--- On Sun, 2/1/09, Jim Garland <4cx2...@muohio.edu wrote: > I've been reading a thread on another email reflector about the K3's audio > response on AM. One of the reflector participants is a serious SWBC listener > and is very dissatisfied with his K3's AM audio. He has pointed out a few > things about the K3 AM response which puzzle me. Here they are: >"My K3 sounds the same as others I've heard which I consider to be absolutely terrible when using an external speaker and beyond absolutely terrible when using the internal speaker! The 6KHz roofing filter in the K3 will provide an audio bandwidth of up to 3KHz and the FM 13KHz roofing filter will enable the DSP to provide the user with an audio bandwidth of approximately 4.5KHz I've been told by Elecraft." > He also says that "Re the bandwidth, a short time ago Elecraft made a mod to > the firmware so that audio bandwidth and not IF bandwidth was displayed on > the K3's screen. If you're seeing 5KHz and you don't have FM filter installed > then it sounds like yet another bug." >His particular complaint is audio clicks on AM when hard consonants (Ts, Ks, >etc) are spoken, a problem that he says has been confirmed by other users. >He has installed all the audio mods in his radio, but they didn't fix the >problem. >This was all news to me. I've not used my K3 extensively on AM, and maybe my >ears aren't as sensitive as his, but I've not noticed any problem. I have the >6 kHz roofing filter in my radio, and the DSP BW goes up to 5 kHz, which he >says is a bug, and that the actual BW is only 3kHz. > So my questions are: (1) Is audio distortion on AM a known problem with the > K3? (2) What does the LCD BW actually read? Is it an IF BW, an audio BW, or > what, and does the displayed BW actually corresond to reality? Tnx, Jim W8ZR
Answer to (1). Yes. At least in my K3 and it's not limited to AM. I've been trying to determine the cause of this for the month that I've been using it. I didn't really want to pay $2,200 for another science project and would much rather be working on antennas though. My experiments now suggest the problem (or at least one of the problems) is AGC related. Following are links to plots that demonstrate some of the "fun" I'm having. The first two are taken using Spectrum Lab to look at the Line Out audio signal from the K3. The input signal is from a homebrew 28.7 MHz crystal oscillator/buffer with about a -70 dBm output level. For these particular plots the K3 mode was set to LSB, although I saw, but didn’t record the same issue on CW. The beat note is 450 Hz. The Line Out is run into the microphone input of the built-in sound card in my Lenovo T400 laptop. Care was taken to insure that the sound card wasn't being overdriven. The K3 DSP bandwidth is set to default values, as are the AGC parameters and the crystal filter is a 2.8 KHz, 8-pole. (Note: all of these adjustments affect the results. Trying to categorize them all would drive one crazy. If you don’t like the answer, turn a knob.) The first plot is with the AGC set to "Slow." http://k6mhe.com/n7ws/K3_AGC_Slow.jpg For the second plot, *nothing* has been changed except the AGC button was toggled to "Fast." http://k6mhe.com/n7ws/K3_AGC_Fast.jpg For a sanity check, the third plot is from the aux audio output of my Kenwood TS-870S while it is tuned to the same signal as the K3. http://k6mhe.com/n7ws/TS_870.jpg Now it might be argued that these artifacts are at such a low level they would not be audible as distortion. Maybe, but they represent essentially a "static" situation. When listening to a string of dots sent with my '870 the difference between fast and slow AGC is clearly audible. When the signal is S4 or 5, with fast AGC there is garbage that essentially sounds like an increased noise level. A simple test such as tuning in WWV and listening to the voice ID and hearing the audio distortion and syllabic "clicking" disappear when simply turning the AGC off and turning the RG gain down appropriately indicates a real problem. If there are K3s out there that don't do this, then I want one of them. Wes N7WS ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
