SpaceLabs MicroTrackers

On 6 October and 24 November 2019 I briefly described a future project using a GNSS receiver module with a microcontroller and a display, to parse GPS NMEA text strings and display my choice of data in an easy-to-read form.
I have now completed 3 versions using different firmware, GNSS receivers, microcontroller development boards, and displays. I call them "SpaceLabs MicroTrackers".

Depending on the version, date, time, latitude, longitude, altitude, nr. satellites acquired, fix quality, and update age are displayed. Features they all have in common are a 25x25mm active ceramic patch antenna, USB interface for programming/power and Ublox Neo-6M compatible GNSS receiver module.

While waiting for enough satellites to be acquired for a position fix, version details, local temperature and battery voltage are displayed. I also designed some graphics using bitmap byte arrays and scrolling animation.

MicroTracker-3 with Arduino board, 128x64px Oled display

MicroTracker-2 with PIC board, 128x32px Oled display

MicroTracker-4 with Arduino board, 84x48px Nokia 5110 display

On MicroTracker-4 I have provided a 1 pulse per second ( 1pps ) signal on an external connector. This is an extremely accurate 1Hz pulse ( 10% duty cycle ) locked to the satellites' atomic clock. When locked-on, a LED on the GNSS receiver module flashes on/off once a second.

Waveform of 1pps signal from MicroTracker-4

A second usb socket on MicroTracker-4 gives access to the GNSS receiver module for changing its settings and displaying complete NMEA sentences on a serial terminal.

 

My recent experience gained with OLED and Nokia 5110 displays has been put to practical use.

Nokia 5110 Liquid Crystal Display

I recently discovered surplus Nokia 5110 Liquid Crystal Displays ( LCD ) which were used in Nokia 5110 phones, circa 1998. Thousands of these interesting display modules are available at on-line auction sites. I bought four, about GBP2.30 each; two having a blue back-light and two with white. The resolution is 84 x 48 pixels; overall dimensions 4.5 x 4.5cms. The controller/driver chip is PCD8544. I searched for and downloaded the data sheet; essential reading !

Nokia 5110 LCD types: back-light colour (L) blue (R) white

Communicating with the display uses the SPI ( Serial Peripheral Interface ) bus specification, which is a synchronous MASTER/SLAVE configuration with the MASTER generating the clock. As well as supply and ground, the following 5 connections to the display are needed; Chip Enable ( CE ), Serial Clock ( CLK ), & Serial Data ( DIN ), and additionally, for control purposes, Data/Command ( DC ), and Reset ( RST ). Back-light ( BL ) connection is optional.

6 config bytes being sent to display on SPI: top CE, mid CLK, bottom DIN

Basically, using the display requires (i) configuring display settings, and (ii) knowing how the addressing of the DDRAM works when writing data for displaying. To try out the display I connected it to the PIC microcontroller ( MSSP module ) on my prototyping board, and ran some code to measure and display temperature and voltage. The bus speed I chose was 1MBit/s.
The display lacks a built-in font. But that was quickly remedied by finding a ASCII character set 5x7pixel font file online, and including it as a header file in my C-code.

Nokia 5110 LCD with white back-light in use

I was disappointed with the blue back-lit display being hard to read, despite spending a lot of time experimenting with contrast, bias and temperature coefficient settings. The white back-light variant is much better in this respect.
I found the display very easy to use; now ready for a suitable future project.
( Click on images to zoom ).
MSSP = Master Synchronous Serial Port, DDRAM = Display Data RAM

Upgrading my design tools for PIC projects

2020 has just begun, and here is my new year's  resolution, which I have already completed !
For my embedded control projects it was time to standardise on PIC microcontrollers, ( e.g., the PIC16F188** family ), belonging to a newer generation than some of those I had previously been using. PIC development in recent years has now led to devices typically having larger memory, supporting higher clock speeds, with more peripherals including core independent peripherals, and new features, such as peripheral pin select, device information area, configurable logic cell, integrated temperature sensors to name a few.

