Thursday, 5 November 2009

USB-Powered Direct-Conversion Receiver



I've built and blogged one of these before, but now I've developed the idea a little. The original suffered with a poor antenna and although it worked, it was more than a little deaf. The new USBrx has eighteen components, including the connectors, antenna and panel.
  1. Loop antenna wire (32feet, 9.75m) - any insulated wire will do.
  2. Loop antenna connectors male (2 required)
  3. Loop antenna connectors female (2 required)
  4. Tuning capacitor, 20-200pF, polyvaricon
  5. Knob to suit tuning capacitor
  6. Ferrite toroid, 9mm 4c65, FT37 type, any small HF ferrite
  7. SA602 mixer / oscillator
  8. Quartz crystal, frequency at band edge
  9. Emitter capacitor, 30pF
  10. Feedback capacitor, 30pF
  11. Bypass capacitor, 3n3 (3300pF)
  12. Coupling capacitor, 100nF
  13. Supply decoupling capacitor, 10uF
  14. USB extension cable
  15. Old earbud cable
  16. Copperclad panel (FR4), 55x55mm
You can save on some parts by permanently mounting the loop wire, without using the connectors. I use my 32-foot loop on other radios, so I need to be able to disconnect it. If you plunder an old AM radio for the tuning capacitor, you may have a knob with it already.

The antenna is a loop, 32 feet long, and works as a quarter-wave magnetic loop for 40 meters. It was first described by Ben Edginton G0CWT, to whom I'm indebted for the idea. It's basically an opened-out version of the 'original' USBrx antenna, itself a copy of the Poundshop antenna, first used by me in 2005 as part of the Poundshop Radio. These antennas are directional, and very compact. An eight-foot square for forty metres is something which can be occasionally strung across a room (I use lengths of nylon string and small bulldog clips), and taken down when business is done. The loop is resonated with the polyvaricon variable capacitor, which can have one side connected to the panel. I made a 64-foot version in 2007, and with extra windings on the transformer I used it on Topband.

The antenna has a very low impedance, in the region of 2.5 ohms, so a transformer is used to match the loop antenna to the 1500 ohms of the SA602. One turn (a single pass through the toroid's hole) for the antenna and 25 turns of 30SWG enamelled copper wire for the SA602 input.

The output, using the tip and body contacts of the 'earbud' cable, will drive the microphone input of a PC soundcard. You can listen to it with headphones directly from the soundcard output, but you won't like it. Everything from the crystal's frequency to 25kHz in both directions will be coming at you, and it's quite a cacophony. To make sense of it, you need a little help from one of my favourite pieces of software, the SAQrx. You can download this from here. This is intended for use with LF antennas for monitoring the famous Alexanderson Alternator transmissions, which use the callsign SAQ, but it has a host of other uses, including rudimentary digital signal processing. Because the SAQrx software can filter out any narrow band from near DC to over 20kHz, you can use it to choose a spot frequency to listen to. You can choose narrow, medium and wide bandwidths, and the gain can be adjusted to suit. The program works well under Wine in a Linux machine, as shown in the photograph. Comment if you need any help with this one!

10 comments:

Vladimir said...

It's good, but how about the second channel?

Pete Morris said...

Hi, Vladimir, thank you for your question.

"The second channel" - audio image or the other output pin of the SA602?

The audio image covers 6975 - 7000Khz, which is used by fixed services, and very quiet. Contrast this with a direct-conversion receiver of 3kHz bandwidth operating within the 40m band; this would recover cw signals from both sidebands, and we all know how confusing and irritating that can be.

I could have used both outputs of the SA602, with perhaps an audio balun transformer to increase the signal recovery and provide better matching to the microphone jack, but then the simplicity of the radio is compromised.

Anonymous said...

I can use in 26,500 mhz?
how would the antenna?

Pete Morris said...

Hi,

You can use an SA602 as an oscillator / mixer with any ceramic resonator or fundamental-mode crystal. I haave used SA602s at 24mHz in my theremin project, and they perform reliably. At that frequency range, reducee the two capacitors associated with the crystal to 5pF or 10pF; experiment!

The antanna is another matter entirely; in the 11m band I woulod use a dipole or j-stick and match the 70-ohm or 50-ohm of the antenna to the 1500-ohm of the SA602 with a transformer. Use 2 turns for the antenna side and 11 turns for the radio side to give the needed 5.5:1 turns ratio (30:1 impedance ratio). Use a yellow Amidon core at this frequency (T37-6).

Good luck, and 73!

Anonymous said...

73´s

I have the NE602N...

Can I´ do it the project ??

my english is no good
sorry !

Pete Morris said...

Yes.

The NE602 and SA602 are the same, but made by different manufacturers. 'NE' is the Signetics code, 'SA' is the Philips code.

Bothe devices can be used successfully in this design.

Good luck, 73, Pete G1inf

Anonymous said...

Peter give me your e-mail please...

I made the circuit with an 26,690 MHz crystal and two capacitors of 8 pf.

I just need to try to see how it works
my e-mail is lsedr@hotmail.com

Anonymous said...

26.690 mhz

Anonymous said...

YEAH ! MY SDR RECEIVER IS READY...

THANKS PETE MORRIS

working 26.690 mhz

remember, my english is no good

Anonymous said...

now, i want use my 40/80 M dipole for receive the 40 and 80 m frecuency..

I will use the crystals of 3.600 MHz and 7.050 MHz for 40 and 80 meters