the softrock signal generator project, part 1

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.

3 Responses to “the softrock signal generator project, part 1”

  1. March 15, 2013 at 8:51 pm

    Perhaps you can write next articles referring to this article. I desire to read more things about it! Excellent post. I was checking constantly this blog and I am impressed! Very useful info specially the last part šŸ™‚

  2. 2 Jack Margolis
    March 25, 2014 at 6:53 pm

    Does the ATtiny85’s firmware come from HDSDR or some other place. I haven’t really seen a good explanation of this point, but then I’m new to all this. Jack

    • March 25, 2014 at 7:04 pm

      Jack, in this case it actually was pre-loaded by Tony Parks. When a single microcontroller is used in applications like the Softrock, it would almost always have to be Flashed in advance because otherwise a boot loaded resident on another microcontroller, or at a minimum a USB chip that provides a host PC USB to ICSP bridge would be needed to Flash the controller from a PC, where something like a Windows driver (common) or an application (less common, other than Integrated Development Environments – IDEs) is doing the transfer of what amounts to a HEX file. As it turns-out, the ATTiny85 on the Softrock is actually what provides the USB device interface to the host in addition to supporting the ‘tuning’ translation of what the user is doing in HDSDR and what voltage is present on the si570 chip to produce the desired frequency. Hope that helped a bit.


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