If you have been following my blog for a while, you know that I have made several starts on a simple Spectrum Analyzer. In most cases, as I progressed in the project I came to the point where I needed to build some other project. Little things, like when I needed to test a band-pass filter, I needed to build a SNA first. Then I found I would probably need something with a higher frequency response for working on a SA with the frequency range I wanted.
Over a year ago I found some information on a very small VNA for around $60. Since this NanoVNA was less than 1/10 the price of any other VNA with a built in display, I jumped at it. It gave me an instrument that had the frequency response I would need to work on a SA.
There was an IO group formed to support the little VNA.
Following that group I found another group about Home Brew Test Equipment. One of the ongoing projects in that group was a Tiny SA. Several different versions of that were being developed and it looked very promising. Around the first of the year a TinySA group was started for several people who were testing a future commercial version of the TinySA. Some of the info on this group looked very interesting, especially a short video on the nearly finished product.
http://tinysa.org/video/intro.mp4 Also a lot of information and specifications at
https://tinysa.org/wiki/pmwiki.php?n=Main.HomePage
Just the other day I saw a post on the TinySA group, that the product was available for order. The TinySa is being produced by the same person who originally produced the NanoVNA. With that as a recommendation I quickly placed my order, since I understand there is only an initial production run of 300 being made.
https://www.alibaba.com/product-detail/Hand-held-tiny-Spectrum-analyzer-TinySA_1600085564565.html
The availability of a less than $100 SA with a range of 100kHz to 350MHz (240- 950 MHz. without bandpass filter )will make the life of the home-builder a lot more fun if not easier.
Now all I have to do is wait #@*& !
I made a circuit board layout for the ESP 32 based VFO-BFO using the Wemos ESP 32 Mini controller module. Sent Gerber files off to China and had 10 made up, they arrived in a little over a week with Express shipping. I built one up and found a few minotr errors I had made in some dimensions, but the boards still worked fine. I decided I would use this with a Bitx 40 board I have laying around. I think I will use Pete N6QWs method for doing Upper and Lower sideband selection. That means adding or subtracting the desired frequency from the IF frequency . For the Bitx 40 that means setting the SI5351 frequency to 5 or 19 MHz range depending on the sideband selected. Only tricky part is getting the exact IF frequency, of the Bitx 40.
The board layout has provisions for a up to 7 pushbutton switches that can be read by a single analog input pin.
I chose to use a double row header so I could add individual switches as desired or a switch array connected through a single ribbon cable. I also brought out the connections, 3.3v, and ground for use by a rotary encoder with switch..
The connector for the SI5351 module is mounted on the bottom, so the boards can be stacked. Also, a female header strip can be mounted on the bottom for a 1.8" TFT display which then can also be stacked. I have provision for an optional 5 volt regulator so you can power the assembly form 7-12 volts if desired, instead of directly with 5 volts
One thing I like about the Wemos module is that I only have to make connections to the inside rows of header pins. If I need additional control pins, I can put header pins in the outside rows of connections and just plug onto them. This means I do not have to worry about those pins in the PCB layout.
After playing around with the options this gives me,I decided to modify the layout to make the small corections in layout positions I found.
I am also going to bring out some of the pins I had not originally used , and add a couple connectors for additional connections to the I2C and SPI signals. This will allow me to use the same board as display-controller for several other projects I have been slowly working on. Now just need to send this off to a board house in China, then wait.
A couple of the projects I have been working on have been put on hold because of the shipping delays caused by the Covid shutdown in many places. Looking around for something that only required what I knew I have on hand. I rembered seeing a YouTube video of a SI5351 based VFO with a small TFT display that very nicely simulated a mechanical dial. Going back I found the video, and there was a link to the website of JF3HZB, the schematic , and Arduino code for the project. It uses the same 128 x 160 TFT display I have in my VFO-BFO, and a ESP-32 dev. board. Looking at what pins are required, the small Wemos Mini I have should work nicely.
I downloaded the software, and got it to compile without any problem. After looking at the code, I can see where having two 240 Mhz. cores doing the processing is nearly a necesity for something like this. And also still has plenty of room and power to addd other features. I quickly built a simple board to see how it looks, and works .
After getting the basic software running for the display, I did some modifications to add a couple things I want to have on the display. a LSB/USB indicator and I will probably add a T/R indiator light of some type. The response of the display is really nice, except for some small jumping around that is coming from the really cheap rotary encoder I used. I have a couple better ones that I will use after I get a board designed, now I will see if I can correct with some filtering capacitors.
