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Wednesday, 31 October 2012

Feel the Power!


Since the progress I discussed last time:

with this power meter project; I've had (yet another) build of the directional coupler:

This time its been constructed from double sided PCB material and uses FT-82-67 toriods.

Connecting the forward and reflected ports to my 'scope (terminated with 50R) with a RF generator (this one is made my Kenwood) connected to the transmitter port and a dummy load connected to the antenna port, here's what I found on the 'scope (I've got the setup wrong and the blue reflected port reading on the 'scope needs to be divided by 10):

I then connected the coupler to the ports on the power meter and also included a 30dB attenuator. The attenuator was constructed using this site here as a Pi attenuator:

The setup for the next phase looks very much like this:

So, once this was put together, I then constructed an excel table with frequency across the top and the AtoD down the side, this table looks like this for the forward port of the coupler:

So, once I had these values I can the look into the equation that will take me from the A-to-D reading back to power - this is basically what the power meter needs to do - here's the equation:

So solving this equation as a pair of simultaneous equations - in fact a pair or more of simultaneous equations - I see this:

and this:

so basically I get different constants depending on what lines of the table I use as the input to my simultaneous equations.

However, despite the different constants, if I then use the equation and the constants to re-generate my powers from the ADC values the curve seems to be a very good fit. Here is the original readings plus the two equation outputs plotted:

So, all in all, I'm rather confused about the whole thing! But whatever the confusion, the device certainly seems to be very frequency stable i.e. very little variation in AtoD values as frequency increases. It's all good.

The software I have for this meter contains a huge set of lookup tables for the AtoD reading to power conversion; clearly these all need to change!

Cat's not helped much:

 All good though egh? I'd love to know what you think.

Sunday, 28 October 2012

It's Becoming a Real Power Struggle

Well, blimey!

How hard can it be to get a piece of PIC assembler software to display some characters on an LCD display? The answer seems to be very.

I've started putting my power meter together, it's the one I started here:

Well, eventually I managed to assemble all the parts and put the boards into an enclosure I have here:

I've actually made quite a good job of the metalwork so far, I've intentionally not cut the hole for the display as I don't yet know which one of the many I have here I would end up using. Here's the view of the front panel as-is now, the meter looks OK but isn't the large meter I found, as that's just too big for the case!

So, PIC programmed, LCD display wired in, power on - nothing. Just some very faint black squares on the display that I can make more evident or go away using the on-board contrast control.

So, still not knowing if the software is intended to interface to an HD4470 compatible or not, the question was what to try next?

I ended up cutting the software back to just the display routines and simply programming the PIC with the code to display just a few characters on the LCD. Result - nothing.

So, next I added a simple loop at the end of the LCD routine calls to turn on and off one of the digital output ports every second. I attached a suitable LED to the port and tried again - as expected the LED started to flash. This told me a few things:

  • My PIC programming was working and my software was making it into the chip;
  • The software was reaching my loop for LED flashing so it wasn't getting stuck anywhere;
  • The routines for the LCD weren't working.
Now, you may (or may not) recall I did quite a bit of fiddling with LCD displays and Pinguino and more recently Arduino back here:

So from that messing about I had a reasonably good idea how the initialisation of these LCDs should look, and the code I was staring at didn't quite look the same. So I changed the sequence of numbers sent to the LCD to initialise it, re-programmed the PIC and tried again. Nothing.

Hmm, next I looked at the delays in the software between writing the Enable line on the LCD high and then low again - you kind of have to do an "open sesame" on the LCD every time you send it either data or a command and the delays in the assembly software I was looking at used the processor "nop" instruction. This is really a null operand and hence takes one (I think) clock tick to execute. I changed this to be a 1ms delay and.... Bingo! We have characters on the display:

So, my next puzzle was the fact that I could display the characters 0-9 OK, but any attempt at alpha characters resulted in, what looked very much like, Japanese characters appearing on the display. After much cursing, general muttering and many re-programming of the PIC to try different things, I found a short between two of the address lines on the board. Doh! It took me over 6 hours to find that as I was only looking in software....

So it does seem that I am moving in the correct direction. I have also found that the Assembly code I downloaded from the QST in-depth website for this project, doesn't assemble on my version of MPLAB. I've had to change a few things:

  • declared names are clearly case sensitive in my assembler and not used in that way in the code - many capitalisation changed;
  • literal values used with the "loadw" command - the decimal declarations used generated value too big warnings, all I have done is declare the values as the hex equivalent and prefixed with "0x" - I don't understand why this is different, but it assembles;
  • The declared names in the .inc file for the processor are also case sensitive so again, a number of changes needed there.
I then incorporated my changes to the display routines into the downloaded software and now I am seeing what I expected to see on the display.....

It's time to test the unit, I'll let you know how I get on. I suspect I need to re-make the directional coupler (again) as I posted a photo on the homebrew forum and asked for some advice, here's a very polite post telling me what a pile of junk I have made:

eHam comments

Hard work, frustrating but all good though, egh?


Tuesday, 23 October 2012

A Bit More of a Power Struggle


What a palavar! You remember the Power Meter project from here:

and here:

well, I ordered the strange surface mount resistors I needed that were 52.3 ohms from Farnel. They have a £20 minimum order value so I bought all sorts of other stuff that I didn't really need; they arrived today and I have completely goofed over the type I have ordered. The ones that arrived today are so small, you need a magnifying glass to even see the component on a white piece of paper! They are less than 0.5mm long - no use to me and my soldering iron at all!

