The Softrock Ensemble RX II is a good base for hacking it into a signal generator. Essentially if you build it out omitting the I/Q sampling and conversion circuit, the audio output, and flow the Si570 output into the Automatic Band Pass Filter sections, you end-up with a nice PC controlled (using CFGSR) signal generator.
In this post I’ll cover the basic starting points and progress into more detail in later posts.
Since I had nearly all of the parts to build a second RX II, I ordered a bare board and Si570 from Tony Parks, KB9YIG. I also ordered a few items from Digi-Key and Mouser that are particular to the way Tony put this design together. For example, the PCB mounted BNC, USB-B, and power connectors are each a little special in that the matching circuit board mates to specific part numbers. Of course if you simply purchase an RX II kit, you have everything you need.
The Si570 used in the Softrock kits is a grade-C part, which means it has an upper frequency limit of 280 MHz, or in the case of the CMOS output part used in the Softrocks, 160 MHz. Still, you easily get frequency coverage from 3.004 to 160 MHz if you follow the HF build instructions. Other grades of Si570 are supported in PE0FKO’s firmware, so it’s possible to go still higher; at least until the board layout begins to be a problem in the GHz range.
I also plan to use a Mini-Circuits ADE-1 to provide for modulation of the output. Once again, obviously if you just build-out an RXTX Ensemble, minus the final output, you can get most of the same functionality. However, the RX is nice because it has the ABPF switching.
One note on programming the ATTiny85, I have an Atmel STK500 development board and Atmel Studio 6.0. The ATTiny85’s come from the factory with default fuse bit settings that will need to be set to 0x5D high, and 0xE1 low bits in order for the chip to work correctly in a Softrock, therefore you must either reset the fuse bits before programming the EEPROM or use HVSP to program both the fuse bits and the EEPROM – an ISP alone is not enough.