The new development platform for my embedded control projects

As result I have had to upgrade my hardware tools as well for compatibility; the most significant change being the MicroChip "Snap" programmer/debugger, to replace my obsolete ICD2, and a different prototyping board, which I could call MyDev3, ( see post dated 2 November 2010 ). The MCU featured in the image is a 40 pin 8-bit device, MicroChip PIC part PIC16F18875/P. ( Click on image to zoom ).
All is working fine. The PIC was a new unused blank device and successfully programed with my TEMPSENS-OLED firmware. ( See post dated 6 October 2019 ).
MCU = MicroController Unit, PIC = Programmable Integrated Circuit from MicroChip Inc.


  

Positioning Satellite Receiver module & NMEA Sentences

In my last post I mentioned a future project using a positioning satellite receiver module, ( GNSS receiver ).  Since then I've been trying out the MakerHawk GPS 51, incorporating the Goouuu Tech GT-U7 module*. It cost just GBP8.99 and includes an external antenna.

MakerHawk GPS 51 receiver with active antenna

For convenience ( least effort ) rather than a good view of the sky, ( which there certainly wasn't ), I positioned it close to my laptop ( in fact on top of the printer ) in the centre of a room far from two small windows, connected it to the laptop via a usb to uart serial converter, and opened a serial terminal program.

(L) Uart (R) MakerHawk GPS 51 in operation

I doubted if there would be enough satellites received in that position; but after just a few seconds "NMEA Sentences" were being streamed every second and a position fix given. I was absolutely amazed  !!

NMEA Sentences ( click on image to zoom in )

These text strings of comma separated values are known as "NMEA Sentences"; NMEA standing for National Marine Electronics Association. There is plenty of information online to explain the complete format in detail. For my project, I am particularly interested in the sentence starting $GPGGA. GP means it is the US GPS system, ( if GL here it would mean the Russian GLONASS system, GB Chinese BEIDOU, and there are others ), and GGA signifies that it is a position, time and fix data sentence. On examining this sentence I can quickly find the time, my latitude (N), and longtitude (E). The sentence also states, ( among other things ), that my altitude was 87.5m and that a total of 10 satellites had been acquired in order to give the position fix; not bad from the middle of the room.

Now I am going to write some software for my microcontroller to parse the $GPGGA sentence and display latitude, longtitude, altitude, and time, in a more user-friendly form on an OLED display.
* Advertised and sold online as being compatible with the UBlox Neo-6M receiver module. If so, then I assume that UBlox software ( eg u-centre ) and data-sheet for the Neo-6M at www.u-blox.com can be used with the Goouuu Tech GT-U7.
GNSS = Global Navigation Satellite System 

      

Digital Temperature Sensor with OLED Display

Temperature Sensor - MCU board & OLED display

Serial terminal window showing count & raw adc samples

I was working on this project in April but forgot to post it until now when I was thinking about a future project using this microcontroller unit ( MCU ) board and OLED display; but more about that later.
This digital "thermometer" uses the temperature sensor and 2 Analogue to Digital Converter ( ADC ) channels, integrated on the MCU board. I am using one ADC in averaging mode for temperature. Raw temperature data are sampled every 1.875s and the parsed result of the average of a count of 8 consecutive samples is displayed. Voltage is sampled by the second ADC in basic mode every 1.875s and displayed. This is not the voltage of the batteries shown in the image but actually the supply voltage to the MCU and OLED display. I also send the count number and raw ADC measurements using a serial interface to a pc running a terminal program.
The display shows what the device is, the version of the TEMPSENS-OLED firmware installed, temperature ( in degrees C & F ) with resolution 0.01 degrees, and "battery" voltage with resolution 0.001V. Of course I can change any of the above settings and parameters in my firmware.
In my recent projects I have only learnt the basics, ( such as displaying plain text ), of using an OLED display, and not attempted any scrolling, graphics or animation.
For the next project I am considering using the MCU and OLED display with a GPS receiver module. These are available on a well known online shopping site for as little as 4.26GBP including antenna. I shall also be buying a slightly larger 64 x 128px two colour OLED display to show altitude and position; the latter with possibly 1cm accuracy !
( Click on images to zoom in ).

