Tuesday, September 21, 2021

Resistive tap built with sharpie PCB etch

I'm keen not to burn out the input to the spectrum analyser. It's 50 ohms in with a maximum of +30dBm. The solution is to transmit into a dummy load and sample the signal with a resistive tap.

A good candidate for a simple PCB and I used Paul Taylor, VK3HN's, technique of drawing resist on the board with a sharpie. Components are all soldered on to the top of the board so laying them out to figure out where the tracks go is easy.

Here's the circuit and the components for the board:

I cleaned the board to remove fingerprints with Isopropyl Alcohol - is that the best thing to use? Using components to lay out the board, tracks were drawn with a standard sharpie. I didn't go to much trouble and could probably have filled in more of the board with earth plane to minimise the amount of copper to be etched.

In to a small bath of Ferric Chloride. While gently rocking we had a magnitude 6 earthquake which was fun. It took about 20 minutes to clear the copper.

After washing under water the etch looks great.

Steel wool was used to rub off the sharpie ink. 

Here's the completed board. 

I've got some more of the nice board edge mounting SMA sockets coming so had to make do with a panel mount for the tap output.

What should I use to protect the copper? I think Paul just tins the whole board but maybe there's a spray lacquer that would still allow solder modifications but prevent oxidation.

Sunday, September 12, 2021

An OWON XSA815-TG Spectrum Analyser for the bench

I've wanted to try a spectrum analyser for many years but they've been prohibitively expensive. Just as oscilloscopes have come down in price, spectrum analysers are also getting cheaper. I bought an OWON XSA815-TG Spectrum Analyser from Banggood for just under US$1,000.

This one is good to 1.5GHz which should be plenty for my use.

So far, I'm really impressed. It's very easy to use - easier than many CROs. Certainly easier than the TinySA. Here it is showing the local FM broadcast band:

Here is a signal from a 2m FM hand held (received off air):

Here is the tracking generator in operation showing a 9MHz low pass filter:

There is lots to learn but I feel that after a CRO, a spectrum analyser is a wonderful piece of test equipment that is now within financial range of some of us lucky hobbyists.

The terrible interference I often see on 40m has just returned. Here's how it looks on the IC-7300:

I tried plugging the antenna into the spectrum analyser in the hope of seeing the overall noise but I'm not:

I think I need to play with attenuation and gain to see what's going on. Much to learn!

Here's the output of my recent Si5351 WSPR transmitter with a fairly poor low pass filter after it. Those second and particularly third harmonics look pretty big.

Tuesday, September 07, 2021

Simple WSPR Beacon using Si5351 and ESP8266 NTP for time sync

Wanting to play with a simple WSPR Beacon in the house where I can't receive GPS for time synchronisation, I've hacked together a beacon using an Si5351 oscillator and an ESP8266 board that joins my Wifi and gets the time from an NTP server.

Here's the rig:

It starts off by joining Wifi, then requests the time from time.nist.gov. Next it figures out how many seconds to wait before the next 2 minute slot and delays that long. The WSPR signal is transmitted followed by a 10 second delay before requesting the time again. Assuming the NTP server replies, WSPR transmission is every second 2 minute slot.

The Si5351 generates a square wave into 5cm of wire and there's massive hum on the signal presumably due to the USB power. Here's how it looks decoding off air:

I got this going using a WeMos D1 R2 board which is basically an ESP8266 on an Arduino style board. I tried to use an ESP32 but for some reason (possibly power voltage) I couldn't get it to talk to the Si5351 although it could see it on the I2C bus.

Just a toy really but might be useful for something. The source code is here.

I'm using Arduino 1.8.15 and the following libraries are installed:

  • Etherkit JTEncode 1.3.1
  • Adafruit Si5351 1.2.1
  • Other libraries for UDP were bundled with the board, in my case LOLIN(WeMos) D1 R1

Wednesday, September 01, 2021

ZM-4 Z-match antenna tuner kit review

Years ago I built the AM-2 Z-Match antenna tuner and found that it could match pretty much any old bit of wire thrown outside. I melted the polyvaricons by running WSPR one day but they were easily replaced.

There's a German made evolution of this design with more robust variable capacitors and a larger toroid that claims to handle up to 15W.

The ZM-4 kit is available from QRP-Shop for 98 Euros. It came with an English manual but it helps to view it in colour here.

Nice spread out circuit board for easy construction.

Unfortunately a couple of component labels are missing from the board’s silk screening.

Instructions fairly clear but I started off reversing the direction of winding on the main toroid. 

It says wind to the right but neglected to say the winding should start on the bottom of the loop. At one point it says to wind over the red wire, but there’s no red wire. (I now see that it's red in the colour instruction manual).

The BNC connectors have substantial thermal mass and my little soldering iron struggled to heat the earth pins to melt temperature.

The metal case is great but it would be wonderful if the front and back panels were pre-drilled. My metal work skills are not great and I found the front and back plates to be quite hard aluminium to drill safely.

Great to see metric units with no comment.

When assembling the front and rear panels, have the circuit board close by as a guide. The instructions don’t suggest switches and plugs in the order they are on the panel and I ended up enlarging a hole that I shouldn’t have by mistake.

The kit comes with two banana sockets which I replaced with terminals so that a wire can be directly connected.

Some of the wiring, in particular the links from the panel switches down to the board is pretty tedious. I guess the solution would be a PCB behind the panel to handle that wiring but extra cost would be added by this.

Only after wiring up front and back panels did I realise that you need to slide the board in to the bottom of the case before connecting the panel wiring. I should have tried fitting the case before this but also the instructions should have mentioned it.

I unscrewed the BNC connectors and luckily had enough length of wire to the switches to be able to lift the panel out of the way and slide the board in to the lower part of the case which I had already lined with gaffer tape to try to minimise likelihood of shorts to the case as warned in the manual.

Hooked up to an end fed wire with a short counterpoise the Z-Match was able to get an excellent match by playing around with the additional capacitance switches and rocking back and forth between the variable capacitors.

Z-Matches are a versatile antenna tuner although I understand they can be lossy. This kit is well done but the switch wiring and panel metalwork takes some effort. When boxed up it's a compact and strong little tuner that will work well for portable QRP operation.