On Jan 22, 2009, at 10:16 AM, Simon (HB9DRV) wrote: > A good soundcard will not suffer from intermod - just like a good > radio.
Not just with intermod, but many mediocre sound cards also suffer from bad second harmonic distortion that could be more troublesome than IMD. I have measured poor ones that have second harmonic distortion as bad as -55 dB, and I have seen traces of the second harmonic of a strong signal from them on a waterfall. I.e., similar to the problem with the K3 Line Output before applying the K8ZOA modifications. Fortunately, unlike IMD, you can nudge the VFO knob a little and get the second harmonic to fall at a different spot in the audio spectrum from a weak signal that you are trying to copy. Simon is right. The reason for using a better quality sound card with narrow band rigs is not necessarily to get a lower noise floor. It is to get lower IMD and harmonic distortion. Even the worst sound card that I have encountered in the past half dozen years had a dynamic range which beats the analog gold standard that we use on HF RTTY, the HAL ST-8000 modem. If you are using filters that only lets through a single signal to the sound card, none of this matters -- you should be able to live with 50 dB of dynamic range if only a single signal reaches the sound card, and the rig's AGC can keep the amplitude of that signal moderately constant. But many of us digital mode operators use at least a 2 kHz bandwidth; that is where good and poor sound cards can make a difference. However IMHO, it is always better to use even a poor external output sound card; to prevent system chimes, music, etc, from getting out on the air. 73 Chen, W7AY _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