More progress with the I2C OLED

OLED displaying real time data from MCU ADC

0.96" 32 x 128 pixel two colour OLED with I2C interface

I think the OLED is very cool. Since my last post I have been doing more with it than just turning it on or off. It quickly became obvious that mine has a 32x128px display area and not as advertised by the seller; however not really a limitation for my present purposes.
Having removed the Si5351 breakout board ( refer to previous post ) incorporating the I2C bus pull-up resistors on the SCL and SDA lines, I then had to provide these resistors separately ( 10K Ohm each ) for the OLED.
In my firmware, written in 'C' language, there is a look-up table of characters I created, with font based on a 8x8 pixel grid which gives exactly 16 characters per 128 pixel row. I have so far included only capital letters, 0 to 9, =, -, +, space and a few common punctuation marks; 41 in total.
It is essential to have the documentation for the Solomon Systech SSD1306 OLED driver. This was located and downloaded.
As in my post of 17 February 2019, the 10 bit analogue to digital converter ( ADC ) on my micro-controller unit ( MCU ) development board was used to sample the voltage on the slider of a potentiometer ( POT ) every 0.5s, and the result sent over I2C bus to be displayed on the OLED.
I now have the basics in place and ready to adapt to a future project which uses an OLED.
I would also like to try a white on black monochrome OLED which I might find easier to read.
( Note : OLED = Organic LED display ). 
     

Demonstrating I2C serial communications bus

Continuing the theme on serial interfaces from the last post, Inter Integrated Circuit, ( I2C ), is another type of serial communications interface, also called '2-wire bus', and is a very common peripheral interface. There is a master device, slave device(s), addressing and also a clock, making communication synchronous. Two connections are required; serial data ( SDA ) and serial clock ( SCL ).
In this post I demonstrate using I2C with a Silicon Labs Si5351 frequency synthesiser chip and a miniature 0.96" 128/64px two colour OLED display. Both have I2C interfaces but different addresses, hex60 and hex3C respectively, and so can be connected on the same bus. 

Master reads 1 byte from slave ( SCL = clock, SDA = data )

 The I2C protocol is more complex than that for RS232/485 in the previous post. The image above shows that a 4 byte transaction is required in order for the master to read in one byte of data from a slave, ( in this case Si5351 ). Compare to RS485 in the previous post.
The master is my usual micro-controller unit ( MCU ) development board which does have embedded I2C modules. I programmed it with my own firmware to enable and setup I2C to communicate with the Si5351 and OLED.

Top MCU, bottom L OLED, bottom R Si5351 board

Using I2C, I tested the OLED by turning the display on/off and switching between normal and reverse video. I configured the Si5351 on the purple coloured break-out board to generate two square-wave output signals, on my favourite frequency 137.7KHz in the 2190m amateur radio longwave band. See image immediately below.

Si5351 generates two separate output signals simultaneously

     

Adding a RS485 interface to an embedded control project

A serial connection to a pc can be used to send/receive data to/from a micro-controller unit ( MCU ) based project. Instead of using usb I have programmed a MCU's serial (E)USART ( (Enhanced) Universal Synchronous Asynchronous Receiver Transmitter ) peripheral and written my own pc software in the past for this purpose.
The USART has receive (RX), transmit (TX) and ground (GND) connections like the RS232 type serial COM ports on older pc's. For higher speeds or longer distances, however, RS485 is a better choice. [ note: RS = Recommended Standard ]. The MCUs and development boards which I use don't have RS485 interfaces. So I made one to test its capabilities. I used a 32MHz MCU clock. Therefore the maximum data rate possible ought to be 4Mbits/s ( 4Mbaud ).
As RS485 is a physical and electrical specification for the bus ( not a protocol ), the protocol used by the USART for RS485 can be the same "start-stop signalling"; i.e., one start bit ( always 0 ), 8 bits ( data byte ), one stop bit (always 1).

TX output from MCU USART. L-R: idle 1, start bit 0, 8 data bits, stop bit 1, idle 1

The image above shows, reading left-right, binary data bits 11001001, with the least significant bit ( LSB ) actually sent first from the left. Reversing the order to put the LSB in the normal position on the far right when writing the value down gives binary 10010011, which is also decimal 147. What really interested me was when I measured the duration of a data bit with the vertical cursor lines; 250nS, equivalent to a data rate of 4Mbaud.