I plan on using one of the Adafruit SI5351 modules I have, so will have to see what if any changes I have to make to use them with 3.3 volt logic instead of 5 volt. I have not looked at the 5351 library used with the original software, so do not know if I will keep it or use the same library I have used before.
Trying to think about what I want to include on the board design. All the pins required are on the inside set of pins on the Wemos module. That means I can just add some header pins to the top of the Wemos module instead of having to route them on the board. This should allow for fairly easy expansion.
I like the single analog pin method for monitoring multiple push buttons. I will probably add the resistor chain and some header pins on the board to make it easy to add several push button controls. Also thinking of having provision to add a retoary encoder directly on the board, or a connector for adding an external encoder.
I will get started on the board layout, then decide if I want to order some from one of the board houses. They all offer DHL shipping, so should not have to wait too long to get them.
I tried to make a video of the board working, but couldn't find a way to keep the camera steady while operating the encoder.
So here is a link to the original YouTubevideo I watched.
https://www.youtube.com/watch?v=3PV2kLOippY
With almost everything being shut down from the virus, there must be something good happening. I think I have found at least one thing. Since we can't gather together the ability to hold meetings via video conferencing has been really taking off.
Last Saturday we held the monthly meeting of the North Georgia QRP Club via Zoom. We had around 25 people show up. Mostly members, but also a couple guests including Hans Summers from QRPLabs checking in from Turkey. He gave a brief history of the QCX transceiver. and informed us that they are only a couple hundred short of having sold 10,000 kits. He also spoke a little about the multi mode, all HF band QSX that is in development. We also heard from some of out local members, and got to see some pictures of their shack, and work bench. Except for a couple little glitches, from not being familiar with the software every thing went quitae well. Now waiting for the next meeting.
NO this is not the FBI's Most wanted List, or is it?
Last evening I got to attend another meeting. This one would be a little frather drive from Atlana than the NOGA meeting. This was with the homebrew group from the Peal Amateur Radio Club located in the Greater Toronto area. I usually only get together with some of them at FDIM, so this is a plus for me.
This meeting used a different software package, but also worked quite well. Except for also being the first time most of have used it. We had 17 people in the meeting including several that joined in via cell phone. As with the NOGA meeting there was a guest calling in. He was Rex Harper from QRP ME, and he spoke about the 'Buildithon' project that has been scheduled at FDIM this year. He is still working on the kits, and is planning on having a virtual Buildithon. Also heard about the progress being made on the Direct Conversion Receiver, which is to be the club 'Buildithon' this year.
Possibly the RCMP most wanted list ?
With the aid of the video conferencing software we are still able to get together with our friends. Of course most of us have more time to get on the air, work on projects, or start on the list of things the Wife has been wanting done 'forever'.
My last comment on the matter is something a friend said in one of his e-mails. " I feel like a teenager again, gas is cheap, and I'm GROUNDED"
I had started on the TinySA from Groups.io HBTE, but am still waiting on some parts that I had orderd than have yet to show up. Looking around I found a UHF version that covers 240 to 940 MHz. This only uses a single si4432 transceiver module, which I have on hand. So I will try to do a similar UHF version of the TinySA. Since this does not need the input LPF, mixer, and a high local oscillator, it should turn out to be a small instrument.
I have previously used a ESP32 development module, but that was quite large. Looking around aroound I found a Wemos ESP 32 mini that looked like it would do nicely.
It gets much of its size from having the IO pins as two rows of 2 wide headers .
Fortunatly the SPI and I2c pins are all located on the inside rows of pins, and there are enough extra pins available for the every thing else I need.
I can also put right angle headers in the outside row of pins and connect directly to them without having to have additional connectors on the circuit board.
I had neen very happy using a joystick for the user input device, but they are a little large for what I had in mind, and also they used 3 IO pins. I also wanted a couple extra control buttons available for the UI. I decided to go to keypad made up of a resistor divider string with switches to ground at each intersection. This can be read using a single ADC input.
Using surface mount components, the entire keypad can be layed out on the top side of the PCB, without taking up any extra space in he cabinet. I designed and 3D printed a small case, and the buttons for the keypad.
The software to read the keypad is quite easy. First you have to find out what ADC vlue is returned with no keys pressed and when each key is pressed. These values are reduced about 10% to prevent false readings. These values are put into an array.
