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.