Tim Wescott <tim@wescottnospamdesign.com> writes:> Randy Yates wrote: >> Tim Wescott <tim@wescottnospamdesign.com> writes: >> >>>PM and FM are essentially the same thing; if you run PM through an FM >>>demodulator and integrate it you should get your desired >>> signal. >> Tim, Would you then say that position and velocity are essentially >> the same >> thing? If you integrate velocity you get position. >> Yes, I'm being a bit smart/cute, but then again I'm not. PM is no >> more >> FM than BPSK is FSK. This is one of those engineering > > Are you arguing with my terminology,Yes.> or the fact that integrating the > output of an FM demodulator result in perfectly good PM demodulation?No. -- % Randy Yates % "So now it's getting late, %% Fuquay-Varina, NC % and those who hesitate %%% 919-577-9882 % got no one..." %%%% <yates@ieee.org> % 'Waterfall', *Face The Music*, ELO http://home.earthlink.net/~yatescr
PM demodulation
Started by ●November 2, 2004
Reply by ●November 3, 20042004-11-03
Reply by ●November 3, 20042004-11-03
Jerry Avins <jya@ieee.org> writes:> Randy Yates wrote: > >> Randy Yates <randy.yates@sonyericsson.com> writes: >> >>>[...] >>>This is one of those engineering >> >> >> ... concepts which are essentially wrong. (Like the >> use of 2/T in the BT, but let's not go there...). > > FM with pre-emphasis is essentially PM above the turnover frequency. > Will you buy that?Doesn't pre-emphasis accentuate the high frequencies? If so, that would be somewhat akin to a differentiator, but differentiating frequency does not give you phase (it's vice-versa). But I see your point - it's not going to sound totally foreign since it's really just a filter. I guess that's why the difference is not distinguished too much, especially in the older, analog modulation textbooks. But it's something that hasn't set well with me for decades. -- % Randy Yates % "Remember the good old 1980's, when %% Fuquay-Varina, NC % things were so uncomplicated?" %%% 919-577-9882 % 'Ticket To The Moon' %%%% <yates@ieee.org> % *Time*, Electric Light Orchestra http://home.earthlink.net/~yatescr
Reply by ●November 3, 20042004-11-03
On Wed, 03 Nov 2004 00:04:55 GMT, "TOM" <noname@noprovider.nodomain> wrote:> > I dQ/dt - Q dI/dt >w = --------------------------- > I^2 + Q^2 > >If the receiver limits, then the denominator can be replaced by 1. >Then need to apply the proper pre-de-emphasis curves. > >The above is from Frerking, "Digital Signal Processing in Communictions >Systems" >equation 6.76. He caustions a bit about the choice of taps for the >differentiator. > > -- TomHi Tom, that expression you're using is for an FM demodulator. As Tim Wescott said, the output of that system would then have to be integrated. However, your suggestion for Sam is a sensible one. As I read Frerking's description, I think he should have shown delay elements in the direct I & Q paths to compensate for the delay in the differentiator paths. I show this idea on page 551 of the 2nd edition of my book. The differentiator I show is a simple two-tap differentiator. It only works well when the input signal is low frequency relative to the Fs sample rate. However, the good news is that the two-tap differentiator is multiplier free. Tom, I'm guessing that you're trying to save Sam from spending $500. Maybe we can do that. If Sam's signal is quadrature (complex with I & Q samples), all he has to do is compute the instantaneous phase of his complex samples by implementing some form of an arctangent algorithm. Depending on the accuracy Sam needs for his phase demodulation, there are moderately-efficient arctan algorithms available. [-Rick-]
Reply by ●November 3, 20042004-11-03
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes:...>>FM with pre-emphasis is essentially PM above the turnover frequency. >>Will you buy that? > > > Doesn't pre-emphasis accentuate the high frequencies? If so, that > would be somewhat akin to a differentiator, but differentiating > frequency does not give you phase (it's vice-versa).I realized my error as soon as I hit Send, but I figured you'd see the switch between FM and PB.> But I see your point - it's not going to sound totally foreign > since it's really just a filter. I guess that's why the difference > is not distinguished too much, especially in the older, analog > modulation textbooks. But it's something that hasn't set well > with me for decades.You got the point I tried to make, however wrongly. One time, when I worked late on the W2HJ transmitter, I found myself locked into the City College without change for a pay phone. I had finished with the 304 TH final, but the modulator wasn't yet installed. My Morse was poor and I couldn't even find a key in the shack. I didn't want to mess with the VFO, so I soldered two tin-can ends to adjacent turns of the driver tank coil and shouted into the nearer one. With a kilowatt to get across Manhattan, even a little phase modulation is enough. A friendly ham called the cops and they came and let me out. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●November 3, 20042004-11-03