The keypad function checks to see if a keyis pressed ( reading lower than none pressed value). If it is a short delay to debounce and another reading is taken ,then simple for loop compares this to the values in the array. The value of the last array item lower then the read value is returned to the calling program.
int readKeypad() {
int i;
int key;
// analog value for each key press measured and adjusted down about 10% to prevent
// wrong readings 0 position is no key pressed
int key_val[] = {3500, 3200, 2900, 2600, 2200, 1700, 900, -1};
if (analogRead(keyPadPin) > key_val[0]) // No key pressed
return 0; // just return
else
delay(5); // debounce
i = analogRead(keyPadPin); // get pressed key reading
for (int n = 0; n < 8; n++) { // check to see which key is pressed
if (i <= key_val[n]) // compare against array of measured values
key = n + 1; // bump up so you can use simple if test for a pressed key
}
if (key > 7 )key = 0; // helps to clean up clean up extended switch bounce
return key;
delay(25); // set max repeat rate
}
This works quite well, except for a little key bounce from the really cheap switches I had on hand. I put everything in the printed case, and loaded a modified version of the menu system I had used with joystick.
Now I need to grab some of the si4432 code from the TinySA on the HBTE group and see if I can get it working in the receive mode, then write some sweep and display functions.
I have been using SMA connectors on most of my projects, and have occasionaly gotten a slightly different reading than I had expected. Using the NanoVNA, this has shown up a little more often. After a some checking, it appears that having the connectors 'finger tight' is not quite good enough for getting consistant readings. After watching several Youtube videos and reading soome instruction manuals on much more expensive VNAs , I decided I need some form of torque wrench to help eleviate the problem.
Looking around I found that even the cheapest torque wrench for SMA connectors cost nearly as much if not much more than the NanoVNA. I wondered if it would be possible to 3D print someting that would be usable. I looked around Thingiverse.com, and found a couple examples of torque wrenches that looked easy enough to model one on for SMA connectors. Looking as the specifications for SMA connectors, I found that they shold be tightened to 5 in. pounds of torque. For my use it would be adequate to have it somewhere near that value as long as it was consistant.
SMA connector nuts are 8mm. across the flats, so I went 8.2mm. to make it easier to get on and off. I also put a wide enough slot in the end of the wrench to allow it to slide over cables easily. While I was working on the design I decided to taper the other end of the wrench so it could be used as a stylus for the touch screen. I also added a small hole 2" from the center of the wrench opening, so you can use a small luggage scale to compute the actual torque.
I printed a couple with different printer settings and tried them out. Just place over the connector nut and turn until the wrench slipps around to the next flat. I measured how much force I had to apply for this slip to happen. Then by adjusting the number of top and bottom layers, along with number of perimiters I got the torque to be somewhere around 4 in. pounds. With this value I appear to be getting more consistant readings than I hade before, and have a handy stylus for the touch screen. It also makes it easier to get the cables on and off than just using my fingers.
My settings for the print is for PLA filament. Three perimiters (wall line count), four top and bottom layers, and 40% infill.
The .stl file is located at.
https://www.dropbox.com/sh/qt816x30fujt3kl/AABbUj5DdeNC5n3VE0RyJD9ra?dl=0
Not sure how long these wrenches will last, but only take 15 minutes to print and use about 3 grams of filament.
When I first started working on a Spectrum Analyzer design I needed some way to check the sweep ranges of the local oscillator, and output of stages. Since some of these were in the 100s of MHz., my oscilloscope would not be enough, and my frequency counter could not follow the frequency when doing a sweep. I had used one of the inexpensive RTl_SDR dongles before, and they had worked fairly well. I recently saw an ad for a 'HackRF One' on AliExpress with a TCXO for under $70. This unit covers from 1MHz. to 6GHz., can do 20 Million Samples per Second, and can be used as a transmitter as well as receiver. I have seen it available bundled with a selection of antennas, accessories and in a case for over $300.
Delivery time was only about two weeks. Looking at the board I was impressed with the quality. and I liked the fact that it used SMA connectors for the antenna, and oscillator in and out connectors.
The one from AliExpress was the board only, so I would need to make a case for it. After some quick measurements, I modified one of my standard case designs to fit.
I decided to use a one piece design with guide slots for the circuit board. I have found this to be very solid, and makes mounting circuit boards very easy. I printed it using some Carbon Fiber filled filament I had on hand. Don't think the Carbon Fiber pieces in the filament are long enough to do any shielding, but it sure looks nice.
Next I designed and printed a front and back for the case. After putting it all together it makes a very nice looking unit. A little taller than the ones I have seen with some of the complete units, but still a very nice case that will give good protection to the board. Still in the process of trying several of the different SDR software packages that support the HackRF. Will write up something on them a little later.
For anyone interested, I have the .stl files available at
https://www.dropbox.com/sh/gi3tbny3le8hhhx/AAAJN17G9Nq0jLIntIF3wk88a?dl=0
Meanwhile still waiting on some of the parts needed for the 'TinySA' to arrive. I think the Chinese holiday delayed everything.
