*I'm having difficulty posting. This may be a repeat.* Tim Wescott wrote: [see previous message if curious] > Huh. Good 'technician' course. Yes. We got into Laplace transforms, vector analysis (for EM fields and waves) and more. A blind engineer named Lieljestrand taught us how to solve AC circuits with vector diagrams. He kept string in his pocket for drawing circles on the blackboard. > I'm still going to stand by the notion that the reality may be described as a high-level 1-quadrant multiplier with built-in offset, but while that works well for modeling it's not necessarily how you want to think about it when you're debugging an AM set. There's more than one way to describe almost anything, but that description is accurate and apt. One of the multipliers is a constant (if you can call RF constant). So gain proportional to plate voltage accomplished the multiplication. If there's enough drive always to drive the output to the rail, that's all that's needed. > Incidentally, I've often wondered if you couldn't get decent SSB performance with an AM modulator that goes all the way down to 0, coupled with an exciter that shifts phase at the appropriate spots. Needing a DC- > coupled audio amp to the RF deck plate is probably the least of your practical difficulties, though. I think you're describing a complicated way to make a balanced modulator. If so, there are still two sidebands. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
modulation: audio DC offset
Started by ●December 23, 2009
Reply by ●December 24, 20092009-12-24
Reply by ●December 24, 20092009-12-24
Another possible repost: Vladimir Vassilevsky wrote: > > > Tim Wescott wrote: > > >> Incidentally, I've often wondered if you couldn't get decent SSB >> performance with an AM modulator that goes all the way down to 0, >> coupled with an exciter that shifts phase at the appropriate spots. >> Needing a DC-coupled audio amp to the RF deck plate is probably the >> least of your practical difficulties, though. > > This is what was referred as synthetic method of SSB generation. The > problem with it is the accuracy of phase and amplitude modulators. I knew it (from RCA Institutes and ARRL handbook) as the phasing method. Did you know that, early on, vestigial sideband TV was accomplished by a filter in the antenna feed? The first filters I designed apart from tone controls were done with waveguide stubs and a Smith chart. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●December 24, 20092009-12-24
hello, and thanks everyone for your replies. I feel like I'm on the same page with everyone. The basic equation you provided validates my concept of the offset and shows me where to place the offset. if I use (a) for audio and (c) for carrier, 2cos(c) + 2cos(a+c) + 2cos(a-c) implies that a is the carrier on the RHS, but the identity: cos(x)=cos(-x) => 2cos(a-c) = 2cos(c-a) fixes thngs. Back to physical representation. If the tube plate of the modulator is connected to top tank circuit, and the tank bottom is in series with the modulator ( transformer ), which then connects to Eb: IOW, If I see a plate circuit, there is the ep, et, em and Eb Eb is the static plate supply em is the modulation et is the plate RF tank ep is the plate voltage itself. Then the plate supply ( static ) Eb = et + em + ep The voltages add. I see no multiplication here. I have been also taught that non-linear effects make the modulation. Unless it's evident in this example, I will ignore it. However, the only addition two things I can think of that might in some way contribute to the explanation are: 1. there is some "power" in the calculation that helps explain the multiplication ( V x I is a multiplication ). 2. the fact the additions above are hiding the "X" effect ( select 1 ) static E's dynamic e's ?? Not to stray too far from the technical aspects of how modulation happens and it's translation to DSP, I would like to know, since I am simulating from a learning experience ( using C#, not Matlab ), what ( in general ) would be the ratio ( range) of digital modulations often used with "computers". For example, would someone use a 2 ghz pentium and a 65536 sized array to simulate a 100 hz signal onto a 5 Mhz ( 50 mhz, 500 Mhz ) carrier ?? Is there a rule of thumb ? ( would it matter if it was some digital modulation or linear ?) Thanks for your advice. The thread has been quite enlightening. Walt......
