Web, KR4WM wrote: I may be oversimplifying the problem, but here's my experience in aligning my K2. I happened to remember reading some very old QST articles about a device used to get stations precisely on the net control's frequency during net operations. It was a very simple 1KHz tone generator. The NCS station was supposed to send out a 1KHz tone, and net stations would listen on frequency while at the same time, playing a 1KHz tone at their location. This would allow stations to zero-beat the NCS station, thereby being precisely on the NCS station's frequency.
----------------------------------------------------------------- That's great for matching frequencies so that no retuning is needed for perfect audio when listening to a round table (or net), Web, but it still leaves the issue of setting the SSB filter bandpass in the best place. Assuming the stations tune each other in correctly on the air, adjusting the BFO frequency has the effect of shifting where the filter bandpass falls across the audio spectrum. As the BFO frequency is changed, it has the effect of moving the RF bandpass up or down so that more low frequencies are cut off in favor of passing more high frequencies or, cutting off more high frequencies in favor of passing more low frequencies. It's a lot like playing with an audio equalizer or set of tone controls in which they are locked together so that increasing the bass automatically cuts the treble and vice versa. The only way to change both the bass and treble independently is to change the filter passband. That happens when we choose the bandwidth of the filter itself. So, when we choose the filter bandwidth we lock the bass and treble controls together then we move them back and forth for best sounding audio by adjusting the BFO frequency. We tend to choose SSB filter bandwidths that restrict the audio range transmitted as much as possible while still maintaining good intelligibility. That reduces the bandwidth needed for the SSB signal while increasing the "punch" of a given RF power by concentrating that power in the filter's bandwidth. But, as the bandwidth of the filter is reduced, how it is positioned to pass the audio spectrum, balancing the "bass" and "treble" becomes more critical, and more dependent upon individual voices. The communications "standard" of about 2.8 kHz is a compromise between those two issues: it's a reasonably narrow bandwidth but wide enough for good intelligibility if the bandpass allows just the right compromise between high and low audio frequencies through. That's where Spectrogram comes in. It lets us "see" exactly how the filter passband sits in relation to the audio spectrum: what it's passing and what it's cutting off. That's great for setting that passband to a specific place, but it does not indicate what place is best for a particular voice and microphone. Don gave some good "rules of thumb" that will put the filter passband in a position that produces good sounding SSB audio under most conditions, but it may not be optimum for every voice and microphone. That's why the SSB module book says to do final checks listening to the signal as transmitted if one wants to do any final "tweaking". Just how critical the actual position of the bandpass becomes depends upon the filter bandwidth. The original K2 used a narrow SSB bandpass of about 2.1 kHz. That's well below the "standard" used by most rigs but still wide enough to produce good audio. I believe the idea was to give the signal the most "punch" by putting the signal into a narrower slice of the RF spectrum while still having enough bandwidth to produce clean, intelligible audio. Wider bandwidths, such as the 2.7 KHz used by most SSB rigs, are far less critical to adjust, passing more of both lows and highs for the vast majority of voices. You can choose either the original narrow or wider 2.7 KHz for your K2 by making minor component changes on the SSB module. Even wider bandwidths (ESSB) are even less critical, but at the cost of winder bandwidths and a lower S/N ratio at the receiving end for a given power, since the RF power is being spread out over more RF spectrum. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