"Jerry Avins" <jya@ieee.org> wrote in message news:2usf0tF2dvudoU1@uni-berlin.de...> Randy Yates wrote: > > One time, when I worked late on the W2HJ transmitter, I found myself > locked into the City College without change for a pay phone. I had > finished with the 304 TH final, but the modulator wasn't yet installed. > My Morse was poor and I couldn't even find a key in the shack. I didn't > want to mess with the VFO, so I soldered two tin-can ends to adjacent > turns of the driver tank coil and shouted into the nearer one. With a > kilowatt to get across Manhattan, even a little phase modulation is > enough. A friendly ham called the cops and they came and let me out.Great MacGyver story, Jerry! For those who don't know MacGyver was an American TV show from the late 80's where the main character routinely solved difficult problems using the common materials he happened to have on hand (chewing gum, clothing, etc.): http://www.tvtome.com/tvtome/servlet/ShowMainServlet/showid-706/.
Reply by ●November 3, 20042004-11-03
Thank you all for these answers ! Well, OK, I'll try to find an arctan algorithm (if anyone could show me one... thanks in advance !) But I have another question : do I need to derivate the I and Q components from two mixer products (one sin and one cos) or is that possible to simply delay the signal from a few samples to get the Q ?? Thanks in advance ! Sam
Reply by ●November 3, 20042004-11-03
Jerry Avins <jya@ieee.org> writes:> Randy Yates wrote: > > > Jerry Avins <jya@ieee.org> writes: > > ... > > >>FM with pre-emphasis is essentially PM above the turnover frequency. > >>Will you buy that? > > > > > > Doesn't pre-emphasis accentuate the high frequencies? If so, that > > would be somewhat akin to a differentiator, but differentiating > > frequency does not give you phase (it's vice-versa). > > I realized my error as soon as I hit Send, but I figured you'd see the > switch between FM and PB. > > > But I see your point - it's not going to sound totally foreign > > since it's really just a filter. I guess that's why the difference > > is not distinguished too much, especially in the older, analog > > modulation textbooks. But it's something that hasn't set well > > with me for decades. > > You got the point I tried to make, however wrongly. > > One time, when I worked late on the W2HJ transmitter, I found myself > locked into the City College without change for a pay phone. I had > finished with the 304 TH final, but the modulator wasn't yet installed. > My Morse was poor and I couldn't even find a key in the shack. I didn't > want to mess with the VFO, so I soldered two tin-can ends to adjacent > turns of the driver tank coil and shouted into the nearer one. With a > kilowatt to get across Manhattan, even a little phase modulation is > enough. A friendly ham called the cops and they came and let me out. > > JerryQuite the story, Jerry!!! But what's the "304 TH final"? -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
Reply by ●November 3, 20042004-11-03
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes:(snip)>>One time, when I worked late on the W2HJ transmitter, I found myself >>locked into the City College without change for a pay phone. I had >>finished with the 304 TH final, but the modulator wasn't yet installed. >>My Morse was poor and I couldn't even find a key in the shack. I didn't >>want to mess with the VFO, so I soldered two tin-can ends to adjacent >>turns of the driver tank coil and shouted into the nearer one. With a >>kilowatt to get across Manhattan, even a little phase modulation is >>enough. A friendly ham called the cops and they came and let me out.> Quite the story, Jerry!!! But what's the "304 TH final"?Knowing only a tiny bit about how they used to build radio transmitters, and even less about how they build them now... There should be a carrier source, modulator, and final amplifier (and probably other important details along the way). If you don't have a modulator, all you could get is unmodulated carrier. Now, what is the result of modulating the L or C of the carrier oscillator? I remember stories about vacuum tube amplifiers (maybe phone preamps) being vibration sensitive enough that you could get feedback placing them too near the speakers. Presumably motion of the tube electrodes changes the electrical properties enough to generate a signal. All the fun that went away with transistorized amplifiers. I presume a 304 TH is a final amplifier. -- glen
