USB Isolators/Ground Loop Eliminators for Software Defined Radio Applications
|By Bruce A.Conti - May 2018|
Intro to USB|
USB stands for Universal Serial Bus. It's an internationally standardized specification designed for easy plug-and-play hot connection between peripherals and a host computer. In other words, the USB connection can be safely plugged in and unplugged from the peripheral and host without powering down the device or computer.
USB data transfer rates, specified in bits per second (bps), are low speed 1.5 Mbps, full speed 12 Mbps, high speed 480 Mbps, super speed 5 Gbps, and 'super speed plus' at 10 to 20 Gbps. The USB 1.1 data rate is specified for low and full speeds. USB 2.0 can operate at low, full, and high speeds, but not all USB 2.0 devices are capable of high speed. USB 3.0 supports data transfer rates up to super speed, 3.1 to 10 Gbps, and 3.2 to 20 Gbps super speed plus. Software defined radio (SDR) receivers covering LW, MW, and SW, such as the Microtelecom Perseus and WiNRADiO Excalibur, typically require USB 2.0 high speed. Some of the extra wideband SDR's for FM broadcast spectrum capture such as the Elad S2 and S3 units may require USB 3.0 super speed.
The typical USB 2.0 high speed cable for connecting a SDR receiver peripheral to a host computer consists of four wires. The power from the host is via red (+5 VDC) and black (ground) wires. The differential data lines of the USB serial interface are green (D+) and white (D-). The more expensive high-end USB 2.0 high speed cables are typically shielded, sometimes including a ferrite bead at each cable end for common mode choking of RF emissions. Cheaper cables may not have a black wire, instead using the shield as the power supply ground. The cheapest USB 2.0 low/full speed cables aren't shielded, and may only consist of three wires, combining the data return line and power supply ground via the same wire - stay away from these! Maximum high speed cable length is specified at 5 meters to limit 'round-trip delay between host and peripheral. If the host sends an instruction to the peripheral and it doesn't receive a reply in time due to excessive cable delay, then USB operation will fail.
The USB 2.0 high speed power supply voltage specification is +5 VDC nominal, +5.25 maximum, and minimum +4.40 at the peripheral to allow for voltage drop across the cable. A low power peripheral requires up to 150 mA of current (0.75 watts at 5 volts). A high power USB 2.0 peripheral can draw as much as 500 mA (2.5 watts at 5 volts), while a USB 3.0 device may demand 900 mA (4.5 watts at 5 volts).
The standard USB 2.0 connector at the host computer is Type A; pin 1 = +5 VDC, pin 2 = D-, pin 3 = D+, pin 4 = ground. While a Type B connector with the same pinout but different style is common for peripheral connection, the type of connector used by a SDR can vary in pinout, shape, and size between several standard types. There are also multiple grades of connectors from cheap consumer types to heavy-duty industrial. Industrial strength connectors for general electronics applications are constructed with higher reliability contacts specified for at least 1,500 mating cycles. Additionally the locking tactile feel of a high retention industrial connector ensures proper insertion and prevents it from coming loose.
The USB Ground Loop Problem|
I learned that the USB ground loop problem existed after stumbling upon a number of pro audio websites and blogs that discuss this as a major problem in the recording industry. According to my internet research, numerous issues involving artifacts and dynamic range of digital audio can be traced to USB ground loops. So USB isolators/ground loop eliminators are extremely popular among audiophile and pro audio circles. They talk about vibrant soundstage and deeper blackness when describing the improved audio performance achieved by breaking the USB ground loop. Then I connected the dots between the pro audio reports and recently experienced computer noise issues during a group DX'pedition. It was one of those "Ah ha!" moments.
A ground loop may actually exist between an SDR and computer, or between the antenna and computer, via the USB cable. The result is a higher noise floor. As indicated earlier, audio performance may also be impacted by a USB ground loop, so it shouldn't be thought of solely as a receiver RF noise issue.
Think of this differently than an antenna noise problem. Ground loop currents can introduce noise not only through the SDR RF input, but also in the computer audio. The USB isolator breaks a ground loop between the computer and SDR. After judiciously reducing noise at the antenna which remains mission critical in any DX setup, breaking the USB ground loop may provide an additional 3 to 6 dB of headroom and improve audio clarity, which could make the difference between observing a weak signal and hearing audio. Testing with the WiNRADiO Excalibur SDR and a laptop computer proved it out, however results may vary with different SDR's and host computers.
Break the Ground Loop Construction Project|
One simple solution to breaking a USB ground loop and reducing noise is to disconnect the often dirty +5 VDC power at the USB output of the host computer. Note that this does not completely isolate the USB between the host computer and SDR because the D+ and D- data lines will still be connected. Ground loop currents may yet flow through the data lines, so results could be insignificant to nil but worth trying for the cost savings versus purchase of a USB isolator. This project also allows the USB power from the host computer to be replaced by an external low-noise power supply via the binding posts which may prove to reduce noise for SDR's that aren't powered by a separate supply.
