Homebrew Challenge 2014

SNW Homebrew Challenge 2014

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Submission of Section UBA SNW 2014 - 1st place

RTL-SDR Radio HF Convertor (including Android devices)

Foreword
In this article we use a normal DVB-T USB stick for TV reception, as a panorama receiver on our laptop, computer, or tablet. This we "abuse" in order to receive other frequencies with it, which it was originally intended.
The received signal can we see on our computer screen in a kind of spectrum analyzer environment and the sound we can hear in different modes like FM, AM, CW, LSB and USB.
The intention is to use HAM software, which exists for both Windows and Linux and Android. We use a modified hardware driver that allows the stick is in a sort of "test" mode, through which the incoming antenna signals after a tuner and a decoder in an I and a Q signal are continuously sent to the USB port . The maximum window width that you can see on your screen is about 2 MHz. For example, in one fell swoop to see the whole 2m band on your screen. Usefull programs are HDSDR and SDR # (SDR Sharp). With HDSDR + Dream is "DRM" (Digital Radio Mondiale) possible. With SDR# there are a variety of "plug-ins" to decode other modes. For Android there is "SDR Touch" with the necessary RTL2832U driver.
All this is only possible on TV sticks containing a particular chipset. You should therefore first sort out before purchasing.
The usable chipsets have always a single chip in common: a Realtek RTL2832U decoder chip, the signal after the chip tuner for DVB-T decodes MPEG-2, but is here in the test mode a raw I and Q stream. Hence one also sometimes called a "RTL stick" or "RTL-SDR" because this chip.

Some existing RTL2832 sticks with different tuners:


FC0013 Tuner	E4000 Tuner	R820T Tuner	R820T Tuner

The tuner in the chipset can be different for each type of stick and determined the receive frequency.
The most common tuner chips are the following:

E4000 Elonics a tuner chip with a range of from 52 MHz to 2200 MHz (now discontinued),
R820T or R820T2 Rafael tuner chip with a range of from 22 MHz to 1766 MHz (it is now common),
older tuners chips are Fitipower FC0012 and FC0013 with a range from 22 MHz to 1100 MHz (with gaps!).

So basically you can standard with this stick receiving VHF, UHF and SHF partially.
But given the reception restriction below 50 MHz on these sticks, we need a receipt for HF converter, the HF band of roughly 0-30 MHz transforms upwards, so that we can hear the signals at a higher frequency band (for example, from 50 to 80 MHz) and which can receive on our stick WEL.

The desired design:
In this case we are going to tackle the design of an RF converter differently than you find in other designs that can be found elsewhere on the Internet.
Next business for example are highly desirable and to adapt and / or resolve:

it must be a bit of a universal circuit for the various types of existing dongles,
you can choose a suitable mixer crystal for the RTL stick used by you,
the circuit must consume little power (eg for Android devices)
it must be made integral with easy to get and inexpensive materials,
no switching parts (relay) between HF and UHF more, as in previous designs
2 antenna inputs remain, one for HF and one for VHF / UHF.

Especially for Tablet or Smartphone usage:

the circuit should be as small as possible (for android = portable use)
an additional USB throughput feature on the converter, because a tablet usually has only one USB port,
the power can be delivered by the Tablet or Smartphone, but also external feed is possible.

Let's look at the frequency domain and reception capabilities:

Fig1

Fig. 1

Of great importance is the choice of the crystal for the RF mixer. This is actually dependent on the RTL Stick used.
I still want to clearly distinguish one. Look so sure what kind of stick that you just got.
For the RTL stick users with E4000 tuner (the older types):
If we look at the upper part of Figure 1, we see that an E4000 tuner only can receive from 52 MHz. If we apply this proposed crystal of 50 MHz to the RF mixer, then we can after mixing, the HF band in the segment received 50 to 80 MHz, but we lose the reception on the first 2 MHz. That's an important part of the HF band (0 to 2 MHz). It is therefore appropriate for owners of an RTL Stick with the E4000, to apply a slightly higher crystal in order to receive the entire HF band. There is not much margin. One possibility is to use a cheap crystal block of 66666 MHz (from an old computer) or better another crystal of 55 to 60 MHz. The exact frequency does not matter. A higher crystal can also, for example 80 MHz, but then you come to the end of the HF band (80 + 30 MHz) after mixing in the radio FM bands that is not the intention. Some use in their design a 100 MHz crystal, but then the HF band is also right in the strong FM band. A good choice is to use, for example, a 125 MHz crystal, but then the HF band pawl falls in the band plane. However, that is not very, because it was pretty quiet there, when you do not connect the VHF antenna. The high-pass filter must be recalculated starting from 110 MHz, so that the strong FM band is largely suppressed.
Here we sacrifice so either the 50 MHz band, or a piece of plane band 125-158 MHz to the HF band to can receive. Are you not sure of your choice, use a 125 MHz crystal. That always works with any RTL stick.
For the users of an RTL stick with the R820T:
For the owners of a R820T stick it is somewhat easier to use a standard 50 MHz crystal.
This stick has been receiving standard wide from 50 MHz and there is some room to use a different crystal, which may be between 40 and 60 MHz.
With a 50MHz crystal mixer, you receive on HF perfect the time MSF60 channels (60 kHz) and DCF 77 (77.5 kHz) !
In the lower part of Figure 1 shows what the intention is: the HF band passes through a low pass filter that only the HF band 0-30 MHz pass. Then the antenna signal is mixed with a 50 MHz crystal. At the output of the mixer there is a 10dB attenuator. Behind the signal goes through a high pass filter, all below 50 MHz blocks to remaine a pure RF signal. The last filter is not strictly necessary, but it helps to keep the mirror frequencies of the crystal under 50 MHz to suppress.

