On 2/2/2011 11:08 AM, Kok Chen wrote: > My own K3 will not do any better than -35 dB transmit IMD
An important source of IMD (and harmonic distortion) is the sound card used to generate the digital signal. The sound cards in IBM T-series Thinkpads are better than average, but their distortion increases by 10dB as you get close to clip level, even without clipping. Reducing the output level by only 6dB reduces distortion by 10dB. Thus, my advice for setting up sound card levels is to first crank the output level up to the point where you just begin to hear distortion (or see it on a scope), then back it off by at least 6dB (half the voltage). Audio transformers can also be a source of IMD (and harmonic distortion), and that distortion increases dramatically if they are driven from a high impedance source, even at relatively low signal levels. The ideal way to drive an audio transformer is from a source having a very low output impedance. Adding resistors in series with the source is a very bad idea, and CAUSES the transformer to produce distortion. This was a problem with the line output in early production K3s. . It's OK to add a 10dB pad (resistive voltage divider) between a sound card and the rig (K3 or other) IF the pad has a low output impedance. That's easy to do -- simply use a low value for the output resistor. Most sound cards are designed to drive headphones to about 1 volt peak, with load impedances between 8 and 600 ohms, so a good 10 dB voltage divider would be something like 220 ohms in series and 68 ohms as the load, or 180 in series and 56 ohms as the load, or 150 ohms in series and 47 ohms as the load. This range of values won't overload the sound card, and will minimize any distortion produced by the input transformer in the K3. BTW -- the same sort of precautions should be taken with ANY input transformer, not only those used in the K3. Finally, one must also be careful not to overdrive the K3 with too much audio gain. Follow instructions in the manual carefully for this. Note that these precautions apply to ALL audio feeds to a ham rig, including all digital modes and voice playback for contesting. 73, Jim Brown K9YC ______________________________________________________________ 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
