Saturday, April 28, 2012

Softrock SDR Kit - Divider and Op-Amps

Yesterday the weather was gloomy. It started off overcast then mid-morning a light drizzle began that lasted all day. By afternoon we were completely fogged in and the temperature had dropped to the low 60s F. In a word, perfect ... for kit building. 

I made good progress on the Softrock kit, finishing both the divider circuit and the op-amps. The mixer IC is also mounted now due to mechanical reasons, but is not tested until later. I think I am over the halfway point on finishing the kit (not counting setup later). 

PCB Top Side


The divider circuit takes the output from the local oscillator, at 13.500 MHz (when building for 30 meters) and divides the frequency by 4 (3.375 MHz). Two signals are output, but each is 90 degrees out of phase from the other. All current and voltage tests passed, but to check the signals you have to continue with the op-amps, which will boost the signals 500 times.

PCB Bottom Side - small push pin for scale

The ICs, again SMD parts, are mounted on the bottom of the board. Mounting the SMD caps was really not difficult, but this was my first try at soldering such small ICs. It really wasn't as hard as I thought, however, and despite a couple of easily cleaned up solder bridges it went well. I used the "drag" method wherein you load up the iron with a small blog of solder and then press and drag it across the pins. The key to success is to coat the pins and pads with plenty of flux beforehand. 

There is more caution expressed in the manual about the op-amp IC than the others so it must be more sensitive to ESD. Once it was in place, and I double-checked the voltage readings on every pin, it was time to make sure the LO, Divider, and Op-Amps were doing their thing. The tests for this are two: 1) using a bridge resistor see that the nominal 2.5V comes up to 3.75V for each op-amp. Check. 2) With power applied, pipe the output from each op-amp via marked test points directly to the antenna input of an HF RX tuned to 3.375 MHz. Again the K2 was the test instrument and the signals were there, equally strong.

I really like this method of building one section then testing it thoroughly before moving on. It not only ensures ultimate success, but it helps your understanding of the entire circuit immensely. The only thing I don't like about this kit is that the manual seems to be overly helpful in places. Especially, it repeats the list of parts and pictures for each section, once for inventory and once while you're building. It can be a little confusing knowing if you are on the right page sometimes. Still, better too much than too little.

Next up is the bandpass filter, winding #30 wire on teeny tiny little toroids ...

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Thanks very much for your comment! 73, Casey

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