One big source for ground loops in systems having multiple AC powered devices is the "protective earth" (the third wire of the power cord). In the PC and many power supplies, the PE of the AC service is tied to the metal chassis and that is tied in turn to the "ground" and/or negative DC terminal. You may wish to investigate and either start clipping shields on audio cables at one end to guarantee only a single return path exists or selectively "lift" the ground on the AC power cord of a single device. The latter is a delicate issue with respect to electric codes, safety, etc. and should only be done for investigatory purposes (wink wink).
The same issue has existed for "centuries" when one is attempting to make precision measurements of small/HF signals using oscilloscopes. Tek used to sell fancy AC supply isolation transformers that would float AC supply and lift the ground of the scope since it was tied to the chassis and PE of the AC service. Nobody used them; they would just clip the PE on the power cord and make sure not to stand in water, barefoot, while using the scope. It's also the reason "real" analog engineers stand around with one hand in their pocket. Everyone just took for granted we were some form of deviant, when in fact it was a based upon a safety issue! (Many still are some form of deviant irrespective of their proper safety habits). Lee Pedlow NG6B Systems Engineering Sony Electronics, Inc. San Diego, CA (858) 942-2538 [EMAIL PROTECTED] CONFIDENTIAL This email is covered by the Electronic Communications Privacy Act, 18 U.S.C. 2510-2521 and is legally privileged. This e-mail is intended only for the personal and confidential use of the individual(s) to which it is addressed and may contain confidential information. If you are not the intended recipient, you are hereby notified that you have received this document in error, and that any review, distribution, copying or disclosure is not authorized. If you have received this communication in error, please notify the sender immediately by telephone and destroy the message. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of n4hy Sent: Tuesday, June 14, 2005 9:31 AM To: Joe - AB1DO Cc: Flexradio Reflector Subject: Re: [Flexradio] FW: Yikes, I got Spikes Before I had the picture blocked. I got a copy. This has nothing to do with the INA but is a horrible DC noise problem. LOOK FOR GROUND LOOPS> Bob Joe - AB1DO wrote: > Hi all, > > I am noticing a "strange" phenomenon, visible in all of the console > versions I have, i.e. from 1.3.4 on. On the far left hand side of the > panadapter display, and _only when preamp is off_ I see the noise > floor rise to a peak. To focus in on this more, I used SPEC to center > what turned out to be an approx 3kHz wide central peak with many > harmonic spikes: > > In the above, the rig is connected to a dummy load. When the preamp > setting is increased, this peak gets smaller, to virtually disappear > at a preamp setting of med or higher. The preamp is the ONLY > adjustment I can make that seems to influence it at all. If I turn the > power to the SDR box off (after having turned the console on with it > powered on), the central peak remains visible, but the harmonic spikes > reduce greatly in value. When tuning up or down, the peak remains on > frequency until it reaches about + or - 1.5kHz, at which point it > flips to the other side, that is from -1.5kHz to +1.5kHz and v.v. It > is also visible on every single band, at every single frequency. Also > the peak value of -110dBm is absolute, that is if I calibrate the > console to show a noise level of say -90dBm, it is not visible. > > Before I go tearing the whole shack apart here in search of the > interfering source, I'd like to know if anyone else has experienced > this, if it is something inherent in the S/W or hardware, or if it has > something to do with my settings (I recalibrated everything using > Elecraft's XG1 signal generator, but to no avail). If none of these, > I'd appreciate a clue as to what I should be looking for. > > My setup uses the Delta 44 and USB adapter. > > Thanks, > > Joe > >----------------------------------------------------------------------- - > >_______________________________________________ >FlexRadio mailing list >[email protected] >http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > > _______________________________________________ FlexRadio mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz
