Category Archives: mods

USB device cable shield connection – grounding it or not?

Colinb, an user from forum post this here:

I have been trying to understand how to ideally handle the cable shield
on a USB device. (Full Speed USB, in this particular instance.)

As seems to be the case with many signal integrity issues,
contradictory recommendations abound, each with its own unsupported
claims. Even authoritative-sounding sources such as Texas Instruments,
Intel, FTDI, and Cypress Semiconductor seem to disagree on the correct
way to handle the cable shield on USB devices.

Contrary to my initial supposition, the purpose of the USB cable shield
is not to protect the USB data lines from outside interference, but
rather to prevent the USB device from radiating EMI.

Here are some of the options that have been recommended.
Note that (2)—series capacitor to pass high frequencies only—seems to
directly contradict (3)—series ferrite bead to block high frequencies

(1) Connect shield directly to signal ground.

– “Full speed devices use a shielded cable which requires that the
connector shell be tied to the ground plane.”
Intel. EMI Design Guidelines for USB Components. Sec 5.4 (p. 9).​

(2) Connect shield to signal ground through a capacitor.
(Possibly with high-value parallel resistor approximately 1 Mohm.)

– Connect shield to signal ground with 0.01 µF to 0.47 µF capacitor.
FTDI. Debugging FT232BM and FT245BM Designs. Section 3.2 (p. 11).​


– Cypress recommends a 1 Mohm resistor in parallel with a 4.7 nF capacitor.
Steve Kolokowsky & Trevor Davis (Cypress Semiconductor). Common USB Development Mistakes – You Don’t Have To Make Them All Yourself! Figure 7 (p. 7).​


– “Tying the shield directly to ground would create a direct path
from the ground plane to the shield, turning the USB cable into
an antenna. To limit the USB cable antenna effect, it is
recommended to connect the shield and ground through an RC
filter. Typically, R = 1MΩ and C = 4.7nF in Figure 3-5.”
Atmel AVR1017: XMEGA – USB Hardware Design Recommendations. Section 3.3.3 (p. 8).​

(3) Connect shield to signal ground through a ferrite bead.

– “Place a ferrite in series with the cable shield pins near the
USB connector socket to keep EMI from getting onto the cable
Texas Instruments Application Report. USB 2.0 Board Design and Layout Guidelines. Sec 2.2.4 (p. 3). SPRAAR7 – December 2007.​

(4) Do not connect cable shield to ground on the device at all.

– As referenced in the AAC thread where to terminate usb cable shield?, Hardware Book says USB devices must
not connect the shield to their own ground.
Hardware Book. Universal Serial Bus: Shielding.

Whether or not the device has a metal chassis, and the handling
of chassis ground and signal grounds, (as well as how the USB cable
ground is connected to either one) is certainly important as well, but
this isn’t clearly discussed in most of the writings on USB cable shield

The device I’m developing is a bus-powered device which will likely be
in an unshielded plastic enclosure.

Thanks in advance for any bits of wisdom on this topic full of
contradictory information. I recently posted this question on si-list,
and even there I got little in the way of answers.



RTL-SDR: Passive radar with $16 dual coherent channel receiver

My previous post describes the $16 dual channel rtl_sdr dongle hack. In the last few days I’ve done some more testing and it turns out I can use the system for passive radar! I didn’t expect this, because the receiver only has 8 bits and passive radar requires a lot of dynamic range.

Airplanes and occasional specular meteor echoes.

I hooked up the two channels into yagi antennas that we have used with Echotek and USRP receivers for passive radar. One of the antennas was measuring the transmit waveform, and the other was measuring the echoes. I ran a measurement, and to my great surprise, it worked just fine.

I did tweak the signal levels a bit in order to ensure that I optimally use the dynamic range. I also had the bandwidth set to 2.4 MHz, giving me about 4.5 bits extra dynamic range after filtering the signal to 100 kHz in single precision floating point.

Two log periodic antennas used to passive radar with the dual coherent RTLSDR R820T dongle.

This really does give us a glimpse of the future where high end digital receivers will cost $10 per channel. The low end ones are already in that price range. Think of all the potential science that can be done!


RTL-SDR: $16 Dual-channel Coherent Digital Receiver

I have been playing around with the cool RTL dongles (more on rtl-sdr dongles on superkuh’s web page or that you can buy on e-bay for about US$8 (including shipping). These are very capable 8-bit digital receivers that have up to 2.4 MHz bandwidth and can tune anywhere between 24 MHz and 1850 MHz
I recently came up with a trivial hack to build a receiver with multiple coherent channels using the RTL dongles. I do this basically by unsoldering the quartz clock on the slave units and cable the clock from the master RTL dongle to the input of the buffer amplifier (Xtal_in) in the slave units (I’ve attached some pictures).

