Softrock RXTX v 6.3: RX Switching and RX Muting

This stage handles the muting of the RX section when I PTT goes high. After soldering in all the component, the testing went according to plan, until taking resistance readings of the power rail.

The initial current readings were as expected, but when I switched the DMM over to read resistance levels, nothing registered. I was expecting to see around 7 Meg ohms at the 12V test point, 950 Ohms at the 5V test point and 10 K Ohms at the 3.3V test point. Resistance readings of the band pass filter’s secondary windings were fine.

That’s where I left things for the night. My philosophy is to sleep on it when things get tough.

It was at around 4am the next morning that I woke suddenly with the answer as to why the resistance readings for the power rail were non-existent: I was taking the readings with the power to the PCB turned on! So I took readings again, this time with the board dead and all was just as it should be.

Still flushed with success, I decided to push on and continue with the rest of the testing. All readings on my DMM were as they should be, so it was time for the best part, to solder in a temporary antenna, connect up a lead to the input of the computer’s sound card, start up the software (Rocky in my case) and see if the rig could detect a test signal on 7.046 MHz (the centre frequency of the 40m band that the rig is tuned to).

This is the spectrum before sending a test signal from my FT-817.

This is the spectrum before sending a test signal from my FT-817.

Now came the moment of truth.

The test signal is clearly visible now. The spike at 7046.7 is the signal. It has a mirror image at roughly the same distance to the left of the centre frequency. This is due to ground loops on the PCB.

The test signal is clearly visible now. The spike at 7046.7 is the signal. It has a mirror image at roughly the same distance to the left of the centre frequency. This is due to ground loops on the PCB.

Once the rig has been completed and installed into an enclosure I will try to filter out any signal images that mayexist.

This completes the build of the main board. Next is the difficult part: the PA filters.

 

 

Softrock RXTX v6.3: PTT circuitry

The PTT circuit is all about connecting the PTT and Keyer inputs up to the SDR software via a serial interface. However, the DB9 connector will be installed in a later part of the build. I will be using instead, a USB I2C interface as both my Surface Pro 4 and my Compaq laptop, which runs Arch Linux, don’t have serial ports.

This stage involved installing four caps, 12 resisters, a diode, an RF choke and four transistors.

All appeared to go well until it was time to carry out some tests. Current tests proved spot on, and so did the initial voltage tests. Until it came time to prove that the transistors were turning on when voltage was applied to the PTT_IN connection.

What is supposed to happen is this: when 12V is supplied to PTT_IN, Q1 turns on, pulling R21 and PTT_IN to a low level. Q2 then turns on and I should be able to measure about 12V at S12V. This means the rig is transmitting.

I did not see this on my DMM. My reading was of the order of 0.02V.

So it was out with the schematic once more (I had become quite familiar with this piece of paper). I started by tracing the power supply to the transistors to see what was wrong and why Q2 wasn’t turning on. After much thought, I noticed that two vital resisters were missing on my PCB.

These resisters were missing on my PCB

These resisters were missing on my PCB

It was then that I decided to check if any other components were also missing. What I discovered was that I had omitted to solder in all the capacitors and all the resisters for this stage! No wonder Q2 wasn’t turning on.

Once this slight oversight had been corrected I ran through the test once more, with perfect results.

Next will be the RX Switching stage.

 

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.

 

Softrock RXTX v6.3: Building the TX Op Amps

This stage has a fairly high component count, so patience was the order of the day. I decided to take it nice and slowly so as to enjoy the process. It would also make sure I didn’t make any mistakes.

The stage consists of four unitary gain op-amps, arranged in pairs. The left channel’s input resolves to two signals: 0° and 180°. The right channel’s input resolves to two signals: 90° and 270°.

Each of the 14 resisters was checked with my DMM to ensure I had the correct ones for insertion into the PCB; it’s easy to mistake brown for red in the colour coding on the tiny resisters.

The Op Amps themselves (IC SOIC-8 dual Op-Amps) were also tiny beasts each with eight pins that required careful soldering so as to ensure no solder bridges or spashover on any of the adjacent empty holes. For this task I used a very handy suction tool that Wallace, VK4CBW, gave me some time ago.

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Positioning U1 on the underside of the board with the suction tool.

Once all the components had been soldered into place and I was certain there were no cold solder joints or solder bridges, it was time to conduct the usual current and voltage tests according to the instructions.

Thankfully my patience paid off with all reading being as expected.

Next is to tackle the TX mixer.

Softrock V6.3 build: The RX Mixer Stage

The mixer stage acts like two traditional direct conversion mixers operating in tandem. It centres around U10, a SOIC-16 Dual 4:1 Mux/Demux Bus Switch. A 2-pin and 3-pin socket also needed to be installed along with three resisters. And for testing the stage, an 80/40m Band Pass Filter board also needed to be built.

Care had to be taken once more when soldering the tiny pins of U10 into position.

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This is the underside of the PCB with U10 top middle.

Building the BPF was relatively simple, with the exception of winding the two coils; these take time and care but aren’t particularly difficult to do.

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The 80/40m BPF board in place.

Testing went well. Current and voltage readings were as expected. The only test I was unable to carry out at this stage was to test it with Rocky, a SDR software package that shows a chunk of spectrum. This was due to my Linux laptop’s sound card not showing up in Rocky.

Next will be to build the TX Op Amps.

 

Softrock RXTX Dividers – Stage 3

The dividers stage takes the local oscillator’s signal and divides it by four, producing two output signals that are said to be ‘in quadrature’. This means they are out of phase with each other by 90 degrees.

The trickiest part of this stage’s construction was soldering in U9, a 74AC74 SOIC-14 SMT that has fourteen small pins (or legs). And even though I took great care soldering them onto their respective pads using flux and a very fine soldering iron, when it came time to test if all was okay, the readings I obtained suggested otherwise. So it was out with my more powerful iron (with a larger tip) and with care, the re-soldering exercise produced near perfect readings on my DMM.

Next on the agenda was something I had been waiting for with anticipation: the frequency output test.

To accomplish this I would need my Hantek 6022BE USB DSO, and both probes.

This is what I observed.

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As can be seen, the two signals are 90 degrees out of phase.

Next to do is to build the RX Op Amp.

Softrock RXTX v6.3 – First Smoke test

It’s always a little daunting but it’s also the part I like best – doing the smoke tests. Testing the power supply was really all about ensuring that the SMT caps had been soldered in correctly and that there were no solder bridges.

First I tested current draw with a 1K resister added to the positive probe of my DMM to restrict the current in case of a bridge. I applied 12V power and took readings.

3.0mA so all was in order.

Next I checked the power at the 12V, 5V and 3.3V rails: all fine there.

Now it was time to build the local oscillator.

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Once all the components were in place, it was time for the next smoke test.

Once more I needed to check current draw but this time I did this with a 100 Ohm resister instead of a 1K Ohm one. The readings I obtained were excellent. I was now safe to test without the protection of the resister and again got great readings, all of which were under 80mA.

Next up was a frequency test. I set the dip switch on the PCB to 7.046 MHz and applied 12V. I then tuned my FT-817 to 28.184 MHz (four times the local oscillator frequency of 7.046 MHz) and set the rig to CW mode. After attaching a length of coax in the 817’s antenna socket and draped it close to the Softrock, I heard a good, solid tone.

Excellent. Another smoke test passed.