This is a kit that was put
together for the smd workshop at the 2010
Finningley microwave roundtable. It was also expected to be an
introduction to SDR receivers and software for some builders.
Anyone familiar with phasing receivers should recognise the circuit blocks used. The pcb generates audio I (in-phase) and Q (quadrature) signals, but relies on a PC to process these, which has the added advantage of extending the coverage to at least 48 KHz - tuning over this range being included in the various software packages available.
The sdr dongle is intended to feed a PC soundard, using the line-in connector. Reception is at 3750 +- 24 KHz for a standard 48 KHz sampling rate card. If the user is lucky enough to have a 96 KHz sampling rate card, then coverage will be 3750 +- 48 KHz.
There are plenty of freeware SDR programmes that can be used. Although Winrad (by I2PHD) is my favorite, you can also try SDRadio, Rocky, M0KGK SDR, PowerSDR and Spectravue. Just use Google to find downloads.
Winrad is downloadable here: http://www.weaksignals.com/
Circuit diagram
This is similar to the early Softrock receivers:
SMD placement
By following the build order below, there will be no components that are difficult to fit because of clutter around them:
Start here!
NOTE: C10 above (yellow) is shown as a tantalum capacitor. If an aluminium electrolytic is supplied, the marker will be at the negative end, so fit with the band to the right as viewed above
Congratulations – that’s all the
SMD devices fitted
Testing
Once the smd build is complete and the few leaded items fitted, the board can be tested. Before doing this, just check that C10 is connected the right way (ie, you read the note above regarding the marking on aluminium electrolytics, which were what were actually supplied in the kit!).
Connect 12v to the power connector. The current drawn should only be about 20mA.
Connect an antenna and also a stereo lead from the PC to the audio output socket. Run the software, as described below. Signals should be heard.
If the board does not seem to work, use a scope to establish that the 15 MHz oscillator is running. A couple of boards at Finningley did not oscillate, and C5 had to be reduced down to 150pF to reduce the loading of the buffer on the oscillator. Likewise, if the oscillator runs, but takes a second or so to start, change C5. In fact, you could do worse than to change C5 by default!.
Operation using Winrad
Apply 12v to the SDR dongle via the supply socket. Do not exceed 15v. There is a series diode on the pcb to prevent damage due to incorrect polarity connection.
Use a stereo 2.5mm back-to back male cable to connect the SDR dongle to the line input connector of the PC soundcard.
Connect an antenna to the SDR dongle via its phono socket.
Assuming that you have already downloaded Winrad (currently v 1.33), you should find the Winrad icon on your desktop. Click icon, after which you should see the following display:
Select the appropriate input and output sound cards (likely to be your default setting), and click the 'OK' button.
Click the 'start' button at the top of the screen - the programme should spring into life. However, you'll probably find that the sound card is selecting its mic input, so click on the 'show options' button in the top left hand corner. You should see the following menue:
 |
Click on 'select input',
then on the new menu click 'open input mixer...'. You will then see
your default mixer panel, from which you need to select 'line' if it
currently is sellecting 'mic'. Exit the mixer panel, and again select 'start' at the top of the Winrad screen. The display should now be showing some band activity. Tune by dragging the blue segment on the spectrum analyser display. Once you have dragged the tuning onto a signal (oh, having ensured that you have selected 'lsb' as below) you should hear appropriate audio.  |
A couple of points:
To get a direct read-out of frequency, go to the 'LO' setting (near top right hand corner). Left/right mouse click on the appropriate digit until the LO displays '03.750,000'. You will now find the frequency scale to be accurate to ten Hz or so.
To get the maximum sideband rejection, find or input a large carrier on the display. If it is greater than 60 dB above the noise floor, you will find its image mirrored about the centre of the screen. View this whilst doing the following:
Click the 'show options' menue and then click on 'channel skew calibration'. You will then see:
You will probably have to drag the new dialogue box to one side in order to see the image signal again. Now use you mouse to drag the coarse and fine phase and amplitude balance sliders until the image is minimised. This software action corrects for tolerances in the hardware component values.
Oh, one last thing: If you receive ssb, but decode it only with the 'usb' mode selected, instead of the 'lsb' selection, just go again to 'show options' and select 'swap I and Q channels'. This will correct the lsb/usb selection error.
There's lots more you can do - enjoy playing with the software!