I originally drove the master crystal with both dongles, which also worked. However, Ian Buckley pointed out to me that a more typical way of doing this is feeding the signal into Xtal_In (in the pictures below). So I tried that too, and it also worked. I’m still not sure what the optimal setup is, as there is no schema for the dongle, but both methods I’ve tried so far have worked in practice.

This is how you make a dual coherent channel digital receiver with $16.  The clock drive probably won’t be enough for many of these, but this can be fixed with a buffer or some other active splitter.


The oscillator is wired using a piece of 75 Ohm antenna coax that came with the dongle. It’s like they designed the dongle for  multi-channel coherent applications.

This has some implications for low cost geophysical instruments. It will be possible to use this receiver for the 150/400 MHz beacon satellite receiver, as this only requires that the receivers have clocks that are locked with each other. Interferometry and passive radar are other application examples. With more than two locked channels, applications such as imaging start to become possible.

I’ve made some relative phase noise measurements, and the systems don’t have detectable sample drift over two hours, and their relative phase is also pretty stable.

Spectrum at 1 Hz sample rate of the relative z_1/z_2 phase signal going into two receivers.


IQ plot of the z_1/z_2 relative phase signal over ~6000 seconds at 1 Hz sample rate.

And oh, by the way, I found this nice usb hub, which I’m going to use to hopefully get a 7 channel coherent rtl system.

Hub with the right usb port orientation for rtl dongles.

Stay tuned for more results. I already have some pretty nice passive radar results using the system, which I’ll be posting in a few days.

Update: Apparently three dongles will also run fine from one master clock. I know the clock isn’t split correctly, but adding any components would increase the total cost and the whole point of this exercise is to determine what the lower bound is for software defined radios.

Three channel coherent RTLSDR receiver.


The “MINNIE”: RTL-SDR and DIY Upconverter in the same enclosure.

By Nick G0CWA

Hi this is my latest design for a RTL-SDR communications receiver. The set amounts to little more than a switchable receive up converter. Although designed for the Realtek RTL2832U Chipset + Elonics E4000 tuner, it will work with any of the other compatible tuners. I would personally recommend SDR# as the control package for the dongle.

The frequencies from DC to ~1.7GHz are covered in two ranges: High range ~60MHz to 1.7GHz (depending on tuner chip) with the normal gap around 1.1GHz And Low range DC to 60MHz.

The main aim was to build a version of my designs to comfortably fit into a Lap top bag and be fully portable and powered from the PC USB port. The completed unit including a dongle fits into a small die-cast instrument case measuring only 55x25x125mm. To keep the circuitry size to a minimum I have not included any extras e.g. pre-amp or antenna switching etc.

SDR Receiver using a tv dongle rear view

There is nothing clever or particularly original in the design with one exception the use of a 125MHz conversion oscillator so avoiding the VHF broadcast band on the output and reducing any sensitivity issues. The 5V supply is taken off the dongles USB connector by opening up the dongle case, carefully, and soldering a fine insulated wire to the appropriate USB connector pin taking this out through a small opening cut in the side of the dongle case.

SDR Receiver using a tv dongle inside

Follow my track layout and use top quality miniature co-ax for maximum sensitivity I used silver plated. Do NOT use single strand wire for any RF signals as these dongles are very sensitive to mis-matches and unless you are very lucky will shut down and work poorly, if at all.

SDR Receiver using a tv dongle schematic

SDR Schematic (Larger View)

You may notice that D1, D2, C11 and C12 do not appear on the PCB top they are actually soldered on the underside of the board to save space. There are also two wire links on the top of the PCB to add.

The RF input to the dongle is via a connector shown in the “real world” PCB layout diagram. This connector was “scavenged” from a cheap plastic bodied co-ax plug which is soldered to the PCB in a small slot. TAKE NOTE the connector is soldered on both sides bridging the two earth pads to complete the ground track to both sides of the PCB; the centre pin is connected via a small piece of copper wire to the PCB.

The only problem I had was when switching between the receive bands sometimes the dongle would lock up, this was due to the current demands of the relays and smoothing caps I managed to “cure this problem” by reducing the values of C11 and C12 anything above 100 nF should be ok.

The actual PCB dimensions and cut-outs suited both my box and gave sufficient clearance for the dongle, antenna socket and switch. Adjust these to suit your case etc.

