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1-10 of 93 messages
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Designing the Ultimate HF Front End
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by ZENKI on June 22, 2009
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Neither of the 3 radios that you mention has the ultimate front end.
The work by PA3AKE has produced the ultimate front end.
http://www.xs4all.nl/~martein/pa3ake/hmode/index.html
If you are going to start dreaming about the ultimate front end, you would be wise to also start dreaming about the ultimate transmitter performance.
The major impediment that prevents us from extracting the maximum performance from our ultimate receivers is the poor state of our transmitter performance specifications.
Until our transmitters IMD, phase noise and keyclicks approach the IMD dynamic range numbers of our receivers at close signal spacing, it would all be a wasted effort.
This is exactly the situation that we have today with radios like the K3 and Orion, great receivers however their below average transmitter performance specifications leaves a lot to be desired. Rob Sherwood has given a presentation on this point.
We need transmitters like the ADAT which have technically perfect and clean transmitters.
http://www.adat.ch/index_e.html
However expecting perfect transmitters from ham manufacturers is just a dream. The only way we will get perfect transmitters is when laws are passed that mandate these specifications under ITU law.
While its nice dreaming about the above radios, all that you are really doing is continuing the receiver performance numbers hyperbole.
BTW the AOR 7070 is due out soon, and according to my spies it is going to blow everything off the planet. A darn shame that it has no matching transmitter with fantastic transmitter specs.
Dream on!
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RE: Designing the Ultimate HF Front End
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by G3RZP on June 22, 2009
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My measurements (published in QEX and NCJ) taken at sunspot max and sunspot min, showed that 100dB of instantaneous dynamic range is about all that is needed - in a place where the noise level is below ITU median quiet rural levels, and 7MHz BC signals can be big. The insidious part of front end performance is phase noise, which may well negate a lot of the attempts at a high spurious free dynamic range front end. Spurs need to be low, too, which can be a problem with DDS. Narrow tuned filters help that, as well as helping to reduce the number of signals giving IMD problems.
Maybe those old designers of tube radios who used three gang tuned circuits actually did know what they were doing!
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RE: Designing the Ultimate HF Front End
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by K0BG on June 22, 2009
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A couple of comments.
The Icom IC-7600 is designed the same way, and once the ARRL, Sherwood Engineering, et. al. test it, I'm sure it will prove to be very close to the 7700 and 7800 in receiver performance (maybe for naught).
I could not agree more with the statement about transmitter IMD levels. The biggest problem is not the transmitters themselves so much, as it is the way they're used. Far too many amateurs, use way too much microphone gain and compression levels, peaky microphones, and other devices to the point that all of the transmitting stages are vastly overloaded. As a result, it is not uncommon for the IMD level to be nearly as high as their fundamental bandpass ones are.
Adding even more injury, they drive their amplifiers so hard, what little overhead is designed into them, is long since flattened out.
The EU and the FCC have rules governing the manufacturers with respect to IMD, yet little if any effort is made to govern on-air activities.
And to paraphrase, until we tackle the self-generated IMD problem, the receivers we have now are more than adequate.
Alan, KØBG
www.k0bg.com
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RE: Designing the Ultimate HF Front End
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by WX7G on June 22, 2009
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While the front end is ever so important the first IF can also make or break receiver performance.
Most modern amateur transceivers (receivers) up convert to the first IF. Because of the high IF frequency (70 MHz) the 'roofing filter' cannot be made with a very narrow bandwidth.
A handful of amateur transceivers, such as the Elecraft K3 down convert to a low IF. This allows a narrow roofing filter. The K3 can be ordered with a 500 Hz roofing filter to provide incredible close-in signal performance. Where this matters most is during CW contesting, especially on 160 meters.
I have not found out if the IC-7800 uses up conversion or down conversion for the first IF. Does anyone know the answer?
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RE: Designing the Ultimate HF Front End
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by G3RZP on June 22, 2009
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The EU limitations on transmitter IMD have no requirement closer than 5kHz from the centre of a transmission at HF. Further out, for an HF tx, they are 43dB + 10logP, not exceeding 50dB.
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RE: Designing the Ultimate HF Front End
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by W9OY on June 22, 2009
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On the SDR front we are now developing radios that have wide band image rejection and brick wall filtering on both transmit and receive.