MCU board with RS485 interface fitted - inset view from above

I made a RS485 interface on a daughter board which plugs into a MCU development board. As this uses 3.3V logic levels, ( 0 = 0V, 1 = 3.3V ), I chose the MAX3485ESA chip for the RS485 line transceiver. This chip has one receiver and one line driver; so works in half-duplex mode. Unlike RS232, the RS485 physical link is a twisted pair of wires. I already had 4.5 metres of this. The data signal I used was the voltage on the slider of a pot ( blue knob ), sampled by the MCU 10bit analogue-to-digital converter peripheral and sent to the USART. So. for example, the value 147 would equate to 474mV.

Terminal RX window showing data received from MCU at 2Mbit/s

This setup worked perfectly up to 2MBaud, as shown above, using my favourite terminal emulation software. Unfortunately the same virtual COM port could not be opened when I tried 4MBd ! In future experiments I will see if this limitation was due to the USB-RS485 converter ( not shown ) which I used to connect to a pc at the other end of the link.   

            

Software Defined Radio - RTL SDR

RTL.SDR hardware - connections for usb and antennas

SDR# software - tuned to VHF FM broadcast station on 105.6MHz

For several years usb 'dongles' for DVB-T reception have been available. The dongle consists of a tuner and down-converter. The frequency coverage of the tuner depends on the dongle variant, and typically from VHF to SHF. A common chip set is R820T tuner, RTL2832 down-converter and ADC ( analogue to digital converter, 8bit ). In-phase and Quadrature ( I - Q ) digital samples of the signal are sent at speeds up to 3.2MSamples/S over a USB2 connection for digital signal processing by a computer running appropriate software.
Using  a similar architecture, although a little different from the dongles in size and appearance, is the RTL SDR ( Software Defined Radio ), sometimes called RTL Downconverter. I recently bought one of this type online for GBP27 (  + free shipping + 10 week wait ) which has the new R820T2 tuner. This particularly interested me as the tuning range is continuous ( no gaps ) from 100KHz to 1.7GHz, ( LF-SHF ); so includes the amateur LF ( 136KHz ) and MF ( 475KHz ) bands. I plan to use the RTL SDR as a spectrum analyser to check some of the equipment I've built for these bands. I downloaded SDRSharp ( SDR# ) software, selected source RTL-SDR ( USB ), connected the supplied antenna and casually tuned and listened. The dynamic range seems to be about 60-65dB; not an impressive figure but should be enough for my purposes.
Further information is available from RTL-SDR.com and I downloaded SDR# from www.sdrsharp.com
There are also many video tutorials at www.youtube.com , e.g., Getting started with SDR# and an RTL SDR tuner - YouTube
An alternative software package which I haven't tried yet is 'SDR Console'.

Class C transmit amplifier for the 630m MF band

I continue to try different amplifiers in order to improve my station for weak signal digital modes, such as JT9 and WSPR, on the 475KHz, 630m band.

unboxed prototype amplifier with space left for antenna relay

The amplifier posted here is a medium power, and physically small, two IRF640N mosfet class C design. I designed the amplifier so that the 13degC/W heatsinks would provide adequate cooling for up to 50 watts output power when run from a 54Vdc power supply, while still retaining the compact size. With 2.5W drive power at the input, I measured 33W output power, ( gain 11.2dB ).  This was deliberate because at that power level, the optimum mosfet drain load is almost 50 Ohm; so I avoided the need for any output matching circuit. It will be interesting to see what I can achieve on digital modes now.

antenna change-over circuit added (lower-right)

Using it for the first time yesterday evening, my WSPR beacon transmission was received at a record distance 1655kms in France, and I was rewarded with a JT9 digital mode contact with Scotland at a similar distance.

 finished amplifier in home-made enclosure

 

3 February 2023

I received a request from a radio amateur in the USA ( Jeff, callsign WB8RJY ) for the circuit diagram, as he wanted to construct the amplifier. A few days later Jeff sent me several pictures of his completed amplifier, and a summary of his WSPR activity using the amplifier detailing some very long distance reception reports.

amplifier built by WB8RJY based on my design

 

 

 

 

 

 

