Tag: radio

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Yet Another WSPR Transmitter

I have joined the ranks of the many who have built a WSPR Transmitter. Why? Mostly so I can have a bit of fun. The problem is difficult enough to be interesting but not so complex as to become boring. Having said that there’s a lot involved.

I can transmit WSPR with my TS590S and presumably any other off-the-shelf transceiver. But these all have fan noise so I wouldn’t want to run them overnight. So the transmitter will be able to be used to test propagation overnight.

But mostly I’ve made it to have the fun of making it.

The transmitter cycles through the amateur bands from 80m to 10m optionally transmitting on each band. An RTC module (DS3231 I2C Real Time Clock Module) is used to keep track of the real time. An AD9850 DDS Signal Generator Module produces the RF signal. Both of these are controlled by a D1 Mini ESP8266 Dev Board. The ESP8266 does all the encoding of the WSPR tones and decides when to send them. The signal from the AD9850 is run through a power amp (10W linear HF PA from QRP Labs) then through an appropriate LPF to the aerial.

WSPR Transmitter Picture

Currently I’ve only had it on air on the 40m band as I only have a home-brew 40m LPF to hand. This works fine and the signals are being picked up in Europe ok. I’m getting a switchable LPF for the other bands but it’s taking a while to arrive from Russia. However the transmitter works on every band as my TS590S close to the transmitter can pick up the stray signals when I run it into a dummy load. The signals are decoded fine using WSJT-X.

More details later.

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SDRplay and dodgy USB connection

I had thought that my SDRplay RSP2 was dead because none of my computers would recognise it. But I noticed that it was still drawing current from the USB port when attached and 50 mA seemed reasonable.

I got it to work under CubicSDR somehow, but it was not reliable. Luckily at some stage it was working (I was listening to a Classic FM broadcast) and I accidentally tapped the RSP2 and the dulcet sounds of the orchestra turned into a loud buzzing noise. I realised that it was a hardware problem and I tried to recover and nothing would work until I tried an old USB cable. The RSP2 works fine with this cable but not with any other!

Maybe I’ve lived a sheltered life but I don’t remember any device having a dodgy USB type-B socket. Micro- and mini-USB fails are common, but type-B?

Anyway, I’m happy now because I can monitor my VHF Q65 transmissions using the SDRplay.

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SignaLink Jumpers for FT290R (correction)

I was playing about with my FT290R and Q65 protocol and realised that I had to configure my SignaLink jumpers in a way different from how I had previously blogged. 

So these are the settings that I now find ok. Apologies if you’ve been misled by my previous post.

SignaLInk Jumpers for FT290R correction

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PIC12F1840 Si5351a VFO

To recap: this is a no-frills VFO intended as a drop-in replacement for a Colpitt’s oscillator in a Direct Conversion receiver. It allows the frequency to be changed within limits set in the code and allows the tuning to be toggled between fast and slow. An LED lights if a frequency limit is reached.

I decided what to use instead of the PICs I usually choose. The answer is: … more modern PICs! A lot of the competition are modules which makes them bigger. 

I bought a PICkit 3 which seems to be a lot cheaper than when I last looked. This allows me to program a PIC12F1840 which is much smaller than a PIC16F887. So I have ported the PIC16F887 Si5351a VFO code to that PIC. I have improved the encoder turn handling. With the encoder I’m using I needed to debounce by adding capacitors to ground. The tuning speed is toggled with a button in the encoder. It may be nicer to have a switch instead, perhaps a DIP switch for the tuning as holding down the button while tuning is a bit clumsy.

VFO Wiring

The PIC12F1840 has a sleep instruction so the code now does all the encoder turn processing in the ISR. The main loop goes to sleep and all that is done in it is to change the frequency of the Si5351a. So the PIC is asleep nearly all the time. It only wakes up when the encoder is turned. The Si5351a is always on, though, as it is generating the clock that the received RF signal is beating against.

A separate button (or DIP switch) allows the user to change the amount the frequency changes each turn. The VFO uses 30 mA when sleeping and 40 mA when the encoder is turning. I don’t know what the Colpitt’s oscillator took.

Here’s the code.

While I was debugging the code I took the following screenshot:

Debug Screenshot

The yellow and blue are decoder turn voltages. The pink is the LED used for debug. It goes high if the code is on the ISR. As you can see it enters the ISR at every encoder turn change. The reason I’d had to go to these lengths to see what was going on was that I’d assumed that the interrupt-on-change flag was for all changes, but it isn’t. There are separate flags for rises and falls. Moral: always read the data sheet!

