The Pic-a-Star project

joined the Pic-a-Star Yahoo group about 8 years ago with the intent of building one of these rigs.  The ‘Star is a DSP transceiver designed by G3XJP, and was first written up in the RSGB Radcom magazine around 2003.  Picastar has been called a software defined radio, but is such only from the final IF stage onwards.   In this regards, its design is similar to many ‘IF DSP’ radios from Icom, Kenwood, and Yaesu.  It’s a dual conversion scheme with a first IF (as designed) of 10.7mhz and a second IF of 15khz.  Users can pick a different first IF frequency (8-9 mhz usually) to suit available roofing filters by swapping out a crystal in the second conversion oscillator and changing some software constants in the DDS unit that provides the first conversion injection.

The project was under continuous development until sometime around 2009.  The project was supported until recently on the Yahoo Picaproject users group, but Peter has pulled down all of the software and hardware design files from the group sometime last year, effectively shutting down his support and involvement in the it.  With many of the parts (especially the DSP processor and the codec chip) reaching end of life, there are few new builders of this rig.  The design might be long in the tooth hardware wise, but a new improved DSP software set was developed about two years ago.  (I wasn’t able to help beta test this as my unit wasn’t finished.  Now with Peter pulling the plug, I don’t know if I can obtain this new software.  The original DSP code from ten years ago is still functional however.)

Originally, building the Picastar required the builder to etch and drill his own printed circuit boards, however VK3PE has designed several different sets of professionally manufactured blank boards which he has made available on a dead cost basis with G3XJP’s permission.  I have built my own boards for both the DSP (three pcb’s) and the IF from Peter’s artwork.

As designed by Peter, the rig is controlled from his Pic-n-mix DDS unit which was also described in Radcom.  The entire article for this was reprinted by Analog Devices as an AP note for their AD9850 DDS chip.  Peter’s UI using the Picnmix controls EVERYTHING in the rig from a standard 12 key ‘telephone’ keypad and a rotary encoder.  The rotary encoder is used to tune the DDS vfo, but it is also used to vary dozens of parameters such as AF and RF gain, keyer speed, vox settings, etc.  Which of these parameters is being adjusted is selected by multiple presses on the keypad.  Some settings can be made using only the keypad via direct numerical entry.  Everything is displayed by a single six digit seven segment LED readout.

While fully functional, this bare bones UI is extremely unconventional, and requires memorizing numerous key press combinations, or constant use of a ‘cheat sheet’ reference to control the radio.  Many builders of the Picastar have opted to use a related project, the TRXAVR controller.  This AVR based DDS unit also has a keypad and a rotary controller, but makes use of an LCD display to present the UI.  Various graphical and textual LCDs can be used, along with a touch screen display.  Multiple additional rotary encoders can be added, each one dedicated to a single parameter.  The front panel of a TRXAVR equipped ‘Star will more closely resemble the usual ‘store bought’ rig then the home brew design of G3XJP, though it will have the same performance factors.

I originally planned on using the TRXAVR controller with a 192×64 graphical LCD display.  This would have required my modifying the code for the 128×64 graphical LCD to make use of the wider display.  However there was a problem, the TRXAVR design makes use of a PC based program called Hobcat to configure and load the Star software and parameter files.  Hobcat is a Windows program based on the Delphi Pascal runtime system.  It won’t run natively on Linux (it might work via WINE though).  Peter’s Picnmix code is also configured via a PC utility, but he wrote his in Quick Basic.  At least I can run QB natively on Linux via dosbox.  TRXAVR is quite a bit more complex than the Picnmix, both in hardware and software.  Using it would have required more work in customizing the software to my liking, as well as finding a solution to the problem of getting Hobcat to run on Linux.  (I could have run a copy of Windows under a VM on Linux, or used a dual boot setup).

For various reasons (including the mental roadblock of the TRXAVR vs Picnmix), I set the STAR project aside for several years.  I had also got involved with 3D printing, designing and building my own 3D printer.  Now that that project has reached a point where only software updates are necessary, I dug the completed Star DSP and IF assembly out of storage.  I was able to re-verify that the DSP module still works, and was finally able to ‘smoke test’ the IF assembly.  I injected a 10.7mhz signal from my GDO into the IF input, and heard the beat note from the DSP board through an external stereo amplifier and speakers.   The next step will be to build the front end BPF and bilateral mixer modules, then I’ll have a working receiver.  The transmit portion comes after that requiring a power amplifier chain and a LPF, along with the necessary TX/RX switching.

I’ve also decided that Peter’s PNM DDS unit will be used as the front panel.  It may be a bit too bare bones for my liking, but it will be a quick way to get the radio working.  I can always build something else later, reverse engineering the DSP API while using the PNM to control the rig.

dspboards_back

dspboards_front

As I mentioned earlier, I etched and drilled the boards for the DSP and IF units myself.  Toner transfer was used for the resist, printed from an HP laser jet printer onto special iron on transfer sheets.  Above are photos of the boards after etching and drilling, before any parts were mounted.  Some additional cleanup with a hobby knife was required before soldering after I checked the traces for shorts and opens.

SONY DSC

Here is a photo of the DSP motherboard and the processor board after soldering the parts on.

IF_DSP

Finally on the right is the DSP motherboard with both the processor and codec board added, and on the left is the assembled IF board.  The DSP board was later mounted with spacers under the IF board, and the two were interconnected via shielded wire.  Several of the trimmer resistors on the IF board have been changed out since the photo was taken.

Advertisements