On Wed, Feb 16, 2011 at 11:17 AM, W6VOL <[email protected]> wrote: > From the previous posts, it is my understanding that PowerSDR generates its > AM modulation by generating a carrier, and then independently generating > the > upper & lower sidebands. > > > > In traditional high-level (plate) AM modulation schemes, variations in > plate > voltage, at audio rates, generated the sidebands, and caused the resultant > inefficiencies between power levels in the carrier vs. sidebands. >
This is something that seems to be an issue with a lot of hams. We see this in the FSK vs. AFSK debate as well. The point is, it doesn't matter how you construct a signal so long as what comes out the antenna is the same in all cases, e.g. FSK vs. AFSK, plate-modulation vs. screen modulation, etc. In the case of AM you can generate it with plate modulation, screen modulation, grid modulation, or cathode modulation. You can generate it with a double-balanced modulator and then reinsert the carrier in a subsequent stage. You can generate it by unbalancing a balanced modulator in order to allow the requisite carrier leakage. The end result will be the same signal coming out the antenna. Any differences are the result of doing the job poorly rather than an advantage of one over another. And everyone at some time in their life, should have a Central Electronics model 10A or 20A "Multiphase Exciter" to see what all this stuff is about. It is amazingly educational to have knobs that allow you to diddle all the coefficients. In AM operation with PowerSDR, is it necessary or suggested to maintain 100% > carrier level during normal operation? If not, what carrier levels have > proven optimal? > This answer depends on the receiver. If the receiver is using an envelope detector, you have to have a pure AM signal where 75% of the power is in the carrier. If the receiver is using exalted carrier detection or synchronous AM detection, you can turn down the carrier to relatively low levels. It all depends on what you want to do. Personally I cannot understand why anyone would *want* to run AM. SSB with a reduced carrier (-20dB or so) used as a pilot tone to allow the receiver recover the frequency and phase of the carrier for demodulation with a single-sideband synchronous detector provides all the fidelity of AM on 1/8 the power and 1/2 the bandwidth. But no one asked me that question. I also tend to say, "RTTY is to digital modes what spark is to CW." For some reason that doesn't win me nany friends either. ;-) -- Brian Lloyd, WB6RQN/J79BPL 3191 Western Dr. Cameron Park, CA 95682 [email protected] +1.767.617.1365 (Dominica) +1.931.492.6776 (USA) (+1.931.4.WB6RQN) _______________________________________________ Flexedge mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for posting topics related to SDR software development and experimentalist who are using beta versions of the software.
