully in AM systems. Those systems already had quadrature mixers for other purposes, so there was no cost impact.
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RF-OM
Joined: 03 Aug 2007
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| 11 Jul 2008 23:14 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| Quadrature mixer always produces two output signals instead of one. It may lead to much undesired consequences in your system. It is not well done design if you deliberately do such things. These mixers were designed for digital modulation and should be used only with digital modulation. Using them for AM is absolutely unacceptable and poor design. Your goal is to suppress all unwanted signals into system but not to created additional.
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echo47
Joined: 07 Apr 2002
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| 11 Jul 2008 23:51 Superheterodyne LO stability,power ,Mixer input impedance? |
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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.
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RF-OM
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| 12 Jul 2008 0:04 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| But IQ format is two signals. One is Sin(wt) and other is Cos(wt).
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echo47
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| 12 Jul 2008 0:10 Superheterodyne LO stability,power ,Mixer input impedance? |
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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.
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RF-OM
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| 12 Jul 2008 0:19 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| 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.
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echo47
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| 12 Jul 2008 3:10 Superheterodyne LO stability,power ,Mixer input impedance? |
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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. 
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RF-OM
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| 12 Jul 2008 4:24 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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| 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?
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echo47
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| 12 Jul 2008 5:14 Superheterodyne LO stability,power ,Mixer input impedance? |
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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.
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RF-OM
Joined: 03 Aug 2007
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| 12 Jul 2008 6:42 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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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.
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echo47
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| 12 Jul 2008 14:25 Superheterodyne LO stability,power ,Mixer input impedance? |
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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.
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RF-OM
Joined: 03 Aug 2007
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| 12 Jul 2008 19:59 Re: Superheterodyne LO stability,power ,Mixer input impedanc |
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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.
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