Well? You must have played a little by now?

Well,

Last time I mentioned that I had bought myself a Rigol DSA815 Spectrum Analyser with the built in Tracking Generator. I'm just about to head back to MAN for a flight to A71 land but I have had a bit of a fiddle this morning. First off, I made myself a very simple SWR bridge using some bits and bobs lying around, it's similar to the one I made back here for the power meter:

http://g0mgx.blogspot.co.uk/2012/10/well-since-progress-i-discussed-last.html

but smaller and not designed for the power, it looks like this:

What we have here at the top of the photo is a LH connector for the RF source, the top RH connector for the load (antenna). On the bottom the 2 parallel 100R resistors form a 50R termination for the forward power sample port and the reflected power sample port is connected to the bottom connector.

So, if I now connect the output of the Spectrum Analyser tracking generator to the top left (RF source), an antenna under test to the top right (RF Load) and the input to the Spectrum Analyser to the Reflected power sample port, we have a very expensive antenna analyser thingamabob!

With my recently constructed 40/80M dipole from here:

http://g0mgx.blogspot.co.uk/2013/04/well-further-to-my-musings-last-time.html

coupled to the RF Load port and the instrument appropriately configured, here's what we see:

Now, I've used the built in marker capability within the Spectrum Analyser to do a few things here:

What does this all tell me?

• The 80M part of the antenna is nicely tuned within the band at 3.533MHz
• The 40M part of the antenna is a mile out at about 7.765MHz
• The 5dB width of the 40M resonant point is about 165KHz

I haven't done the maths yet so I don't know if my selection of a +5dB measurement is appropriate to see the bandwidth of decent SWR - I will do that later. But a summary of the summary?

It's too damn short!

Interesting though, egh?

Calibration Finaly Complete (Really)

Well,

You may remember the musings from a while ago where I made the dBm meter for the shack:

http://g0mgx.blogspot.co.uk/2012/11/calibration-complete.html

I wasn't overly convinced of my calibration, the 'scope, spectrum analyser and this meter didn't align, especially at low power levels. There is always a risk that the old test equipment I have (the spectrum analyser) is well out of calibration, but I was curious. The calibrator that I made (Design courtesy of Mr Kopski, K3NHI) didn't quite look correct on the 'scope either - it was described as a square wave, but mine looked kind of triangular like this:

But it did look full of harmonics when viewed on the spectrum analyser:

So, I started by asking some suitably dumb questions on a forum or two, and ended up in a very educational and excellent email exchange with my new found chum Kerry who lives down under.

To cut a long story short, Kerry has some very accurate test kit (accurate read expensive!) and has kindly fed some known signals into an AD8307 RF front end (like the one in my power meter) and noted the voltage output by the device itself, here are the values:

So, I took these values and plotted them as a X-Y scatter plot in good old Excel, then added a trend line to each set of data and got the spreadsheet to include the equation on the graph:

Now, hopefully you will be able to see that using the equation on the graph, we can now calculate any dBm value that the meter should be displaying from the actual voltage on the device output (pin 4).

So, using my good faithful Avo signal generator and some of the attenuators I made earlier, plus the attenuator on the Avo itself, I have fed a load of different RF signals into the meter, read the voltage on pin 4 of the AD8307 and then used the equation to calculate the dBm value that the voltage represents:

Once this was complete (ignore the yellow above for now), I then used the lowest and nearest to 0dBm to calibrate the meter. The yellow values are the meter actual display readings once the calibration was complete.

My confidence is now very high in this homebrew instrument and I suspect it may become the most useful device yet.

So, feeding a -30dBm signal (as measured with the calibrated meter) into my Spectrum Analyser with the reference level (top line on the display) set to 0dBm, the fundamental 5MHz signal in this photo is shown -30dBm down from the 0dBm reference level i.e. they agree! Sorry about the wonky photo:

Here is the dBm meter with the Avo signal generator set at 0dBm and a -10dB attenuator (pad) in line with the feeder; 0.3dBm is really very close indeed:

Oh, and the suspect Kopski calibrator? That now reads 19.9dBm - can't be bad! It was the -50dBm version that I concocted myself that was highly suspect - it reads -39dBm!

I've started an Analogue equivalent of the meter, initially because this would only require a single calibration point, but now I have found a better way to calibrate the digital meter, the project is rather redundant. I'm rather pleased with the meter scale though:

And all this because I became suspicious of the readings I was seeing from the attenuators I created as part of this project:

http://g0mgx.blogspot.co.uk/2012/12/ddsing-once-more.html

Hey, Ho. Perhaps now I am happy with the dBm meter, I can progress that project? Unfortunately I've run out of holiday and have to go back to work. I suppose there's always next Christmas.

Fun though, egh?

Calibration Complete

Well,

Following my musings back here:

http://g0mgx.blogspot.co.uk/2012/11/the-art-of-calibration.html

about making some known RF sources, I seem to have been successful. I have created a -20dBm and -50dBm RF source which I have used to calibrate my bench power meter:

As well as the two RF sources I've also made two pads in little die-cast boxes; one of 20dB and one of 30dB.

This should provide me with the tools I need for power measurement and calibration.

Good, egh?

 

The Art of Calibration

Well,

Been thinking some more about power meters and the calibration of them. Quite tricky really, as you need an known RF source to calibrate a meter against it, but to have an accurate and known RF source, you need a calibrated meter to measure it. Recursion: see recursion.

The first thing I have done is to build a power meter that was published in QST, it's designed by Roger Hayward, KA7EXM and is PIC based. This is a stand alone meter for simply measuring from about -80dBm to +7dBm:

Then to calibrate this baby the text says... "assume for a moment that you have an accurate signal generator capable of -70dBm to +7dBm"..... well I don't!

So, a newly found homebrew chum in VK land pointed me at an article by Bob Kopski, K3NHI where he presents a simple calibration source using a CMOS clock oscillator. It kind of looks like this:

So the idea here is that you set the variable resistor such that a digital volt meter connected to the test point reads exactly 158mV. Then in the example above the output goes through a attenuator pad such that the output port is a known -20dBm signal at 10MHz. As I need at least two reference points to calibrate anything (because of the interpolation needed in software) I figured I could use a different pad at the output to create another one of these at, say, -30dBm or even -50dBm. Here's one of them on the bench under test:

Creating a pad is quite simple and there are a number of calculators on the Internet, here's the one I used:

http://chemandy.com/calculators/pi-attenuator-calculator.htm

I'm one BNC socket short of the party right now, so I'll post more to let you know how I get on, but here's one of them in a box ready for use:

Fun, egh?