SDR radios are phasing rigs and as such can attain very clean transmit characteristics and receive characteristics. The new programming does its magic in milliseconds as you tune across the bands giving you 100dB image rejection and brick wall transmit and receive filtering. In other words the radio configures itself to best performance on the fly for what ever frequency you choose to operate.
As the quality of DDS and A/D improves these radios will become the only rational choice on a cost vs performance basis alone
73 W9OY
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RE: Designing the Ultimate HF Front End
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by AB7E on June 22, 2009
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I agree with K0BG. While many rigs may have weak transmitter performance, the much greater problem lies with how people use them.
It's pretty obvious to the user when a receiver isn't performing well, but in my opinion, most hams are fairly clueless whether they are transmitting a clean signal or not ... and most hams are reluctant to provide honest feedback that might help the situation. I've heard hams with horrible SSB signals ask how they sound, only for them to be told by the guy on the other end that they sound fine. I've almost never heard someone with 2KHz-wide key clicks receive anything other than a 559 or 579 or 599 report.
My experience is that most hams really appreciate honest feedback. During last January's NAQP SSB a fellow in the midwest with a 59+20db signal was calling CQ but getting rather few callers because his signal was so overdriven it was difficult to understand him. Two hams on frequency commented amongst themselves how badly he sounded but they didn't tell him, so I gave the guy a call and suggested he back down the mic gain and compression. He cranked it down a bit, then some more, and then some more, until finally he sounded pretty good. His signal was literally 15 db weaker (59+5db) but he was far more understandable and immediately started getting replies.
Here's another example. During the recent CQ WPX CW contest, several stations had horrible key clicks, and one large M/M station was so bad on 20m I was convinced he was hot switching the amplifier. So I sent them a carefully-worded email after the contest to describe what I heard. I received a nice reply a few days later saying they had been using two different rigs during the contest, and after checking them out they discovered that one of them (an unmodified Yaesu) was generating bad clicks. They thanked me for the report and said they had already ordered a mod kit from W8JI.
No ... I don't make a point of playing signal quality policeman, but I think if we all gave honestly constructive feedback to others we meet on the air we'd have less trash to complain about.
73,
Dave AB7E
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RE: Designing the Ultimate HF Front End
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by KB2FCV on June 22, 2009
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Perhaps a LID filter. So that when there are those 1 or 2 guys that don't understand what 'up' means on the DX station and just keep calling and calling and calling and calling completely wiping out the DX station. That would be awesome.
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RE: Designing the Ultimate HF Front End
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by K7LRB on June 22, 2009
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Dave AB7E),
Very nice post and very true. Probably the biggest problem with being "honest" and forthcoming with people is that, for the most part, people are very "covetuous" of their own "stuff" (ideas, equipment, signal quality, etc.). Therefore, rather than accepting "constructive" criticism, most people get very defensive when they receive what they consider "criticism", constructive or not.
That is probably why most people prefer NOT to say anything rather than provoke someone's ire. This appears to be true of society in general as well as ham radio.
Just look at some of the postings here on eHam. Personally, I think we should exercise at least a modicum of care with grammar and spelling. Sometimes the spelling and grammar is absolutely atrocious, to the point that some posts are literally impossible to "interpret". Yet, "I pity the fool" who makes any comment regarding the apparent lake of literacy. You are labeled "the spelling police", or worse. Two things come to mind: one is that, apparently, this site is checking my spelling, even as I type this; secondly, surely everyone knows a literate person who can post FOR them.
You can apply this to virtually any activity or topic in life in general, again, as well as ham radio. If you mention you own a Ford, there are thousands of people who will gladly jump on you and tell you what a fool you are. Oh, you ride a Honda motorcycle? What an idiot! Oh, you bought a Yaesu? What a moron!!
So the obvious question is why would you subject yourself to such "backlash" and vitriol? Is it really worth it? Sometimes, as you point out, yes it is. My guess is, more often not.
Sometimes people can't understand what you are saying anyway. Give a report of 599C. Don't be surprised if the response is, "Thanks for the great report!" Conversely, send 579X. Hey, there must be something wrong with my signal!
Again Dave, I appreciate your thoughtful, and civil, reply. We should all be so courteous!
73 to all,
de Larry
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