(1) short-length USB 2.0 type A male to USB 2.0 type A female high speed extension cable
(1) double pole double throw on/on toggle switch
(1) red binding post
(1) black binding post
(1) non-polarized ceramic bypass capacitor, 0.1 µf
(1) small plastic project box
1. Cut the USB extension cable in half. Dejacket a few inches of the outer insulation from each half of the cable.
2. If the cable is shielded, cut away the shield to expose the internal wires. There should be four internal wires; red, black, green, white.
3. Drill holes of appropriate size for mounting the switch and two binding posts in the project box.
4. Cut two slots on the sides of the project box to accommodate the USB cable ends.
5. Solder the USB cable red wire of the female end (peripheral SDR) to the common terminal of one switch, solder the USB cable red wire from the male end (host computer) to one of the on positions, and wire the red binding post to the other on position. Refer to the schematic diagram.
6. Follow the same wiring procedure for the USB cable black wires and binding post to the remaining switch.
7. Solder the 0.1 µf bypass capacitor between the black and red wires of the switch common terminals.
8. Solder the green wire from the cable female end to green of the male end, then solder white to white.
9. Complete assembly by marking USB/isolated switch positions appropriately, secure the cables and close the box.
Installation and Operation
1. Plug the USB isolator into the host computer and into the USB cable of the SDR.
2. Set the switch to the USB position.
3. If the SDR requires USB power, i.e. the SDR doesn't have its own external power supply, then an external +5V low-noise power supply must be connected to the binding posts.
4. Power up the SDR and start the SDR software as you would normally.
5. Once the SDR is in operation, toggle the switch from USB to isolated position.
Note: If the SDR has its own separate power supply (not powered by USB), this start-up procedure must still be followed. In most cases, the +5V from the USB host computer is required to initialize the USB circuitry of the SDR. Without +5V, the SDR may fail to start up properly. Once the SDR is powered up and operating, then the switch can be toggled to the isolated position.
The short length of USB extension cable is used to simplify wiring and prevent miswiring. Alternatively, bulkhead/panel mount male and female connectors can be mounted on the project box for a cleaner more professional looking unit, but extra caution must be taken not to miswire the connector pins. It's much easier and less confusing to take advantage of the color coded wires of the halved cable.|
If this homebrew USB ground loop breaker reduces the noise floor by 3 dB or more, awesome! You've saved yourself hundreds of dollars. If it doesn't produce the desired results, then a true USB isolator might be necessary to get the job done. Read on...
Intona USB 2.0 Hi-Speed Isolator|
The USB isolators used in pro audio are typically only full speed (12 Mbps), Analog Devices ADuM chip-based designs, not fast enough for SDR applications. The high speed (480 Mbps) isolators are much more expensive as they require more elaborate circuitry. Found and purchased the Intona USB 2.0 Hi-Speed Isolator (https://intona.eu/en/products), pricey but not too far out of reach, and it works! Reduced the noise floor by 3 to 6 dB. This could make a huge difference in trying to dig out weak signal DX on clear frequencies such as 828 South Africa or 1206 Mozambique, or in my case getting audio from weak transpacific signals at dawn. The Intona is plug and play. Simply insert the unit between the host computer and SDR with high speed USB cables, then power up the SDR normally.
It isn't perfect. Discovered an open carrier at approximately 488 kHz intervals; 731.9, 1219.9, 1707.9 kHz, etc. It's a very quiet spike like an open carrier, no noise, buzzes, or blips. My guess is that it's related to the galvanic isolation 5VDC to 5VDC internal switching power supply frequency, although adding bypass capacitors to filter out the noise was unsuccessful. The Intona also has a Xilinx Spartan XC3S50A FPGA on each side, so the clock frequency could be the source unless it's synchronized with the switcher frequency. To remove USB jitter, a primary cause for concern in pro audio applications, the FPGA on the host computer side unpacks the USB data for the isolators, and then the FPGA on the peripheral SDR side repacks/reclocks the USB data. Since USB communication is bidirectional, the same repacking is done from the SDR to host. Apparently this also eliminates latency problems encountered when isolating USB data, a limiting factor in cheaper USB 2.0 full speed devices. Serial data isolation is through Si8660 6-channel digital isolator chips.
The Intona is manufactured in Germany. It's available in consumer and industrial grade models for under $400 from U.S. distributor Kitsune HiFi (KitsuneHiFi.com) and Canada distributor L'Atelier Audio (atelier-audio.com).
iFi iGalvanic 3.0|
The iFi iGalvanic 3.0 USB isolator (ifi-audio.com) is promoted as the world's first USB 3.0 galvanic isolation device. The iGalvanic 3.0 meets USB 3.0 standards with USB 3.0 connectors and it operates up to super speed 5.0 Gbps. (USB 3.0 connectors have more pins/wires than USB 2.0, but the USB 3.0 connectors are backward compatible with USB 2.0 cable connectors.)
Like the Intona USB 2.0 Hi-Speed Isolator, the iFi iGalvanic 3.0 uses an internal switching power supply to provide isolation between the host computer 5V USB power and the device, resulting in RF noise spikes at regular intervals. The iFi and Intona USB isolators are designed primarily for professional audio applications. Although extremely effective at elimination of ground loop hum and noise issues at audio frequencies, apparently the designers don't pay much attention to RF noise.
