SDR Receive Converter & Pre-Amplifier

By G0CWA – August 2012

Nick has built a new SDR project, the Mark 2, with improved circuitry, which supersedes this design.

Introduction

Hi welcome to my latest project a combined receive converter and pre amplifier for use with an RTL2832U-based DVB-T USB dongle with the Elonics E4000 tuner.

I claim no originality for this as I came into the game late on. The basic idea of using these TV dongles as a broad multimode receiver was first “discovered” by Antti Palosaari. This project is a change from my usual offerings in that it is at the other end of radio technology and uses silicon fuses !!

The idea was probably first mentioned on Hackaday . In 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.

The price of the dongles is comparatively cheap at £10 to £20 from many sources including the inevitable E-Bay. Please note double check the dongle is compatible a this brief list. The list of software available includes HDSDR, WINRAD and my personal favorite 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 Forums for further details. The idea of a receive converter was, I believe first broached by George (M1GEO) / and Chris (G8OCV). Thanks to them for getting me started.

My Contribution

I will write the descriptions etc as though they were totally my idea for ease although other people’s articles helped and inspired me.

The converter is needed to enable reception of frequencies below around 60MHz, as this is the lowest these tuners are capable of.

The circuit and PCB layouts are included and are described in reasonable detail below. I have built two versions one the stand alone set presented here and a “sawn” off prototype version that slots into a spare pc drive bay using the PC power supplies. My external version was built in an external 3.5inch hard drive case with included PSU.

However take note this is usually a switched mode unit and may be too noisy at RF frequencies to be of any use. I ended up using a cheap wall wart PSU and on board rectifier/regulator. I would recommend you do the same don’t trust the regulation on the cheaper wall warts. The more expensive ones tend to be switched mode supplies and as mentioned earlier generally are not suitable.

The whole idea was to build a modular setup where you only build the component circuits you need.

Circuit Component Modules

The Power Supply Nothing special here this is just a simple 12V regulated supply based on the inevitable 7812 chip regulator, the circuit is built on the main board, no heat sink is included as the power levels involved are so low as not to warrant needing them. (This is not needed if building a PC version, use the PC’s internal supply with filtering) The resulting 12V output is used to DC select and power the various functions.

Receive Converter (VHF MODE) The VHF receive path is straight through the system to the pre-amp/dongle with the exception of the switchable VHF broadcast band notch filter no additional filtering is included, none is needed, the E4000 has its own internal ones built in. This filter is only included to optionally decrease the tendency for desensisation of the input by any very strong BB signals when receiving at other frequencies. A series tuned circuit is used across the co-ax so no losses will be incurred due to signals passing through relay contacts, any contact losses will only reduce the filters Q.

Receive Converter (HF MODE) The HF (I know it is also low vhf) is fed from its own separate antenna via a switchable VHF broadcast band notch filter as before and a 60MHz low pass filter to reject unwanted signals. The resulting output is the fed into an SBL-1 (or similar)double balanced mixer’s IF port the output being taken off via the RF port, this may seem strange at first but it allows reception down to DC instead of around 1MHz. No attempt is made to tie the output impedance to 50Ohms as the following stage(s) will do this. The local oscillator input is fed from a 125MHz crystal oscillator module via a 3dB pad to tie down the impedances to 50 ohms. The onboard LM1117v33 regulator supplies the 3.3V supply for the oscillator. The board is designed to accept either size of module.

The use of a 125MHz “Local Oscillator” means that DC to 60MHz will be received between 125 and 185MHz, avoiding the VHF broadcast band. No attempt has been made to reduce the image response of the converter as it did not seem to be significant above around 200KHz. Saying this any oscillator module can be used as long as it is above 60MHz, but problems may be encountered with the VHF BB.

NOTE: check the orientation of coils L1, L2, L3 they should be at right angles to each other for best filter performance (see photos).

Receive Pre-Amplifier This is just a simple amplifier based on a MAR-6 MMIC amplifier chip and is common to both signal paths, in theory it should give 22dB of gain at up to 2 GHz with a typical low noise figure of 3.0dB, if you believe the specification. The addition of the variable resistor, VR1 is just to make it easier to set the biasing for the MAR-6 amplifier. I measured the gain of my version at around 15 to 18dB at 100MHz. Please note on most of the SDR dongle software correct operation is indicated on the waterfall plots by an improvement in signal to noise ratio NOT an increase in signal levels.

FINALLY: Although not as sensitive as my Yupiteru 7100 the use of the TV dongles is an excellent cost effective introduction to SDR radio. I hope you find this converter a useful addition to the TV dongle SDR’s or even your fun-cube. Any comments will be gratefully received.

All the best for now and happy building de G0CWA (Nick)

Because the project is being linked to from so many sources and other forums please take note I can only guarantee to see your questions and answer if you ask directly via e-mail, The Radio Board or QRZ forums. Sorry if this causes any problems Nick G0CWA Thanks to the others who are spreading the word.

Addendum

One of my friends built this design and said he had a minor problem with a slight loss in overall sensitivity, probably due to the relays causing a mismatch with the antennas he used. He got round the problem by introducing another "pre-amp" before any filtering. If you have this problem try using an ATU first and if all else fails graft another low gain broadband pre-amp/buffer before any filtering to boost the signal levels to compensate. I personally did not have any problems. In my prototype circuits I didn't even use rf relays but used good quality multi-pole switches and didn't notice any excessive losses.

This problem is sorted out in the improved MK 2 version. Therefore I have removed the schematics and board layouts from this page and you are invited to go to the new page. .

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.