Why Apple’s iMac May be the Best PC for SDR Applications
To illustrate the type of RFI interference we are talking about, a couple of different PC’s configurations were tested against the iMac and some screen shots taken show the results of how much RFI was being generated from a PC vs iMac. The SDR platform used was the SDRPlay and it’s accompanying SDRUno software. The reason the SDRPlay was used was because the SDRPlay often takes some flak about being more susceptible to RFI interface because of it’s plastic case. This is simply not true when it comes to computer generated RFI. The same tests were done on an Elad FM-Duo which is housed in a metal case and the results were pretty much identical. The testing was done on 20 meters with the receiver connected to an outside dipole with nearest leg located about 25 ft away from the SDR / Computer installation. To illustrate that this is not a PC cable RFI issue, an Icom 7300 which was not connected to a PC and connected to the same antenna showed the same type of interference problems in it’s waterfall / spectrum display. When the offending computer was shutdown the Icom 7300’s scope showed that the problem goes away. With the iMac in operation, there was no noticeable interference present on the Icom 7300’s screen. If the SDR’s antenna was removed there was no RFI present or anything else when the offending PC was running. The monitors tested are current models that can still be purchased. The approximate same spectrum slice was also used for each test. The screen captures below taken from the SDRPlay and SDRUno setup are representative of what was seen on the Elad FDM-DUO’s spectrum / waterfall display as well as the Icom 7300’s waterfall / spectrum display.
Results (click on the images to make larger)
In figure 1 you can see with the SDRPlay connected to the Core i5 iMac with a 2560 x 1440 display that there isn’t a lot of RFI interference present in the spectrum / waterfall display. The power line interference is always present on just about all frequencies. This seems to be related to neighborhoods that have underground power systems in the area.
Figure 2 shows the SDR play connected to a Core i5 PC using a HP 34″ inch 3440 x 1440. You can clearly see that in this setup we see some CPU noise along with some monitor noise showing up in the waterfall. The CPU noise can be generally identified as a fairly wide band of interference on the waterfall. Monitor noise often shows up with very closely space small spikes that go completely across the waterfall. The HP monitor actually is not to bad when compared with other monitors. This particular monitor use a SVA type panel and a large power supply which may or may not have anything to do with it’s lower RFI output. To be fair though, once you get to around 80 meters then the background noise will significantly increase, but not many spurious spikes show up. This setup was also was tested with another HP 22 inch 1080p IPS panel (not show) which was significantly noisier than the 34 inch panel. So basically size doesn’t matter or the manufacturer when it comes displays or display types. For example the test iMac also uses an IPS monitor with completely different results. So when buying a monitor for the shack it is best to try before you buy if you can.
Figure 3 shows what happens when the same Core I5 PC is connected to what is considered a high end Samsung 27″ 1080p IPS panel. You can see that the Samsung is a hot mess with tons of RFI spikes everywhere. This is what way too many SDR hobbyist see in their waterfall display and go running for the ferrite and tin foil hats. Remember anytime this thing is on it transmitting directly to your antenna so just put the ferrite and tinfoil away. It’s understandable that most SDR hobbyist really really don’t want to blame their troubles on that sweet new $275 monitor they just bought. Just take it back before its too late. The Samsung is so bad that it it is not allowed anywhere close to the shack.
As you can see computer generated RFI can be a real pain and often very hard to resolve. Finding a commercially built RFI quite PC and monitor setup can require a lot of trial error and frustration. Remember. first always try to relocate your antenna far away from your SDR setup to see if that helps. Moving the antenna may not help if the computer system you are using emits a lot of RFI. The RFI may still be able to reach your antenna anyway or leach into the feed line. If you have a SDR that features a network connection like the Anan or Flex series of SDRs then you may also be able to locate the SDR and the Antenna far away from the PC setup. If you have a USB connected SDR, then you want to consider trying a 25ft. USB extension cable and try locating the SDR and antenna as far away as possible. If you can’t do all of this or take the steps mention above and you still have serious RFI problems then you might want to consider making an iMac the heart of your SDR system or central shack computer.