How to tame a USB stick SDR

Recently there have been a number of articles on how to use low-cost (and here we mean really low-cost, typically about 20€ to 30€) USB sticks as low performance SDR’s. Using these (outwardly simple) devices and some free SW, it is possible to make a receiver that can cover between about 22 MHz and at least 1 GHz. Some “conversions” of the units have been described, to try to alleviate the normal RF interference issues associated with an unshielded small PCB connected to an antenna and USB cable (read second uncontrolled antenna and interference source). This article shows another approach to accomplish the needed shielding and small performance enhancements.

Figure 1. The RTL based SDR in it’s shielded box connected to PC, receiving the OH2VHF beacon on 144.443 MHz. Two windows on the PC screen, one being SDR# to the right, in the background, the other being the DM780 Digital master program from the Ham Radio DeLuxe suite (free version).

Background.

There are a large number of USB “sticks” for receiving either DVB-T and/or DAB broadcasts, based on using the RTL2832U converter chip from Realtek and various silicon tuner chips. A Finnish developer (Antti Palosaari) of SW for the Linux V4L/DVB project has found a mode of operation of the RTL2832U chip which allows it to work as a high-speed dual channel A/D converter, where the I/Q samples can be moved over the USB bus for processing in a suitable PC (either Windows or Linux). There are several free software packages which have recently been written or adapted to be used with these USB receivers. Some packages which the author has found useful are referenced later in this article.

So now you can buy a cheap USB stick, download and install some free SW, and instantly have a SDR that covers from about 22 MHz to about 1 GHz, in 2 MHz “bands”. But these small USB devices really aren’t built for high performance SDR’s. They have only 8 bits resolution (most SDR’s use 12, 14, 16 or more, to give a wide dynamic range), but anyway are still useable. But what seems a more difficult problem is the fact that they are not shielded in any way and have a high noise floor because of leakage of RF into the “antenna” port of the silicon tuner. Note that the frequency range quoted above is totally dependent on the

tuner chipset in the USB stick, and can vary especially in the lower limit. See the references for more information.

The “noise” may be USB noise, or local FM/GSM/etc transmitter noise. Remember these are wideband receivers, and have no selective filters in front of them. In the authors case (living in Espoo within sight of the main TV/FM transmitter tower), it didn’t really matter whether an antenna was connected or not. The FM local FM stations were almost full strength, the distant ones (with antenna connected) were unusable. The 144 MHz band wasn’t too bad, but the general noise background was not as low as the author thought it should be.

The Improvement Project

So what could be done, and what was worthwhile ? This kind of SDR chip will never be the heart of a high performance SDR, but for many uses can be pretty attractive. As a tunable IF (at say 144 MHz or 28 MHz) for use with UHF or SHF converters, where generally the dynamic range of the signals within the bandwidth of the receiver may be controlled, it can provide a low power, small physical format and portable solution. And it’s a wonderful and cheap solution for new hams or other interested people who want to “get their feet wet” at low cost. Downside is that some sort of converter is needed to “up-convert” the HF bands.

So the decision was made to basically shield the unit, keeping the USB noise and unintentional antenna noise paths controlled. Also a 28 MHz low-gain pre-amp and bandpass filter were added, and a bypass for the filter so it could be used at other frequencies. The design, simulation of the filter, pcb design and construction of the whole unit took about 2 days in total. So it was not a big project.

The project was constructed in a 55 mm by 74 mm by 30 mm tinplate box, using a double sided PCB. One side of the PCB is a complete ground-plane, the other side is mainly a ground-plane, with a few components.

Figure 2. Top view of the USB tuner mounted in its shielding box, with the two SMA connectors, 1 for wideband use, the other for 28 to 30 MHz use. The “black box” next to the USB stick is the changeover relay for the antenna connection (from 28 MHz filtered to broadband)

Figure 3 Underside of the unit. Small copper rivets were used to interconnect the top and bottom of the PCB and provide ground plane connections to isolated areas.

The author required a 28 to 30 MHz small tuner, to be used a a tunable IF. So the first thing was to design a bandpass filter plus a moderate gain pre-amp. The end selection was to use a MMIC, followed by the filter. As there would always be some selectivity in front of this receiver, in the form of a 144 MHz to 28 MHz transverter, it was not thought necessary to add filtering in front of the LNA. But to protect the USB receiver chip from further broadband noise, the filter was placed between the LNA and the USB stick front-end. In order to allow for usage of the Rx. at other frequencies, a high isolation RF relay was added, plus a second connector for the broadband input. To simplify interconnection, the unit was powered from the 28 MHz co-axial input, which also switches the relay.

Figure 4. Simulated response plot of the filter, driven from and terminated in 50 ohms. The basic design is a fifth order elliptic low pass filter, but inductors L101, L103 and L105 are added across the low pass C elements to provide a low-frequency cut-off. The measured response with the LNA chip agreed with the simulation.

The schematic was captured using the Eagle PCB design tool, and the PCB designed as a two-sided board, with complete ground-plane on the topside of the PCB. A reasonably large number of vias were made between the two sides of the board, and small copper rivets were soldered in place to provide the interconnections. The PCB was placed approx. 10 mm from the bottom of the box, and soldered in place along all edges to ensure good earthing and a complete faraday shield between the two sides of the board.