The footprint of the VFO is only slightly larger than the Colpitt’s oscillator and if that used a beefier variable capacitor the new VFO would be smaller. I’m hopeful it will be useful in a lot of projects.

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ISS SSTV Image

I finally got a decent image from the space station. The recent ISS passes have been plagued with interference from somewhere, but this image seems fine.

PD120 20210624 092404

I’m using this setup — the aerial is my Cebik Moxon.

ISS 6 of 12

The Raspberry Pi 3B plus is running QSSTV. The USB Audio dongle is a 33051D. The pre-amp is an M-100.

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Si5351a VFO for IC Based Receiver

This is a drop-in replacement for a Colpitt’s oscillator as the local oscillator for the mixer in a direct conversion receiver using a NE602. So it has no frills — it just generates a square wave at the required frequencies. The tuning is done with a rotary encoder. The receiver I want it for works on the CW part of the 40m band so the VFO tunes from 7.000 MHz to 7.020 MHz. An LED lights when a frequency limit has been hit. The encoder button is used to toggle the tuning rate between fast (1000 Hz per click) and slow (100 Hz per click). These can be changed to any pair of values, as can the frequency limits.

A Si5351a synthesiser is used to generate the square wave. It is controlled by a PIC micro-controller.

PIC 887 VFO Layout

I wanted to use a small 8-pin PIC for this to make the VFO have a small footprint. I chose a PIC12F1840 which has the I2C built-in needed to drive the Si5351a. But I found that I couldn’t program this PIC with my ageing PICkit2. So I tried my even more ancient K150 programmer — no joy here either. So I’ve had to use a PIC16F887 for now — even though it must be twenty times bigger. I considered bit-banging the I2C with a PIC12F683 or a PIC12F688, but it looks pretty onerous and I don’t want to spend my time chasing I2C errors. It may be the time has come to move on from PICs though I haven’t seen any alternatives that appeal yet.

The code traps encoder turns as interrupts and works out which way the encoder is turning. The debounced encoder button and frequency changes are done in the main (endless) loop. The Si5351a I2C is driven using a port of Hans Summer’s Si5351a Arduino demo. Thanks Hans! I’m also using one of Hans’ Si5351a module kits. I ‘trimmed’ the 27 MHz crystal by changing the value of SI_XTAL_FREQ in si5351a.h to 27003934 which gave the expected frequencies as seen on my ancient RACAL-DANA 9916 frequency counter.

Here are the source files and make file to build the VFO.

I haven’t tried this in the receiver yet, but it’s hopefully useful as it is.

Sometimes an encoder turn isn’t seen, but this doesn’t seem to be too obvious in use. I intend to fix that when I port the code to whatever PIC replacement I choose.

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Rotary Encoder Test for PIC

I’ve been playing about making simple direct conversion receivers using an NE602 for the mixer. These receivers have some sort of input network for the signal captured by the aerial, mix that with a VFO and amplify the (possibly low pass filtered) audio output from the mixer.

The “classic” version uses a Colpitt’s oscillator for the VFO. This is an analogue oscillator that feeds back via a voltage divider made by a pair of capacitors. This works well and tunes nicely and is magical in its own way. However, the tuning is quite coarse with the variable capacitors I have, and the cost of replacement variable capacitors is getting eye-watering. They can also be quite large.

So I’m trying a replacement VFO made from an Si5351a clock generator. The Si5351a will be controlled by a PIC, being my microcontroller of choice (and available in my junk box). The tuning will be done by turning a rotary encoder. 

I had a rotary encoder of the right size in my junk box and but had no way to find out how it performs. So I wrote a small test program for a PIC16F887 which drives the rotary encoder and shows on LEDs which way the encoder was turned, whether it was turned recently and whether the button on the encoder was pressed or not. I often use a PIC16F887 for tests like this as it has many i/o pins that can be digital or analogue and can be pulled up at will. I don’t think I’ll be using it for the VFO as it is quite big (40 pins!) but I’ll be able to re-use the code amended for whichever PIC is choose.

The schematic is simple: pins on the PIC PORTB are weakly pulled-up and are used for input from the encoder. Four LEDs are connected to PORTD each with a current-limiting resistor. One is the power on LED, the others show what is happening (or what the PC thinks is happening) when you play with the encoder. All the pins are digital.

