"The AFSK TX Filter cleans up any hum on your sound card from bad connections, grounding issues, or distortion from over drive. However, with the K3 one can get the same narrow transmit spectrum by using FSK without the issues of AFSK." Written by Joe Subich, W4TV
Not as I read the article - but I may be wrong. " Effect of the K3 AFSK Transmit Filter Rather than hacking the K3 to transmit through a narrow roofing filter, an equal or better result can be had by simply enabling the AFSK transmit filter in the configuration menu. This places a 400 Hz filter before transmit audio arrives at the RF modulator (this only applies to AFSK-A mode, not DATA-A mode). This filter is centered around the tones configured under the radio's PITCH menu. The effect is similar to what MMTTY and other programs do in their software, but this is done in the radio's DSP firmware. The radio's manual says this filter can serve to filter any noise that might be on the audio input, and it certainly will do that, but it has the additional benefit of filtering keying sideband energy. Figure 9 shows the result of MMTTY's unfiltered phasecontinuous AFSK audio into the K3 with the K3's AFSK filter enabled. Comparing Figure 9 with Figure 3 demonstrates the effect of the K3's AFSK filter, as both have exactly the same audio input to the K3. Attempts to overdrive the radio resulted in no significant change in signal bandwidth. Even when transmitting wideband noise into the K3, the AFSK filter limits the bandwidth. While it is possible to transmit trash that is difficult or impossible to copy, the K3's AFSK filter makes it unlikely that it would generate much interference on adjacent channels." "An important point: Even given the K3's transmit IMD, the occupied spectrum using shaped FSK is much narrower than that when using the internally generated FSK synthesizer. FSK keying sidebands are not unique to the K3; every radio that uses phase-coherent for its internal FSK generator will generate a wider spectrum at essentially any power level. The only differences will be in cases where the IF filter cuts off the sidebands. As an experiment, I decided to see just how wide I could make my signal by overdriving the K3's line input. I cranked the PC's headphone output to 100 percent and drove the ALC as hard as I could by setting the line input gain to maximum (see Figure 7). Some strange spurs show up in the spectrum, but even those are below the keying sidebands of the FSK transmitter (see Figure 2). Even trying to transmit absolute trash I wasn't able to make the AFSK Figure 4 - AFSK with MMTTY using a 512 tap TX BPF with a passband of from 2000 to 2400 Hz, 0 dBm Figure 5 - AFSK with MMTTY with 512 tap TX BPF, 100 W signal as wide as that of the internal FSK generator. The rise in the noise floor over the 2.8 kHz bandwidth occurs, because the K3 amplifies noise from somewhere; at this gain setting it is present whether or not anything is plugged into the K3's line in jack. This behavior is almost certainly specific to the K3, because the K3 scales audio in DSP before it ever enters the RF stages. Other radios are not going to protect you from yourself nearly as well, so you might expect harmonic distortion and other bad things with no limiting prior to the point of RF modulation." Oz1ccm Kel ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