Circuit:

Fig. 2 Click on it for bigger view

Clarification and options in the diagram (for the R820T version)
On the RF input to the first filter is a 7 pole Lowpass filter used. C1 * and L1 * were provided to make a 9-pole filter. This was not necessary in practice. The filter is asymmetrical, needed for the SA602A.
Because some values in my filters were based on the famous "FUNcube Converter", I later found an article from F5RCT who made firewood of the design for the FUNcube. Many amateurs complained about poor sensitivity at low frequencies whith the FUNcube Converter. So I recalculated my filters at 50 ohm input and 1500 Ohm output for the input of the SA602. And so I have found a better filter than proposing F5RCT.
The filters are calculated with the program RFSim99, still proves its good service, even in the era Win7. The component values are optimized in order to use as much as possible default values of the coils.
The filter graph (input filter) is for the R820T version:
The red line is the frequency-pass curve. Each division is 5 MHz wide. Begin is 1 Hz and the end is 50 MHz.

The low pass input filter of 0 to 30 MHz

Fig. 3 Click on it for bigger view

We have chosen an active mixer with a SA602A chip. This for 3 reasons:

1) Because there is already a supply voltage present (from the USB throughput) for the feeding of the crystal block. 2) because SA602A is a lot cheaper than for instance a SBL1 or IE500 or modern ADE-1 passive mixer. 3) And now because, in principle, also be a passive crystal can be used, directly in the oscillator circuit of the SA602A chip.
At the output of the SA602A hangs a 9-pole Chebychef Lowpass filter (optional) for filtering of the possible image frequencies below the 50MHz. This filter is not really adapted to the 1500 ohm output SA602A.
But in practice, it gave no problems because the filter over the entire frequency range will never be 1500 ohms and 50 ohms Out. Also remember that the entrance of the stick is actually 75 ohms. Of the output filter, I have shown no graphics here.
At the end of this filter can also be connected a VHF antenna so that switching between HF-VHF do not need a relay. HF antenna should then be quite disconnected.
The USB throughput provides a number of advantages. There is a reduction of the necessary cabling to connect the whole and saves one USB port on the computer, or tablet.
The USB power supply has an additional smoothing capacitor and an extra decoupling capacitor . This can lead to a reduction in interference and noise on the usually already poor USB power from a computer or tablet.

There is a jumper after the USB connector placed. One can do two things with it: on the location of the jumper, you can feed the RF converter with an external 5V DC, or the entire HF happen off by removing the jumper. Or place a switch. For example, the RF converter remains switched off, but also the throughput for the RTL USB stick, so that in principle it is also possible to feed these externally with 5 volts.

The crystal oscillator needs to be an active type according to the schedule: requires 3.3 Volt . Crystal cubes from computers usually need 5 volts. Takes this into account. All types of crystal cubes can fit on the board.
At the output of the crystal is an attenuator of 10dB, otherwise the signal from the crystal cube is much too high to mix with the sometimes weak signals in the HF band. A 2nd reason is the following: the crystal block produces square waves, with potentially strong harmonics. The attenuator partially reduced harmonics and prevents overdriving the SA602A.
The LED in the power on the PCB is optional and does not make much sense if the print built in a closed cabinet. If you do that, it is recommended that the whole in a metal box (copper or tin) is to install.

It is a huge convenience for connecting all on just one USB port, and can stay connected to everything, and can receive both HF and VHF / UHF. By disconnecting the HF antenna temporarily, or remove the jumper, one can also receive only VHF / UHF.

Tip : For the first experiments to receive something in HF is a single insulated wire of 6 to 10m indoors or outdoors enough. You can also use your VHF antenna by de-coupling the mass (RCA plug only half insert). It is then floating and so will behave like a loose wire antenna.

The board

The PCB is double sided (1 side full mass) to have adequate shielding. But it can also be one-sided, possibly taped with some copper foil or other metal foil. I have found, however, no radiation problems in practice in earlier versions.

Some components may be at the bottom (ground side) and so some holes are milled to prevent shorting to ground. Building and placing the parts is easier if you start with the smallest and lowest parts first and then the larger components and the connectors.
For the right C values you can be two solder together. For coils in series goes that bit harder.

A crystal in SMD is mounted on the front, a crystal block is mounted on the underside as well as the chassis USB connector.