Test jig for DDS module and Dongle

AD9850 DDS module on breakout board with interface dongle

CH1 sine wave output, CH2 comparator output +DUT 25%

This post is about using those cheap DDS, ( Direct Digital Synthesizer ), modules based on the AD9850 or AD9851 DDS chips, and found on auction sites for only a few dollars. I have been programming and using them for several years, going back as far as August 2013 ( see post 27/08/2013 ). I am now using the interface dongle, ( USB-SPI v1.00 ), and software from www.spectecs.com which makes the modules extremely quick and easy to use; the tuning word is created and uploaded to the DDS over USB with just a few mouse clicks.
The modules are ideal as a signal source for testing amplifiers etc or embedded in projects which is what I have done recently with the phasing exciter ( 02/11/2017 ) using an AD9851 DDS module on that occasion.
I made a simple jig, ( break-out board ), for speed and convenience when connecting the module for use as a stand-alone signal source. The SPI and output connectors are extended out to pin-strip headers. I also provided LEDs for testing the two GPOs from the dongle.
The module has two sine wave outputs, ( OUT1 & OUT2 ), two square wave outputs ( VO_P & VO_N ). Sine output OUT2 is present on the disconnected black jumper. Referring to the top picture, I've set the jumpers as follows:
J1 green, connected = serial data upload, ( disconnected = parallel data upload ).
J2 blue, connected = enable square wave output, ( disconnected = no square wave output ).
J3 yellow, connected = digital to analogue converter ( DAC ) full scale current set internally, ( disconnected = DAC current set externally ).
I uploaded a tuning word ( hex 010624DD ) for the DDS to generate an output signal at 500KHz. The duty cycle of the square wave is adjusted using the trim-pot next to J2. I found the range of adjustment to be about 12%-87%. The lower picture shows the signal on OUT1 ( 1V peak to peak ), and VO_P ( 5V peak to peak ), which I adjusted for 25% duty cycle.
Although the top picture features the AD9850 DDS module, the AD9851 module is identical in appearance apart from the chip type of course.  

10 watt amplifier for MF phasing exciter

Internal view, cover off - amplifier built inside old pc psu box

Bias circuit bottom left, antenna changeover circuit top right

In my last post, ( 2nd November ), I described the phasing exciter I made especially for data mode communications on the 630m 475KHz MF band. I have been using it for beacon transmissions and 2-way contacts. I tweeted news of one such contact using JT9 mode. All this has been achieved with an output power of just 100mW. For consistent results, however, particularly when propagation conditions are not favourable, ( e.g., long periods of fading ), more power would be beneficial. So I have built a single-ended class AB rf amplifier rated for 10W max output which is connected externally between the phasing exciter, low pass filter and antenna.
Once again I have used a metal box from an old pc power supply, complete with cooling fan, for the enclosure. A type 2SC3039 power transistor is fitted to a 5K/W heatsink directly in front of the fan. I have included a variable bias circuit, based on a 2SC2958 transistor; actually it's the same transistor type as the driver in the exciter as I had spares. I have used this to set the current for my chosen no signal operating point for the 2SC3039 for class AB operation.
The amplifier has made a huge improvement to the reception range of my WSPR signal; so far up to 1632kms.

Phasing exciter for the MF 630m band

DDS interface dongle left, antenna c/o top, main board below

antenna change-over circuit removed to reveal main circuit board

So that I can join in the fun of using weak signal data modes such as JT9, JT65, FT8 and WSPR on the 475KHz 630m band I have built a phasing exciter for a low power single-sideband transmitter. Audio input signal is generated by pc data mode software and sound card. The rf signal source is a cheap AD9851 DDS module bought at an online auction site. Its frequency of operation is changed using the usb dongle and interface software obtained from www.spectecs.com. The rf phase shifter is a dual J-K flip-flop chip, af phase shifter comprises two quad opamps, and the mixer is a dual 1to4 mux/demux fet bus switch.
To give the project a smart appearance I chose a Hammond type 1455N1201BK box for it. Even before the prototype was finished, I couldn't resist the temptation of connecting it to my inverted 'L' antenna and trying it out. The exciter on its own produces only 1mW ( 1 milli watt, 0dBm ); but this was sufficient for my WSPR beacon signal on 474.2KHz to be received with SNR -23dB at a distance of 11kms by a monitoring station.
The prototype was completed with the addition of a class A rf driver amplifier to increase the output power to 100mW, and an automatic rf sensed antenna change-over switching circuit. Since then my signal has been received in Norway, distance 1071kms, Estonia, distance 816kms and Germany, distance 700kms, which surprised me as the antenna's directionality favours the east. Sadly there don't seem to be any foreign receiving stations in the east at the moment.

