Mike, The center noise is due to two things, ground loops picking up AC and the quality of your sound card.
The ground loops can be solved by using an RF isolation transformer on the antenna and by only using one ground wire from your PC to the RXTX. In the case of a Delta 44, a good quality sound card widely used for SDR work, if you only use the audio grounds from the RXTX to the Sound Card for your ground connection, in other words break the ground from the Rocky Serial Interface Board PTT line to the RXTX, then that will get rid of that ground loop. Some also use a battery instead of an AC power supply to get rid of that ground loop as well. But I use an AC power supply and it works fine. It has to be a good quality power supply however. In the case of the quality of the sound card you generally get what you pay for. With a Delta 44, I get no center spike at all, just a slight cusp in the noise floor at the center frequency, once the ground loops are eliminated. This is due to the frequency response of the Delta 44 falling off near DC, which is as it should be. Many sound cards are a lot worse than the Delta 44. With my AC97 built in sound card on an ASUS motherboard the center spike was so bad I was getting audio feedback. Also apparently some sound cards do not have the audio ground isolated from the case of the computer. So some have used audio isolation transformers as well. See: http://ph.groups.yahoo.com/group/softrock40/photos/browse/9d29 For some sample displays. Bill WA9PWR Ìé÷áÞë Åìì. ÌðáëáóêÜò wrote: > Hello to all. > I just start to play with my Softrock v6.2 RXTX 80/40. > I have two things niticed and need your help. > > 1. Noise around center frequency on both bands. A crazy QRM 5KHz +/- . > 2. Receiver is deaf about 50 percent than an old HF radio. A little > difference to better using MGK software. > > Any help ?? > > (sorry for my English) > > SV5BYR > Mike > > > > ------------------------------------ > > Yahoo! Groups Links > > > > > > > >