I've nearly finished the power supply for the project today and the main board is also now fully populated - the PSU just needs a mains input on the back:

The power supply is in a case I found in a really neat shop in the centre of Derby called RF Potts; they sell all sorts of great electronic bits and bobs in there; so this Verobox was £1.90 (so I bought 5!):

Here's the main board:

So, progress is slow and steady! I've emailed the project designer today to ask what type the LCD display needs to be - I'm sure it'll be a HD4470 compatible, but it would be nice to know for sure - the less unknowns in these scenarios the better!

Looking good though, egh?

Sunday, 21 October 2012

JARTS - You What? Oh, JARTS


Had a little dabble in the JARTS RTTY contest today and yesterday, conditions seemed to be really good, but there wasn't as much activity as I had hoped for. Here's my log from the contest in map format:

There were a few new countries for me, but NOTHING in Africa at all! And also no VK (Australia) or ZL (New Zealand) but hey, ho, it was fun all the same!
Still, rather impressive, egh?


A Bit of a Power Struggle


You may remember that I started a Power/SWR meter project back here:

as I said then, I had ordered the circuit boards for the project from Far Circuits, and they have duly arrived.

Unfortunately, I am a few bits short of a full kit within my vast collection of components, and have had to order a couple of SMT components (52.3R resistors!) and a 2.5V reference plus a couple of other small parts.

I should be able to complete the project when those bits arrive; I'm also struggling to figure out if the display used is a HD4470 compatible device, or something else completely. I cant actually find a way to figure that out.

The boards look like this so far:

The two smaller boards underneath the meter are the A to D converters, these take the voltage out of the directional coupler I made in the last post and convert it to a digital data stream; the devices used are Analogue Devices 8307 and they are logarithmic. The main board is at the bottom of the photo on the left hand side and the PSU is at the bottom right.

The meter you can see is really neat; this was an eBay purchase, although it's bigger than I anticipated (it's actually exactly the size the seller said it was but I didn't bother reading it). So I may need to rummage for something suitable to enclose this project when its complete as I haven't got anything here bog enough to house the meter neatly - I do have a smaller meter of the correct value so I may end up using that.

Cats been helping again:

I'll post more when the missing pieces of the jigsaw arrive.

Good though, egh?

Sunday, 14 October 2012

I wonder how much power is there?


Since I got my linear amplifier that I told you about here:

I've been wondering about power measurement. Now, I have the power meter on the linear, a power meter made by Diawa, one made by some other manufacturer.... the point being that they all read different values for the same output!

So, this got me wondering about how to really measure power.

So, in simple terms if we measure the Peak-to-Peak voltage of a waveform, we need to calculate the RMS voltage by:

(P-to-P / 2) * (Sqrt (2) / 2) which is:

(P-to-P / 2) * 0.707

So, for example, lets say we have 100 V peak-to-peak, thats the same as 35.35 V RMS.

Now, to get power in Watts we need to:

(V RMS ^ 2) / R where R is the load impedance (in my case nearly always 50 ohms)

So, my 100V P-to-P or 35.35V RMS is:

(35.35 ^ 2) / 50

which is equal to, or as near as damn it, 25 Watts.

So, making a simple plot of P-to-P vs. RMS vs. Power (Watts) is quite simple, and it looks like this:

So, hopefully you will be able to see that the range of power or voltage we plot doesn't matter, the line will always be the same shape.

So, theory says, that if I connect an RF generator (perhaps one made by Yaesu) to a dummy load, then connect my x10 scope probe across the dummy load, I should be able to measure an accurate P-to-P voltage and quickly calculate the Power in Watts. This didn't seem to be the case when I tried it.

I used the FT-857 I have here and set it to its 20 Watt output setting, I was reading about 124V P-to-P which is more like 38 Watts, whilst I expected the radio setting to be an approximation, I never expected it to be this far out!

So, another experiment was called for. This time I connected the 'scope in the same way, but plotted different power and frequencies. Here is what I found:

You will notice that the reading for 40 Watts at 50 MHz is missing from the graph above. Just as I applied the power was exactly the start of the smoke emanating from the 'scope probe. That probe is RIP.

Now, whilst I don't claim to understand why, it is clear from the measurements above that at low power & low frequency my measurements are close to as expected. As the power increases and certainly as the frequency increases then the measurements become wildly inaccurate!

So, this made me rummage on the interweb for some kind of accurate power measurement project. This is what I found:

Now this looks like just the ticket!

So, today I have started to make the directional coupler - this is the bit that sniffs the RF in a feeder and produces a forward and reflected waveform... mine looks like this:

Far Circuits in the USA:

do the circuit boards for this power meter project so I've ordered a set. I've bought from them before and the service is usually excellent. So, when I have the boards and all the other bits, I'll post some more on my power measurement conundrum.

Good egh?

Tuesday, 9 October 2012

It's a busy place 30M!


Been fiddling with wire antennas again over the weekend. Decided to make a good old-fashioned dipole for 30M.

Here is the dullest photograph ever:

But you can, at least, see that when I say an old-fashioned dipole, I really do mean two bits of wire with coax in the middle.

Now, over the last 24 hours I have left one of the radios WSPRing on 30M, here is the general map of the time period:

looks like a very busy place 30M WSPR!

And here is the map showing just people I am hearing or people who are hearing me (or both!):

So, it very much looks like my two bits of wire are working well! Now, looking in detail at the unique contacts involving my callsign, we find this:

Now, the two contacts to VK land (Australia), seem to be between 7 and 8 AM and also 7 and 8 PM; now that just about during the transition between day and night (or vice versa) so I just wonder if there is some grey line influence here?

What do you recon?

Fun though, egh?