Current affairs

One of the problems that I really want to improve upon is PA current measurement. Reliably measuring up to 20A digitally in what is an electrically rather noisy environment has proved to be problematic, with a mix of common mode rejection issues and RF ingress.

I tried providing a separate 5V power line for the ACS712 current sensor and that did make an improvement, presumably tackling the common mode rejection problem. Still not really good enough though. Next I tried taking the mathematical mean of a series of measurements to try to smooth out noise on the sensor output. Rather surprisingly this didn't help noticeably at all. 

More research turned up a new device, the Texas Instruments INA253, which uses differential amplifiers rather than a hall effect device. This did make a noticeable improvement but the thing that really made the biggest difference was putting a choke/capacitor to ground RF filter very close to the microcomputer input pin. Obviously RF was still getting in there.

The TI INA253 also confers another significant advantage on the design. The differential amplifiers mean that it is possible to reference 0A to anything, including ground. That removes the need to determine the half-Vcc point, something that was a real pain with the ACS712.

First attempt at Mk II PCB circuit

So my current thinking is that I will use the INA253 and that pushes me firmly towards a new PCB, as the INA253 is a surface mount device. There are quite a few other changes that I've made by cutting tracks but there is no way I can do that to bring an SMD into the existing PCB.

Whilst I am at it I will also introduce a potential divider circuit into the temperature sensor input line because the 3.3V maximum input to the microcomputer limits the temperature range to 60°C.

I've made a first stab at the new circuit diagram and will do the PCB layout once I am as sure as I can be that it's correct and as good as I can make it for this iteration. This is normal - the first PCB shows up all the design faults and the second attempt is much better. Occasionally it takes three attempts!

Improvements...

Well I did say that my projects are never really finished...

Paraphrasing Field Marshal Helmuth von Moltke's famous saying "No plan of operations reaches with any certainty beyond the first encounter with the enemy's main force", I have long since learned that however cunning the design, on the air experience will soon reveal its weaknesses. And so it is with the amplifier. 

Metering

The first problem is metering in general and 50V current monitoring in particular. The readings jump around like a kangaroo even when nothing much is going on (e.g. a steady carrier). Partly this was down to how I displayed the current - there really is little point displaying the load to two decimal places when it is 18 amps! So now, once the current is above 2A the resolution is reduced to 0.1A and above 5A it is further reduced to 1A. That helped.

The real problem though is the measurement methodology. I am using an ACS712 Hall effect device and that produces a voltage output that changes proportionally by 100mV per amp. That's easy enough to measure with the analogue input on the processor but the real problem is jitter on the 5V power supply to the device. A small amount of noise directly translates to changed readings, so, say 50mV of noise on the 5V line is equivalent to 0.25A of jitter. 

5V supply noise

I hooked up the 'scope and sure enough there is some jitter as loads come and go. There's also a lot of general crud on the 5V line as the 'scope image shows. I was rather surprised by how noisy the supply line was but I suppose the fact is that we're not generally too bothered about <100mV ripple on a 5V supply.

5V supply with decoupling

Next I tried putting 470nF + 10uF decoupling capacitors right by the ACS712 and that improved things slightly but it only really removed the high frequency spikes, see image. Most of the noise appears to be coming from the Arduino processor and it seems to be more or less impossible to filter it all out.

I think the solution to this problem is to provide the ACS712 with its own stabilised 5V power source. This is cheap and easy to try out with a low noise precision regulator device such as an MCP1702. I have some on order and will report back in due course.

Bias (again)

I know that I said I wasn't going to bother switching the bias. U turns seem to be quite the rage these days and if it's good enough for our politicians then it's good enough for me. And I think my reasons are better than theirs too.

Whilst I was messing around with the metering problem I decided I should accurately measure the amplifier's quiescent current consumption, which are 33mA unbiased and 225mA with the bias on. I had noticed that the heatsink temperature increased slightly when the amp was on but not transmitting. Well 225mA at 50V is 11.25W and that is not an insignificant amount.

So I decided to connect up the bias control line and write the few lines of code to bring the bias on when TX is requested by the radio and hold it on for five seconds on return to RX. The radio already inserts a 25ms delay before producing RF to give the changeover relays time to change state, so my thinking is that this will be sufficient time for the bias to stabilise as well.

