Softrock RXTX v6.3: TX Mixer

The next stage in the project was to construct the TX Mixer stage. The job of this stage is to provide the modulation of the Dividers’ output signals by the four I and Q signals from the Op Amps. The result is a double sideband RF waveform that will be coupled into the PA stage.

This stage centres around U3: FST3253 which is a SOIC-16 Dual 4:1 Mux/Demux Bus Switch. There were also four resisters, a capacitor and two connector sockets that completed the build.

When that was done it was time to test if all was as it should be.

This was when I hit a snag.

First, I had to jumper the hairpin bend of R26 (which hadn’t yet been installed) to ground and then jumper pins two, three and four of socket J1.

R26 needed to be jumpered to ground.

R26 needed to be jumpered to ground.

Current readings were fine, and so were the initial voltage readings. But when it came to measuring the voltage on pins 7 and 9 of U3, instead of getting around 2v I was reading 0.01v.

I tried again but this time noticed that when I turned on my power supply (12V DC) the current surged to around 2A before settling down to more normal levels. I cut the power and began scratching my head.

All the solder joints looked fine as did the components, which were all in the right places. That’s when I decided to take a break and sleep on it.

After the dust had settled, I decided to consult the schematic. I started by tracing the 5V power route, through U4 and into U3. I could see its path to ground was through pins 1 and 15, then on to the as yet uninstalled R26 to ground via C43. So that’s why I needed to insert a jumper.

I then connected up my DMM and swung the switch to the continuity setting. Probing the jumper connection I had inserted into R26 produced nothing. So I probed R26’s other hole and bingo. I had jumpered the wrong whole!

My mistake was immediately apparent.

I had presumed the jumper needed to be in the whole marked with a white circle around it (to indicate that’s where the body of the resister fits). The instructions called for jumpering the hairpin of R26, which I suddenly realised was not the hole indicated by the circle, but the other one.

Resoldering the jumper took only a few seconds, but the satisfaction I received from a full set of good readings lasted quite a lot longer.

This little exercise highlighted to me the importance of being able to read a schematic diagram.

 

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3 thoughts on “Softrock RXTX v6.3: TX Mixer

  1. Nice pickup Grant, I am fascinated by your calm approach to this kit build. Once again thanks for sharing. I’m enjoying the journey. What did we do before blogs?

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