Initial tests show that the iGalvanic 3.0 isn't as good as the Intona USB 2.0 Hi-Speed Isolator, perhaps due to the higher operating speed and higher current power supply required for USB 3.0. The iGalvanic 3.0 internal switching power supply is noisier than the Intona USB 2.0 especially at frequencies above 20 MHz.
iFi iDefender 3.0 and iPower 5V|
The iFi iGalvanic 3.0 USB isolator was purchased as part of a package deal for under $400 from Music Direct that included the iDefender 3.0 Ground Loop Breaker and iPower 5V (www.musicdirect.com/accessories/iFi-Nano-iGalvanic30iDefender30iPower-Bundle). The iDefender 3.0 operates similar to the "Break the Ground Loop Construction Project" by disconnecting the USB 5V power from the host computer and replacing it with a low noise external power supply, in this case the iPower 5V. The iDefender 3.0 automatically switches from USB power to the external low noise power source when that external power is applied via a micro USB connector on the iDefender. The iPower 5V power supply has an audio band noise floor spec of 1 µV utilizing active noise cancellation technology with a "super-silent" 12 element output array. The iDefender 3.0 and iPower 5V combo would be an ideal alternative for those who aren't equipped to build the homebrew project.
Installation of the iDefender 3.0 and iPower 5V on the output of the iGalvanic 3.0 isolator eliminated much of the switching power supply noise of the iGalvanic, bringing it closer to the performance level of the Intona. As of this writing, the iFi iGalvanic 3.0, iDefender 3.0, and iPower 5V package deal is probably the only USB 3.0 super speed isolation available in the under $400 price range, but if the SDR only requires USB 2.0 high speed then definitely get the Intona.
Ciunas Audio USB-HUB|
While test and documentation for this article were well underway, I became aware of another candidate for SDR applications; the USB-HUB from Ciunas Audio in Ireland (www.ciunas.biz). It uses an internal battery to provide USB 5 volts on the isolated peripheral side instead of a 5 VDC to 5 VDC switch mode power supply as used in the Intona and iFi units. The up side is that it eliminates switching power supply noise. On the down side, it will only run for 8 to 10 hours before the battery needs to be recharged which is unacceptable for overnight automated SDR operation that can exceed 14 hours. However the USB-HUB will run continuously on an included external trickle charger.
"The ISO-HUB is powered directly from Lithium Phosphate battery, no voltage regulators which impose their own noise signature," said John Kenny at Ciunas. "The high speed USB isolation is achieved through a chip dedicated to this purpose rather than FPGA's, and finally the isolated USB signal is reclocked/reformed to remove the USB jitter that all isolators introduce. All is housed in an aluminum case for RF shielding, and there are 4 isolated high speed USB 2.0 output ports, not one."
The USB-HUB sells for twice the cost of the Intona; $746 in U.S. dollars, and it's only available direct from the manufacturer with no North American sales reps. Ciunas Audio contacted me after reading news in a shortwave bulletin about my USB isolation investigation. A unit was reported to be under test by a SDR user (WiNRADiO and SDRSpy), and Ciunas Audio promised to forward the test results to me. That was months ago and still no test results have been promulgated, so the performance of this unit for SDR applications remains unknown but promising.
Highly recommended for AM HF (LW, MW, SW) hardcore SDR DX'ing: Combination of the Intona USB 2.0 Hi-Speed Isolator with either the homebrew USB ground loop breaker or the iFi iDefender 3.0 Ground Loop Breaker and iPower 5V installed on the peripheral side of the Intona. This eliminates much of the Intona switching power supply/clock noise in the higher frequencies of shortwave, but the 'open carrier' signals at 488 kHz intervals, three of which are in the broadcast band, still show up. Not a problem. It's the best performing device found so far in the emerging technology of USB ground loop/noise elimination devices.
As more and more peripherals are requiring high speed and super speed USB data rates, the development of USB isolation devices is trying to catch-up. One to watch in the under $100 price range is the HiFiMe High Speed USB Isolator (https://hifimediy.com/high-speed-usb-isolator-480Mbps) using the recently developed Silanna ICE08USB chip for data isolation. The HiFiMe high speed isolator was announced nearly a year ago, perhaps prematurely as it has yet to become available, currently listed as delayed.|
Another worth considering at $199.99 is the CoolGear USB 2.0 High Speed 480 Mbps Isolator Adapter (http://www.coolgear.com/product/usb-isolator-high-speed). It's the first I've found to use the new Silanna chip. Promoted as "the first of its kind to fully support USB 2.0 480 Mbps data transfer speeds," little else is known about the performance of this isolator. It can be powered by USB or an external supply if like other designs an internal switch mode supply generates RF noise.
The design of USB isolation devices is driven primarily by the pro audio market, where demand is increasing for higher speed USB to operate higher sample rate DAC's and processors. So it's expected that in the future more high speed and super speed USB isolation devices will become available at lower cost for SDR noise reduction applications. Otherwise early adopters of the technology unfortunately will find that the choices are limited and the entry price is steep, but well worth the investment for hardcore DX'ing not to mention the added benefits of improved audio.
73 and Good DX!