It seems to work OK but I need to do some proper on the air tests to be sure.

There will be more to follow, no doubt.

Hardware finished?

I think the hardware is more or less finished now but my projects are seldom ever completely finished, especially when it comes to software. Here's some pictures of the finished article

 

 

There are still various "improvements" to make but most of these are in the software and it's tricky to take pictures of software...

• Removal of the tacho monitoring software - I can't find a way to keep RF out of the fans, which are, after all, right up against the "hot" parts of the amp. This is no great loss - I had vague plans of monitoring fan speed to create a control feedback loop but that is provided perfectly well by monitoring the heatsink temperature and setting fan speed accordingly.
• Consolidation of fan speed control to a single output for both front and rear fans. In the end I couldn't think of any situation where I would want to control them separately, so the code can be simplified. If I do produce Mk II controller PCB it too can be simplified somewhat with the removal of a couple of transistors and associated passive components.
• Removal of bias control. In fact I never wrote any code for this as in the end I couldn't see any situation when I would want to have the amp powered up (50V supply on) but not biased. There would be a small reduction in power consumption on receive if the bias was turned off but then there is the question of stabilisation of the bias at the same instant as RF is being applied. I don't feel the need to go there.
  
• Calibration of the power meter. This really needs me to gain access to a calibrated RF power meter somehow because I have nothing that can measure 400W at 144MHz. If needs be I will rent one some time. It's not really too important, as I have an approximate indication, probably within 10% derived by measuring input power and multiplying by device gain. The 50V power input current provides a useful cross check and seems to be consistent.
  
• General ongoing software improvements as I get more experience with actually using the amp - probably a never-ending project!

I fixed the problem of RF getting into temperature sensor logic by cutting a track on the PCB and inserting a 100uH choke with 10nF to ground. Good as gold now. Not worth commissioning a new PCB for that!

So I think I have a usable amplifier. It runs reasonably quiet and cool and local reports are that it is clean, so I shall install it as part of the station once I can find a space for it!

Updates to this Blog will likely be rather less frequent from now on, though I will try to remember to provide occasional updates as the software changes or other improvements come along.

I hope you have found the Blog interesting and comments are always welcome. 

Something's happening

 Yes! We have progress. 

I finally finished the garage workbench upgrade and very splendid it is too. With the nice new heavy duty vice and bench drill installed, it was time to drill large holes for the fans and finish off the mechanical engineering side of the project. This was, more or less, completed just before the new year and since then I have been testing the amplifier in its almost finished state.

I quickly discovered that the sensors are a problem. RF gets into the wiring and gives silly readings, especially on SSB where the RF output level is continuously changing. The temperature sensor is especially prone to difficulties and I spent some time trying to analyse what was going on. 

Eventually, after much faffing around, I attached the oscilloscope to the temperature sense line and found much RF on the line and, oddly, a fairly significant level shift, suggesting that the RF was getting rectified along the way. This is problematic, as the fans are supposed to be speed controlled by temperature and, of course, that really wasn't working. Worse, because the RF interference made the processor think that the temperature had gone down, the fan logic was working the wrong way round!

Well it turned out that it was my wiring. I had naively assumed that the very well filtered temperature sensor output from the amplifier module together with the high level of screening in the case would mean I could get away with unshielded wires. Nope.

Replacing the sensor connections with shielded wire made a big difference but the temperature display was still slightly unstable. So I decided to go the whole hog and instal a filter comprising a 100uH series inductance and 10nF capacitance to ground at the PCB. That fixed it!

With hindsight, I should have put the input filtering on the PCB at the outset but I was too trusting of the amplifier module filtering and simply didn't think the RF could get around my carefully designed screening quite so easily. As I've said before, I'm not very good at RF engineering, especially the fast wiggles of VHF and beyond.

So I've had a bit of a redesign of the control circuit, putting choke/capacitor filters on all input lines, as shown in the circuit fragment to the left. It won't be possible to modify the existing PCB to accommodate these changes and I also have a couple of  other issues that I commented on earlier, so I reckon I am heading for a Mk II PCB. It's so inexpensive to get PCBs made these days that it's hardly worth doing otherwise.

I'm working on the new PCB layout and will probably get it into production in the next week or so. 

Meanwhile the amplifier has been getting some use in the UKAC contests, which seem to be the only time that there is any actual activity on 2m.