All components are on the PCB except the SMA or RCA inputs, The can be placed on a box made from copper PCB or thin around the PCB. Holes are for RCA's 6,2mm, the USB output 13mm x 6mm, 3.2mm USB cable and the OUT 3mm. The print must be around soldered to the cabinet.


The component layout	The print mask (Older RCA Version)	The finished board (RCA Version)

The central connections if it RCA plugs are by short piece of wire on the print soldered.

For the OUT jack on the PCB, I refer later in the construction and finish.

Construction and finishing

To relieve the USB port (especially on Tablet and Smartphone), the converter is connected with a short USB cable to the computer USB port . This cable is for example from an old USB mouse, or in my case a USB GSM adapter cable (to get very cheap via the Internet). Please note that sometimes you need a Mini USB cable instead of a standard type-A USB connector for your tablet.

The RTL stick itself is then plugin to the USB port on the RF converter.

In order to keep the whole a very compact, can be used as an antenna inputs RCA chassis, and this for the following reasons. In many existing HF converters, you'll find as IN and OUT , the chassis SMA connectors back. But there you are. Most of us will not have these SMA connectors in the house. If you can find them all, or buy, they are almost already fitted with a wire. They can not practically self soldering, or you need special tools. Moreover, these SMA connectors are not made to frequently connect and deconnecting. There must be another way. One possible solution is the use of cheap RCA chassis inputs .

RCA chassis used here are well suited for VHF / UHF and some of us will even know that these were also found on former TV tuners. Everyone has referred himself the choice to go with gradient plug gently into 75 ohm or 50 ohm cable to its antenna. The entrance to the RTL-Stick is always 75 ohm (TV stick!).

It is also free to use other or expensive input connectors such as BNC or MCX.

With an RTL TV stick is always a little black stock antene with magnetic foot supplied , with the right connector (MCX or PAL 75 ohms). This antene is usually tuned to about 550 MHz. Not much to begin and to use our RTL panorama receiver. This gives us the opportunity to cut the plug with the 75 ohm wire to a length of about 20 cm and connecting it to (soldering) to the output of our RF Converter PCB. See the pictures below. So we already avoid connection problems of the RF converter to the RTL stick.

To complete this story, I note that there are always improvements will be possible.

Everyone is free to do this or adapt.

Here it was just supposed to be a small, simple and inexpensive RF converter to imagine, with components fairly easy to obtain.

This design can then also compete with other commercial design.

Connections in practice


The new R820T2 with MCX connector	The E4000 with PAL connector (Older model)

In the pictures above you can also see some useful connectors to connect an antenna of 50 ohms or 75 ohms. The blue RTL stick has a PAL to MCX adaptor needed.

Results with PC on HF band

The best program to use is HDSDR. Please inquire about this. Installation retrieve with a small HF antenna (single wire à 10m) are all pretty good results. A good outdoor antnenna is the "Mini Whip" from pa0rdt.

RTL settings in "ExtIO_RTL2832.DLL"

Set the Sample Rate to about 0.96. This gives up to a 1 MHz spectrum screen width. Set also the right Frequency Correction of your dongle, if you known (for callibration with a known frequency). Set Tuner gain manually to 40 a 49 dBm to avoid input overload.

You can use the "zoom-in" slider in the program. On HF bands, all is much closer together and it is otherwise too busy to see the signals on the screen. Also set the offset frequency of your crystal in the configuration of the program. So you can read the real frequency directly, without conversion from the alignment.

An example of RF spectrum from 0 to 1 MHz . From the clock-time channels to a part of the Medium Wave.
The screen, however, magnified even better with the "zoom-in" from 0 to 500 kHz.
The cursor is here on AM station RTL at 621 kHz. Also notice the fine red line , on DCF-77 kHz.

Screen1

Example of the amateur 80 meter band:
The cursor is here on an LSB voice broadcast station.

Screen2

The Android story

Research tells us that for Android devices there is not so much available to a single "generic" DVB-T stick to use as RTL stick on OTG USB port on an Android device. The only programs known to me or "App" is SDR Touch, and RF Analyzer, both available on Google Play. There is a trial version, with some limitations, which you can start. You also must have the 2nd App, the necessary RTL 2832U driver to control the RTL chip.
All in all, it works fairly well and you can listen to stereo FM fine with it and other services, but you feel that the other modes as NFM, USB and AM must be some by-developed. In order to be able to receive on HF it is even possible to set an offset frequency, so that you use an RF converter under 50 MHz can listen. My experience is that you must have an expert HF antenna, otherwise the presence of noise and false signals are immensely.
A disadvantage with Android Touch is also the fact that only a window of 1 MHz can look wide.
All in all though it is interesting to experiment with and to look for possible improvements.

Results with Android (SDR Touch)

Below: the cursor is on "Radio 2" in stereo FM.
At the back, you see a hand made "Slim Jim" antenna for 70cm band, mounted on the originally supplied magnet base ( Modifyed Stock Antenna ! ).

Screen3