--- In [email protected], Jose Amador <[EMAIL PROTECTED]> wrote: > > > --- jgorman01 <[EMAIL PROTECTED]> wrote: > > > I've been reading all the posts over the last > > several weeks about > > single tone/multi-tone, baud/bps, narrow/wide, etc. > > digital > > modes/modems. The one thing I see missing is any > > discussion of the > > actual RADIO's being used in these systems. Kind of > > funny in a > > digital RADIO forum populated by amateur RADIO > > operators. Perhaps I > > am just too impatient and the discussions will > > eventually > > end up discussing the radios being used. > > At least I am assuming a COTS radio, 300 to 2700 Hz > audio bandwidth. That's what I have. > > > Some of my observations. > > > > Lots of discussions about specifications for > > different modems and > > their operation. I haven't seen any discussion > > about the > > corresponding specifications of the radios they are > > connected to. > > Just wonder how ham radio transceiver audio chains, > > mixers, filters, > > and RF chains, both transmit and receive, compare to > > the mil spec > > transceivers used with mil spec modems? > > > > Audio purists spend thousands of dollars on > > pink/white noise > > generators, equalizers, audio amps, speakers, etc. > > to insure flat > > responses. What kind of distortions do you think > > are allowed in ham > > radio audio chains. > > So far....10%....- 20 dB at worst case. Should be > better for complex modulation formats. That's 10% at one end. Add another 10% for the other end. That is, 10% distortion in the transmit audio and another 10% in the receive chain. Not very good! > > > Are there amplitude/frequency > > variances > > introduced in the preamps and amps? Are there > > phase/frequency > > variations introduced in the mixers, amps, and > > filters? Do these > > have any effect on trying to cram high speed modem > > tones through the > > system? I suspect that there can be as much or more > > effects due to > > these variables as propagation introduces. > > If you stay near the center of the passband and shy > away from the slopes, shouldn't create serious > problems I guess my point is, do you know what your passband is in your radio. Are the 3 db points really at 300 and 2700 Hz? Are there any fluctuations at other frequencies in the passband? What about phase variances throughout the passband? Your answer assumes that the design of the amps are adequate to begin with. Have you actually measured this? > > > Has anyone measured the difference between a $50,000 > > Harris HF > > tranceiver designed for this purpose and a $500 Icom > > tranceiver to > > see if the throughput is better? > > Maybe the performance difference does not justify the > price difference...makes me remember the $500 > hammer... Maybe the cost difference isn't justified, but that wasn't my question. Is there a substantial difference in performance? > > > Perhaps I'm just an old fogey who still enjoys > > messing with and > > building radios and am not competent enough in the > > digital modes to > > add much to this discussion. However, I can assure > > you building a > > mic preamp or a receive audio amp that has a flat > > amplitude response > > over 300 Hz to 3500 Hz is no mean feat let alone the > > phase > > differences that can be introduced by using heavy > > feedback to > > acheive this response. I can also tell you from > > first hand > > experience that building a crystal filter without > > amplitude/phase > > variances is very difficult. > > Well, the problem in the past has been more of > inapropiate modulation and coding formats rather than > radio insufficiencies, in general. You say this in a manner that leads me to believe you have made actual performance measurements of different stages in your transceiver so that you are sure it has no significant effects on the transmitted digital signal. Would you mind sharing these? I'll mention just one item to consider. Do you run PSK31 at full output or do you have to reduce your output in order to minimize the IMD distortion. Do you think the military high dollar rigs require their power to be turned down to 20% of rated output? What does this tell you about the design of the high power RF stages in an amateur transceiver? > > > I just wonder if some of this shouldn't also be > > discussed along with > > the "digital" side of the conversation. A system > > must have all of > > its individual components discussed and designed. > > The RADIO is a > > very important part of this. > > Of course. Certainly my old HRO or a Viking Valiant is > insufficient for this. It is generally accepted that > it takes a synthesized radio to start with. Why does it take a synthesized radio to start with? Even a synthesized radio can have frequency stability issues. Frequency stability is only one issue in a whole slew of them. > > 73 de Jose, CO2JA > > > > __________________________________________________ > Do You Yahoo!? > Tired of spam? Yahoo! Mail has the best spam protection around > http://mail.yahoo.com >
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