Most radio hobbyist these days have a PC in the shack these days. Even though convenient and sometimes necessary these PCs can be the source of annoying RFI (Radio Frequency Interference). Of course with many SDR systems, PC’s are a necessary accessory. PC RFI is not limited to affecting SDR systems but also affect analog radios also. It’s just more obvious on SDR radio systems because the RFI can been seen on the spectrum / waterfall display. What causes this? Modern PCs can generate a great deal of local noise via it’s CPU and possibly it’s power supply. The other large RFI generator is the LCD display. Between the two, they can wreak havoc on radio reception. Modern PCs now usually contain multi-core processors that can generate a good deal of RFI. It really doesn’t help matters at all that most PC manufacturers use a large amount of plastic in the cases which allows RFI to leak out more easily. LCD monitors another large source of RFI are all over the place. Some LCD monitors only generate a small amount of RFI while others generate a good deal. All this interference can be seen and heard on your SDR radio system.
What can you do about it?
Frankly not much, but there are a few things that you can do to help mitigate the problem. First of all don’t waste your time putting ferrite cores on everything or wrapping all the cables in tin foil. The RFI is being transmitted over the air directly to your SDR’s antenna from your PC and Monitor and not the cables. Depending on your PC / Monitor setup this RFI can be transmitted a lot further than you think especially if you are using a large receiving antenna. Here are some things to try.
Move Your Antenna – If possible try moving your receiving antenna as far as way as possible from your PC setup. This may help remove most of the RFI problem. However if your PC setup is generating tons of RFI, you may still have some problems. If you can’t move your antenna far away, ideally outside then may have to deal with some level of PC RFI all the time.
Use a Laptop or All In One Computer – This is not always the best solution. Most laptops and some All in One computers use mobile parts for the CPU and graphics subsystem. The mobile parts do use less power and more energy efficient. Some laptops and AIO systems will generate a bit less RFI. The downside to the mobile parts are that they are less powerful than their desktop counterparts. Laptops are not the most efficient type of PC to use for SDR because of the limited screen real estate. So most laptop users want to plug them into an external monitor. Unfortunately doing this may wipe out any gains that you have made with the RFI issue.
Find an RFI Quiet LCD Monitor – Good luck with this. Most LCD monitors are going to generate some level of RFI. Some are tolerable and some are truly awful. If possible, you may want to try a few different monitors with the understanding that you can return the ones that are pretty bad. However, this could take awhile unless you get lucky.
Build You own Custom PC – By doing this you have far more control over what’s inside and outside. It’s not that difficult to do and can save you a few bucks over commercially available PCs. You can choose a high quality power supply and a steel case or aluminum case that will greatly reduce RFI coming from the PC. For example the DIY build of the Quad Core i5 PC used for the RFI test was originally installed in a plastic case. The PC would generate RFI in another PC SDR system located 75 feet away. The power supply and motherboard of this system was recently installed into a dual chamber steel PC case and the radiated RFI was tremendously reduced. The dual chamber case design locates the power supply and associated cables in it’s own separate steel enclosed compartment. The dual case design was never intended to help reduce RFI. The main purpose of the dual chamber design was to hide all those unsightly cables running through the PC. Apparently locating all these cables between another layer of steel helps dampen some of the RFI.
Use an iMac – If you can not do any of the above then you might want to try using an iMac. After testing a 6 year old iMac and a newer 2 year old iMac it appears that these are some of the most RFI quite commercially available PCs around. The older iMac showed a little RFI leakage, but not significant. The newer 27 inch iMac was virtually silent. The reason the iMacs may a bit cleaner than the off the shelf PC is that the whole unit is incased in aluminum. It is also possible that because the high quality LCD is located millimeters away from the main board Apple has gone to some effort to suppress as much RFI leakage as possible so the main board and LCD panel won’t interfere with one another. For whatever reason the later model of the iMac was very clean when it came to producing RFI. To be fair, the latest iMacs with the new 5k screens were not tested so your milage my vary. If you can get one to try out, you might find that the iMac may greatly reduce you in shack RFI problems. One hesitation that radio hobbyist run into with iMacs is that most amateur radio software and SDR software requires Windows to run. Even though iMacs run Apple’s own operating system OS X, the iMac can run Windows software very easily. Using an included program called Bootcamp will allow you to install Windows 7 and up in it’s own bootable hard drive partition. The default boot up operating system can be selected via the keyboard during the startup process. The iMac will always boot up in the default operating system that is selected until changed again via the keyboard during the next startup sequence. Since most iMacs these days run Intel hardware there is absolutely no difference between running Windows on an iMac versus a regular PC. Another possibility that some users like is running Windows under a software package called Parallels. Parallels will allow the iMac user to create Virtual Machines that can run Windows, Linux, and other operating systems that can be run simultaneously along side OS X applications . This allows the user to use their iMac productivity software an Windows applications at the same time. With new beefier multi-core processors used in most iMacs, the Parallels / Windows combination works very well. Since Parallels works by sharing the processor cycles between OS X and Windows, you could see an occasional slowdown if an OS X application and Windows application gets really busy at the same time. Fortunately this is usually fairly rare. The banner at the beginning of this article shows Parallels be used to run a Windows based SDR program and OS X’s Safari browser simataneously. About the only downside to running Windows on an iMac is that you have to buy your own copy of Windows. Another issue that comes up is cost. It is generally thought that iMacs are more expensive than PCs. The answer to that is yes and no. The new Core i5 iMacs with a gorgeous 5K screen clock in at around $1800 before any discounts. A similarly configured PC with a 4k display will come in at close to the same price. However you do get what you pay for. The iMacs are extremely well constructed and very attractive, Also OS X has gained a reputation over the years as being very stable and far more secure than Windows when it comes to being resistant to malware and hacking which is more important than ever these days. Not to mention OS X now has very tight integration with Apples other devices such as iPhones and iPads. This is something that is just not available in Windows. Also compared to Windows PC’s the iMacs are better long term investments. For example that $1000 PC you bought after 6 months will be worth about $300 if your lucky. After a year maybe $100 and after that about $0. The iMac in comparison after 6 months will retain about 2/3 or more of it’s value and about the same after a year. Even when it gets 5 or 6 years old there will be some remaining value compared to a comparable PC.