Another Electronics Blog

After receiving the BN-43-2402 binocular cores from Kits and Parts, I was able to add the T1 and T2 bifiler wound transformers.  I then chose to do the Band 3 inductors L10-L12 since they required the least effort to wind and install in order to test the end-to-end signal path. Lastly, I added a 51Ω 1/4 watt resistor between the junction of pins 7 and 9 on U9, and pin 9 on U6 – the only significant change to the stock design of the RX II in order for the divided output of the Si570 to flow into the ABPF section in reverse. In other words, having the signal flow out of the Ensemble RX II instead of flowing into it as a receiver.

The results were as good as I expected. From 16 MHz to 40 MHz, I measured a roughly -3db response out up to zero db and then back down again to -3db in the RMS output that peaked around 4.7 volts. The sine output appears clean on the scope, but I’d like to get it measured on a Spectrum Analyzer if I can hook up with another local Ham that has one. Lastly, I did the A and B calibrations using the CFGSR tool and was easily able to set the output to agree with my frequency counter to 5 digits after the decimal point, or 10 Hz resolution.

Next I’ll be building out the rest of the ABPF band sections to see the full response from 3 MHz up.

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.