Reply by ●December 24, 20092009-12-24
waltech wrote:> hello, and thanks everyone for your replies. > I feel like I'm on the same page with everyone. > > The basic equation you provided validates my concept of the > offset and shows me where to place the offset. > > if I use (a) for audio and (c) for carrier, > 2cos(c) + 2cos(a+c) + 2cos(a-c) > implies that a is the carrier on the RHS, but the identity: > cos(x)=cos(-x) => > 2cos(a-c) = 2cos(c-a) > fixes thngs. > > Back to physical representation. > > If the tube plate of the modulator is connected to top tank > circuit, and the tank bottom is in series with the modulator > ( transformer ), which then connects to Eb: > > IOW, > If I see a plate circuit, there is the ep, et, em and Eb > Eb is the static plate supply > em is the modulation > et is the plate RF tank > ep is the plate voltage itself. > > Then the plate supply ( static ) > Eb = et + em + ep > > The voltages add. I see no multiplication here.The modulated stage operated Class-C with a tank Q of about 3. That's damped enough to allow cycle-to-cycle changes of amplitude, but live enough to swing the plate up to the power supply when the tube id cut off. Therefore, the peak RF amplitude follows the modulation voltage coming out of the transformer (if there is one. See "Heising modulator")> I have been also taught that non-linear effects make the > modulation. Unless it's evident in this example, I will > ignore it.Multiplication is a non-linear operation. One can also modulate with diodes.> However, the only addition two things I can think of that might > in some way contribute to the explanation are: > 1. there is some "power" in the calculation that helps explain > the multiplication ( V x I is a multiplication ). > 2. the fact the additions above are hiding the "X" effect ( select 1 ) > static E's > dynamic e's > ??Don't stretch it.> Not to stray too far from the technical aspects of > how modulation happens and it's translation to DSP, > I would like to know, since I am simulating from a > learning experience ( using C#, not Matlab ), what ( in general ) > would be the ratio ( range) of digital modulations often used > with "computers". For example, would someone use a 2 ghz pentium > and a 65536 sized array to simulate a 100 hz signal onto a > 5 Mhz ( 50 mhz, 500 Mhz ) carrier ?? Is there a rule of thumb ? > ( would it matter if it was some digital modulation or linear ?)AM is rarely used with digital modulation. It is primarily an analog medium.> Thanks for your advice. > The thread has been quite enlightening.You're welcome. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●December 24, 20092009-12-24
On Thu, 24 Dec 2009 10:07:23 -0500, Jerry Avins wrote:> *I'm having difficulty posting. This may be a repeat.* > > Tim Wescott wrote: > > [see previous message if curious] > > > Huh. Good 'technician' course. > > Yes. We got into Laplace transforms, vector analysis (for EM fields and > waves) and more. A blind engineer named Lieljestrand taught us how to > solve AC circuits with vector diagrams. He kept string in his pocket for > drawing circles on the blackboard. > > > I'm still going to stand by the notion that the reality may be > described as a high-level 1-quadrant multiplier with built-in offset, > but while that works well for modeling it's not necessarily how you want > to think about it when you're debugging an AM set. > > There's more than one way to describe almost anything, but that > description is accurate and apt. One of the multipliers is a constant > (if you can call RF constant). So gain proportional to plate voltage > accomplished the multiplication. If there's enough drive always to drive > the output to the rail, that's all that's needed. > > > Incidentally, I've often wondered if you couldn't get decent SSB > performance with an AM modulator that goes all the way down to 0, > coupled with an exciter that shifts phase at the appropriate spots. > Needing a DC- > > coupled audio amp to the RF deck plate is probably the least of your > practical difficulties, though. > > I think you're describing a complicated way to make a balanced > modulator. If so, there are still two sidebands.Only if the exciter phase is limited to 0 and 180 degrees. If you take the low-level SSB signal and separately run it through a limiter and find it's envelope, then use the limited signal to drive the RF input to the final and the envelope to drive the power to the final, it would work in theory. -- www.wescottdesign.com
Reply by ●December 24, 20092009-12-24
> AM is rarely used with digital modulation. It is primarily an analog medium. > >Um, what abut QAM? :-) Mark
Reply by ●December 24, 20092009-12-24
> One of the multipliers is a constant > (if you can call RF constant). So gain proportional to plate voltage > accomplished the multiplication.I kinda like the way the above sounds. I want to think on it a bit more.> >> AM is rarely used with digital modulation. It is primarily an analogmedium.>> >> >Um, what abut QAM? :-) > >MarkAt first, I thought of asking about a BPSK or such as a radio transmission. QAM will do .. thanks for mentioning it. By the way, I ran across a term that suggested low level modulation using phase and then at the power amp, converting to AM. The term was ampliphase. Didn't know if it was related to the discussion on SSB but it sounded a bit like it. Walt......
Reply by ●December 24, 20092009-12-24
Mark wrote:>> AM is rarely used with digital modulation. It is primarily an analog medium. >> >> > Um, what abut QAM? :-):-) indeed! You don't make QAM with a plate modulator, nor do you decode it with a peak detector. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●December 25, 20092009-12-25
On Thu, 24 Dec 2009 14:43:58 -0500, Jerry Avins <jya@ieee.org> wrote:> >AM is rarely used with digital modulation. It is primarily an analog medium.This maybe true for carrier based (and/or wireless) systems but for base-band systems almost all modulation is AM. Best examples are 1GbE (PAM5) and 10GbE (over 100m copper) which is 16 PAM. -- Muzaffer Kal DSPIA INC. ASIC/FPGA Design Services http://www.dspia.com
Reply by ●December 25, 20092009-12-25
Muzaffer Kal wrote:> On Thu, 24 Dec 2009 14:43:58 -0500, Jerry Avins <jya@ieee.org> wrote: >> AM is rarely used with digital modulation. It is primarily an analog medium. > > This maybe true for carrier based (and/or wireless) systems but for > base-band systems almost all modulation is AM. Best examples are 1GbE > (PAM5) and 10GbE (over 100m copper) which is 16 PAM.I should clarify. By AM, I meant the system in which a carrier is varied in amplitude to create an envelope that follows the modulating signal, and can as a result be demodulated with an envelope detector. Variations that require taking phase into account lie outside the scope of my statement. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