The coils are all 6.5 turns of ~1mm diameter copper wire 0.25 inch diameter by 0.4 inch long, the relays are both NEC/TOKIN UB2-5NE 5V Coil DP Ch-over types although any similar equivalent ones may be used. The regulator is an LM1117T 3.3V unit. The crystal oscillator is a 3.3V 125MHz crystal oscillator module, any oscillator module frequency above 60MHz can be used but I can’t guarantee the receivers performance or immunity to de-sentisation/interference from the VHF broadcast bands.

For any further information about operation and performance check my MK2 design write up also on this site.

Enjoy this design hope you find it of use.

The Radio board and QRZ forums With any questions, please only contact me via these routes if you have any questions, I can’t guarantee to reply otherwise as I can’t see every reference to my sets. Enjoy the design 73 for now Nick G0CWA

Take a look at Mike’s RTL SDR website. This is quite a popular project!

Please note I do not supply kits, parts, PCB’s or build boards for my projects but am more than willing to help talk you through a build or fault finding via my normal contact methods, or even SKYPE if required for direct contact.

Recommendation for poor contact: “USB connector exchange, DVB-T USB Stick (R820T)”

Contact of the USB connector TV28Tv2DVB-T USB Stice of (R820T) is … this is the worst, it’s the problem you have pointed out before, but feel when plugged anyway Ya adhesion of flux in passing Ska … almost there is also a thing that has come out of rust or discoloration. It can be used if you just use only, Hey What a quality of country C, this is I feel bad the quality is very recent ones, especially compared to initial. Solder also seen many things quite sloppy.
Well, sometimes you have disconnected all too soon, such as reception of ADS-B, and it has been extended with a USB repeater cable and USB extension cable tuners original. It is the effective means to directly below the antenna reception, but it will not make very troublesome need to be recognized again connect or disconnect the tuner at the site contact failure occurs once. When you received by the antenna directly under, let’s enough measures.
Refer to a tuner directly to the PC It is big no-no. You can find information fitted in. However, you should consider using a USB extension cable tuner also serves as prevention of leak noise from the PC of course … is loose.
I’ll try to raise the issue of the original USB connector.
Contact instability ⑤ disconnect number of times, which is also what is being corrosion flux adheres to the plating of ④ cover software freezes backlash ② connection after contact ① connector weak touches the big ③ running tuner will increase it will not recognize it becomes
Basic performance … the problem is good Hey there and well, and will teach you the improvements we want to try them. Earlier to say that defect report TV28Tv2DVB-T USB Stick of (R820T) Some of them are introduced but.
I’ll try to increase the improvement of the USB connector.
In the tape winding fixed (degree of difficulty ★) ① USB connector connection part (USB extension cable)
This will not fix the problem, but it is about prevention of freeze touch.
You are using a USB extension cable ② brand item (Difficulty ★ ★)
Resolve only budget, but the degree of contact is slightly better
Will be replaced with a high-quality ③ USB connector (Difficulty ★ ★ ★ ★ ★)
The difficulty is in removing the USB connector …
The connection to the solder and the USB cable ④ tuner board (Difficulty ★ ★ ★ ★ ★)
Most reliable if there is confidence in the arm! And to say that, I tried to replace the USB connector most solid.
(A type, male) was used as a replacement is a surface mount USB connector Akizuki Denshi Tsusho. Actual product photos and product introduction of the HP is slightly different.
(Shape of the rib is different, but is independent of the connection. Reprint from HP Akizuki e’s)
USB connector (A)
Press working (rib), differences in appearance, comes with a form etc. Chigae in general, those that do not come with a part of country C made often seen. The connector just for exchange … It is cost down, pressing of the cover, plating is also beautiful to resin portion of the interior has also been shaped clean.
R820T USB ①
Terminal differences are: (contact). The USB connector of the original, contact will be really weak for terminal is flat, but the terminal of the USB connector you have replaced, contact resistance has improved contact part has a convex shape. It looks terminal is lower than the resin part If you look at the terminal part of the original. Contact also I think in a weak for that? The backlash seems to be less for a replacement, resin part because it can be slightly larger for the original.
Software freezes in haste to the touch even a little USB part in running the case of the original tuner and try to actually use. However tuner after the replacement, did not freeze after compressing or pry or twist a little. Also, I had to freeze or between always If used for a long time, but does not have it! It was not a poor contact even after repeated insertion and removal. And I realized feeling when plugged anyway and is solid.
R820T USB ②
Why not? It is the important part to perform data communication and power supply, but the accuracy is too bad too. It’s not that say it can not be used in its original state, but it is a very important part as the electronic equipment. It is recommended that you try to check once. Such as there often is that the tuner can not be recognized in particular? Maybe you no longer recognize? I think if symptoms get such, it is good when I suspect the USB connector.
You will be omitted replacement procedure, but because it only install by removing the connector at the end.


//~~lo0king for the waves~~//