Figure 5. Schematic of the LNA, BPF and change-over relay. The relay chosen has better than 90 db isolation at low frequencies (< 100 MHz). Additional places were provided on the PCB for extra capacitors in case other uses were made of the same design.

Copper foil soldered to underside of USB “dongle”

pcb before mounting (3 places)

USB connector should be a tight mechanical fit in the tinplate box

hole

USB connector soldered to

tinplate box in 3 places (top and

each side)

Antenna wire soldered to pcb on reverse side and

directly to USB stick antenna connection place

Figure 6. Close-up of the mounting of the USB tuner in the tinplate box. It is about 3 mm above the pcb. After carefully cutting the hole for the USB connector (nominally 12 mm by 4.5 mm), and before mounting the tuner, 3 small pieces of copper foil were soldered to the tuner pcb on its underside. These provide a low-impedance ground connection for the tuner area of the PCB. One is directly under the antenna connection pcb trace, and one on each of the earthed “prongs” which formerly held the large antenna connector, which is removed.

The Results

After building the unit, the first thing was to make some subjective measurements using the broadband input. Although the prime interest of the author was not to make yet another FM receiver for the broadcast band (88 to 108 MHz), this would be the first subjective test. The receiver was powered up with terminations on the co-axial inputs and tuned to the middle of the band. There were basically no FM signals in evidence. Nothing could be heard. Figure 7 shows the situation, using the SDR# (SDRSharp) SW receiver. The “signal” in the

middle of the trace is an artifact of the imbalance between the I and Q signals, which is almost totally suppressed by the automatic balance scheme in SDR#.

Figure 7. Spectrum and waterfall display of part of the FM band as seen by SDR# SDR SW. No antenna connected.

The next step was to put an antenna onto the Rx. and see how the “distant” FM stations, which previously couldn’t be heard, were received. It suffices to say that they were. The signal strength was lower of course, but they were no longer overlaid by the enormous “hash” from the local stations.

Figure 8. Spectrum and display of part of the 144 MHz ham band. The receiver is tuned into the Inkoo 144.443 MHz beacon. Distance is about 34 km.

The next test was to check the wideband performance. Figure 8 shows the Inkoo beacon, the morse keying of the beacon can be clearly seen. The code can be cleanly decoded by, for example, the HRD DM 780 module, or by ear.

A final test was to measure the sensitivity of the 28 MHz input, where the pre-amp is taken into use. No attempt was made to measure the various intermodulation intercepts. The result was that the minimum detectable signal was approximately -150 dbm* (*see below).

The table below shows the measured performance of the SDR for 144 MHz and 28 MHz without the pre-amp and filter, and for 28 MHz with the filter and pre-amp. *(Note that at -130 dbm to -150 dbm, there may be a bit of doubt as to what the signal level really is). All the measurements were made using double-screened co-axial cable to attempt to minimize leakage. A Marconi 2022A signal generator was used as the source, together with high-quality external co-axial attenuators. All the RF path connectors were either SMA, TNC or precision type N.

Measurement ConditionSignal Level (dbm) 144 MHz Broadband 28 MHz Broadband 28 MHz LNA

-80 0 > 0 >> 0-90 -10 -7 >>0

-100 -20 -17 0-110 -30 -28 -11-120 -40 -37 -21-130* -50 -47 -30-140* Beat Note Beat Note -40-150* Beat Note

Figure 9. DM780 plus SDR# plus 10 GHz to 144 MHz transverter receiving the 10 GHz beacon from Pasila (indirect path, small horn antenna 1 metre above ground).

Figure 10. DM780 plus an Icom IC-R75 plus 10 GHz transverter receiving the same 10 GHz beacon from Pasila.

The final figures 9 and 10 show the reception of the Pasila 10 GHz beacon, using the same 10 GHz transverter and similar conditions, but using two different IF setups. Figure 9 is with the RTL SDR USB stick receiver using SDR#, whereas Figure 10 is done using an Icom IC-R75, interfaced to the computer by a fairly high quality USB audio interface (not a USB stick !). The connection between SDR# and HRD was done using inbuilt SW in the portable PC, no cables needed.

SW and References.

All the SW used by the author in this project is Open Source SW and/or is free for Amateur use. So the only cost (assuming you have a PC with USB interface capability) is for the USB DVB-T or DAB stick, and the small number of parts for the box and possible amplifier and filter. Even the PCB design SW is free, if you are only designing boards for your own use and they are less than 80mm by 100mm.

PCB Design SW: Eagle version 5.11

HRD SW: Version 5.24.0.36 (Free version, downloadable from HRD Software LLC)

SDR# SW: Version 1.0.0.1135

Filter Design:

Filter Handbook, A Practical Design Guide, by Stefan Niewiadomski, published by Heinemann-Newnes.

Using RTL 2832 USB Sticks:

http://rtlsdr.org/

http://www.rtl-sdr.com/

http://sdr.osmocom.org/trac/

SDR Receivers:

http://sdrsharp.com/

General articles covering use of RTL USB sticks and how to configure SDR SW:

“Cheap and Easy SDR”, Robert Nickels, W6RAN, QST for January 2013.

“Digital Modes for your SDR”, Robert Nickels, W6RAN, QST for May 2013