Reply by ●November 3, 20042004-11-03
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes: > > >>Randy Yates wrote: >> >> >>>Jerry Avins <jya@ieee.org> writes: >> >> ... >> >> >>>>FM with pre-emphasis is essentially PM above the turnover frequency. >>>>Will you buy that? >>> >>> >>>Doesn't pre-emphasis accentuate the high frequencies? If so, that >>>would be somewhat akin to a differentiator, but differentiating >>>frequency does not give you phase (it's vice-versa). >> >>I realized my error as soon as I hit Send, but I figured you'd see the >>switch between FM and PB. >> >> >>>But I see your point - it's not going to sound totally foreign >>>since it's really just a filter. I guess that's why the difference >>>is not distinguished too much, especially in the older, analog >>>modulation textbooks. But it's something that hasn't set well >>>with me for decades. >> >>You got the point I tried to make, however wrongly. >> >>One time, when I worked late on the W2HJ transmitter, I found myself >>locked into the City College without change for a pay phone. I had >>finished with the 304 TH final, but the modulator wasn't yet installed. >>My Morse was poor and I couldn't even find a key in the shack. I didn't >>want to mess with the VFO, so I soldered two tin-can ends to adjacent >>turns of the driver tank coil and shouted into the nearer one. With a >>kilowatt to get across Manhattan, even a little phase modulation is >>enough. A friendly ham called the cops and they came and let me out. >> >>Jerry > > > Quite the story, Jerry!!! But what's the "304 TH final"?304 TH is a high-power vacuum tube; a "final" is a rig's final amplifier. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●November 3, 20042004-11-03
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes: > > >>Randy Yates wrote: >> >> >>>Jerry Avins <jya@ieee.org> writes: >> >> ... >> >> >>>>FM with pre-emphasis is essentially PM above the turnover frequency. >>>>Will you buy that? >>> >>> >>>Doesn't pre-emphasis accentuate the high frequencies? If so, that >>>would be somewhat akin to a differentiator, but differentiating >>>frequency does not give you phase (it's vice-versa). >> >>I realized my error as soon as I hit Send, but I figured you'd see the >>switch between FM and PB. >> >> >>>But I see your point - it's not going to sound totally foreign >>>since it's really just a filter. I guess that's why the difference >>>is not distinguished too much, especially in the older, analog >>>modulation textbooks. But it's something that hasn't set well >>>with me for decades. >> >>You got the point I tried to make, however wrongly. >> >>One time, when I worked late on the W2HJ transmitter, I found myself >>locked into the City College without change for a pay phone. I had >>finished with the 304 TH final, but the modulator wasn't yet installed. >>My Morse was poor and I couldn't even find a key in the shack. I didn't >>want to mess with the VFO, so I soldered two tin-can ends to adjacent >>turns of the driver tank coil and shouted into the nearer one. With a >>kilowatt to get across Manhattan, even a little phase modulation is >>enough. A friendly ham called the cops and they came and let me out. >> >>Jerry > > > Quite the story, Jerry!!! But what's the "304 TH final"?"Final" is the final amplifier, between the driver (an 807) and the antenna. 304 TH is the wrong number. The tube was in fact an Eimac 250 TH. It had zirconium plates that glowed dull red when tuned for max output and brighter if the antenna coupler was misadjusted. If you could see the shadow of the screen grids on the plates (it was a cluster of 4 tubes in one envelope), you knew it needed help STAT! It ran with a class-C efficiency of 75% and supported 250 Watts of plate dissipation. Zirconium has a self gettering action when it is red hot, but the tube still glowed blue much of the time. The filament was thoriated tungsten and had to be hot before plate voltage was applied or electrostatic force might pull it and short out the grid. Accordingly, the power supplt had two switches. Either would turn on the filament and both had to be on for the plate supply. The Mercury-vapor rectifiers were actually more sensitive to cold filaments; they would develop internal arcs and die of hot spots unless the cathodes were emitting copiously. (250 was the rated dissipation, T stood for triode, and H for high mu. There was a low mu version, the 250 TL.) That was in 1952. I might have forgotten some details and misremembered others. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
