September 2012

Software Defined Radio project

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For use with RTL2832U-based DVB-T USB dongle with the Elonics E4000 tuner.

Hi, welcome to my latest project my improved version of my SDR radio.

Firstly an apology due to file size and the number of photographs the full document is downloadable as a PDF file including constructional details, photographs and pcb layouts.

This is based on my original design but has taken into account problems mentioned to me (with the original design) in particular, some people have had sensitivity issues. This seems to have been due in the main to the relays. I used the two sets of contacts to control one of the building blocks and inter contact set capacitance was killing the signal. I didn’t notice any major problems myself, although I did lose a bit of signal due to this effect. The problem has been overcome by using separate relays for the input and output signals on all the modules.

Although designed for use with the Elonics E4000 tuner it works with any of the available tuners, provided the software you use can control them. The major differences being the maximum receive frequency. Take note that the current rumour is the E4000u tuner is no longer being made and will become scarce and substitutes are being supplied. For example the eZcap EzTV668 is still for sale but may be supplied with a lower spec tuner.

The modules consist of
1. A switching unit with a VHF broadcast band filter
2. A 300 KHz to ~60MHz pre-amplifier of around 18dB gain
3. An HF to VHF converter DC to 60 MHz input up-converted to 125 to 185MHz output
4. A VHF 50MHz to ~2GHz pre-amplifier of up to 18dB gain
5. A PC power rail filter
6. A voltage regulator for an external unit

The modules are all stand alone and you just need to build the ones you want and the four major units are built on the same size of PCB to make them easier to fit together in a case.

All the units are also designed to use a 12V switching voltage. This is not only to control the relays but also supply the unit, the on-board regulators taking care of the rest.

I stated in my previous design that switched mode PSUs are generally not suitable but, in the case of a PC supply, they are usually very well filtered and should not cause major problems. I have, however included a simple PSU filter in the design if needed. The major RF noise source will be the internal PC clocks driving the processor and other internal circuitry.

Switching unit and VHF broadcast band filter

The switching unit is the only dedicated part of the design and is used to control the signal path through the converter and amplifiers. There is nothing really special in this part of the design with one exception, the VHF broadcast band notch filter.

HF pre-amplifier 300 KHz to ~60MHz

This can be used as a stand-alone unit with any receiver having a 50 Ohm input impedance. The HF pre-amplifier is designed to have a minimum -3dB frequency of operation of 300KHz with a maximum -3dB point of around 60-65MHz on a 9V supply. The inclusion of the 60MHz low pass filter helps to eliminate higher frequencies, without it the top frequency could be up to the maximum of the MAR-6. The inclusion of a 3dB pad on the output is to ensure the amplifier sees 50 ohms impedance.

HF to VHF converter

This can also be used as a stand-alone unit with any receiver having a 50 Ohm input impedance. The use of the LDO regulator supplying the crystal oscillator module means that although in my complete design it is supplied by 12V it can actually be powered from 5V up to around 20V. This makes it an ideal candidate for powering off the USB bus if you only want to make this “module”.

The HF converter converts the RF signals from DC to 60MHz to an output signal in the range 125 to 185 MHz. Please note it will work with any of the dongles providing the software you are using will drive them.

The two “clever bits” of this design are:
the use of a 125MHz “local oscillator” which means the output signal appears above the VHF broadcast band reducing any effects it has on both sensitivity and desensitisation of the dongle,
the swapping of the RF and IF ports on the SBL-1 (I used an HPF-505 which is identical) allowing tuning down to DC.

VHF pre-amplifier

This is again another stand-alone unit for use with any receiver having a 50 Ohm input impedance. The VHF pre-amplifier is designed to have a minimum -3dB frequency of operation of 50MHz with a maximum -3dB point of around 2GHz on a 9V supply. The loading coil for the MAR-6 chip is made from a four turn coil wound on a ferrite bead.

The inclusion of the 50MHz high pass filter helps to eliminate lower frequencies, without it they could cause interference and de-sensitisation problems. The inclusion of a 3dB pad on the output is to ensure the amplifier sees 50 Ohms impedance. This is particularly important as I have not followed the normal strip-line design method of construction.

PC power rail filter

This is included for completeness of design, I have not found a need for it on my PC as the internal supply was “clean enough” to use directly. It is only needed if you have problems. There are no pictures of its’ build as it is a paper design only. You may have to p[lay with component values to eliminate/minimise any problems from your PC supply.

Voltage regulator

This is included for completeness of design. I have not built it as I used the internal PC supplies. It is only needed if you use an external supply. There are no pictures of its build as it is a paper design only. You may have to play with component values to eliminate/minimise any problems from your supply.

Avoid external switched mode power supplies as they are notorious for being noisy.

Summarised hints for use

The use of the complete system is relatively straightforward and only the broadest of advice is given as a lot will depend on your antennae and other variables to do with your system.

I found my complete system and dongle gave the most sensitivity with the dongle gain set to manual and at a minimum level when using the pre-amplifiers. Turn the dongle gain up slowly till desensitisation starts to occur then back off the gain a little bit. This is the most sensitive setting. In most cases this is more sensitive than the dongle alone.

If possible turn off the receiver AGC. This again increases the overall sensitivity. From the performance of my dongle the AGC appears to be audio generated but I could be wrong on this point.

Try to use an appropriate antenna on the system with an ATU. This again will give a better signal. I use a long-wire up to around 20MHz and a disk-cone above. This is not ideal as resonant antennas are by far the best particularly as you go up in frequency.

Useful Links

Hackaday An article by Brian Benchoff , I first came across this very cheap communications receiver idea on the QRZ forum in a posting by Sue AF6LJ.

For The SDR Fans Please note double check the dongle is compatible. A brief list is at the link below

SDR Wiki The list of software available includes HDSDR, WINRAD and my personal favourite, SDR Sharp

SDR Sharp The site takes a little bit of exploring to find everything but is well worth the effort as at present I have not found any problems and the people running the site and writing the software seem to be very approachable for help and advice. Just make sure you download the latest version.

See my posting on the radio board for further details at: The RadioBoard Forums

The idea of a receive converter was, I believe, first broached by George (M1GEO) / and Chris (G8OCV) on Fun Cube Upconverter

I cannot guarantee to see your questions and answer them if posted elsewhere. All the best for now and happy building de G0CWA (Nick)

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.

Here are the complete plans in a 2.5mb PDF File. (Updated 13 Sept 2012) Enjoy!

Software Defined Radio project

Updated Pictures

Software Defined Radio project

Software Defined Radio project

Rebuilt on a separate chassis.