Encoder Test Schematic

The built test is shown below. The wires you can see that are not shown in the schematic are connections to allow the programming of the PIC using a PICkit2. Ignore the LED on the bottom left as it shouldn’t be there.

Encoder Test on Breadboard

The code and make file is in this zip file. Changes made at the encoder by turning the knob are seen via interrupt. The button changes are polled. The LEDs are lit (or not) in the same polling loop.

Tune in later for the VFO itself.

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Morse Sending Tutor Mock-up

I realised that now I’ve made a CW Sidetone Oscillator, a Morse Keyer and a Morse Decoder, I could make a useful tutor for testing my sent morse.

The trick of sending good morse is in the timing. Getting the inter-character and inter-word gaps the correct length is quite difficult especially at higher speeds. So a tutor to check this could be a big help.

Morse Sending Tutor Diagram

So I lashed this together using a breadboard to do the connections. 

IMG 0070

And tested it as seen in this video: Morse Tutor Demo.mov.

As you can see it’s very rough and ready. If I were to make a proper version I’d remove the WPM and tuning aid on the top line which aren’t necessary. It would fit into a small box quite easily.

It was fun to throw together!

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Simple Morse Keyer

My home-brew 40m transmitter only handles a straight key at the moment, but I use a paddle. I currently use a separate keyer but this is a bit clumsy. I found a simple keyer circuit that should do the job. The keyer is by N1HFX: see http://www.rason.org/Projects/cwkeyer/cwkeyer.htm. Thanks, Mike!

The keyer is not iambic, but that’s ok for me as I usually use a single lever paddle. When I’m using a dual paddle key I don’t squeeze.

I didn’t have the exact components so I tweaked the design to cope with what I did have. Here is the new schematic and layout. I guess you shouldn’t mix IC families, but it works!

40mtx Keyer Schematic

40mtx Keyer Layout

The circuit works like this. Firstly, the oscillator. 

R2 is the variable resistor between SPEED_POT_1 and SPEED_POT_2.

Assume pins 5 and 6 are 0 V, and C1 has no charge. Pin 4 will be at 5 V.
C1 will get charged through R2 and R1. 0nce most of the 5 V is across C1, pins 1 and 2 will be at 0 V and pin 3 will change to 5 V. Pin 3 is directly connected to pins 5 and 6 so pin 4 will go to 0 V. C1 will discharge and once discharged enough pins 1 and 2 will be at 5 V and pin 3 will become 0 V which is where we came in.
The speed of the oscillation depends on C1, R2 and R1. R2 is variable and is used to change the speed. The speed could be calculated by using the RC curve and seeing where it hits logic high and low. But I decided to do the calculations in MATLAB using these scripts. This gives a plot for R2:

Sidetone WPM

This suggests you can’t key faster than about 27 WPM. This isn’t yet a problem for me as I can barely do 17 WPM.

The keying logic is like this.  Assume the both paddles are up initially. The logic levels are like this for dots:

^ marks dot clock transition to high

                                               
CLK  N0R8     N0R9     D    Q    DOT Paddle
-    1        x        0    x    up
^    1        0        0    0    up
-    0        0        1    0    down
^    0        0        1    1    down
-    0        1        0    1    down
^    0        1        0    0    down
-    0        0        1    0    down
^    0        0        1    1    down
-    0        1        0    1    down
^    0        1        0    0    down
and so on giving equally spaced dots
-    1        1        0    0    up
^    1        0        0    0    up
and so on giving no dots.

Or, less analytically: the dot paddle grounds one input to the NOR gate and the output of this is clocked into the dot flip-flop. The Q output of the flip-flop is fed back into the other NOR gate input. This gives clocked dots.

Single Dot Screenshot
The dash flip-flop uses the Q signal from the dot flip-flop as its clock. So the dash ‘stays on’ for two dits. To make it three dits as required to make it a proper dash the ~Q from the dash flip-flop is fed to the dot paddle input making the dash have an extra dit (as ~Q will be low when Q is high thus pulling the dot paddle low).
Single Dash Screenshot

Keyer on Pad board

All this works fine. Breadboarding was fun, but I made the mistake of putting it on pad board and the soldering was onerous — there’s just too many connections between the two chips hidden under the board. Next time I’ll brush up my coding skills and use a micro-controller.