An UN-UN transformer for my end-fed wire antenna

Prior to installation; coax cable ( left ), insulated wire ( right )

Construction detail - eye bolt is for installation purposes

Raised into position above house roof ridge

Method of installation and connection

For many years, my general purpose antenna for short wave has been a horizontal 46m long end-fed wire; see post dated 12 August 2011. However it had always been difficult to match it's feed-point impedance to my transceiver's 50 Ohm antenna connector on some bands using my station antenna tuning unit, ( ATU ); the impedance being too high. For those particular bands I would sometimes temporarily change the length of the wire; but that wasn't very convenient. So instead I have now made an UNbalanced to UNbalanced ( UN-UN ) impedance step-down transformer, placed it in a small plastic food storage box, connected it to the feed-point of the antenna and to the ATU with coax cable.
The transformer windings consist of 9 turns, trifilar wound, on a T200-2 iron powder toroid core, using insulated wire. The core is held in place with a nylon bolt and a small piece of acrylic. The slots shown in the picture are not required; it had been used for something else. The coax cable is secured inside the box with a 'P' clip. The windings were interconnected to produce a turns ratio secondary:primary of 3:1; hence impedance ratio 9:1. This had the desired effect of transforming the impedance to lower values within the tuning range of the ATU on all bands.
With this modification I converted my simple end-fed wire antenna from a multi-band antenna to an all-band antenna !

New mosfets for the dual band amplifier

Four IRFP360 mosfets mounted on heatsinks inside the dual band amplifier

In March I bought new mosfets for the dual band amplifier ( see post 6 December 2015), but only recently had the time to fit them. I always knew that the original IRF640 types were underrated when I started running the amplifier from a 54V power supply, and there were reliability issues with several of them failing with a loud bang. The new type I've now fitted is the IRFP360 which is a 400V mosfet. During the last couple of evenings I've been transmitting with the amplifier for long periods without any further mishaps occurring. I had to fit these mosfets with a different orientation from the IRF640 ( see previous amplifier images ) as the mounting hole is insulated ( no insulating collar required for the bolt ) and the drain connection was made to the centre pin, not the case. A mica insulator, however, was still necessary under each mosfet between it and the heatsink. The insulator required is slightly larger than the standard TO-220 size. To begin with I didn't have any suitable until I found that Farnell stock them, ( item code 520-214 ). The mosfet and data sheet can also be found at Farnell, ( item code 864-9359 ). 

Vacuum pen for pick 'n place

vacuum pen apparatus

pen and an assortment of nozzles

As I get older my hand is becoming too unsteady for using tweezers to pick and place certain types of surface mount components onto a pcb ready for reflow-soldering. I wanted to see if a vacuum pen would help.
I converted a really cheap aquarium compressor pump to work as a vacuum pump, by reversing the valve action. The 'pen' is a 5ml syringe; plunger not required. The 4mm bore tube for the air-line was passed through two rubber grommets and forced into the syringe. Hot-melt glue was applied inside the barrel of the syringe at the entry point of the tube to form another seal and reduce pipe movement. A small hole drilled into the syringe near the nozzle end makes a simple finger operated vacuum switch.

T-piece vacuum switch

For even better precision and control I subsequently fitted a T-piece for the vacuum switch, close to the pump, to be operated by my other hand. And I made another pen; of course with no hole. 

Half-wave filter for the MF amplifier

Two T200-26 iron powder toroids are used

It's called a half-wave filter because it functions like a half-wavelength of transmission line, ( e.g., coax cable ), regarding the terminating impedances at the design frequency of 475.5KHz. Using lumped circuit elements is a much more practical proposition than 200m of cable. It also has a low-pass frequency response; theoretically 2nd harmonic 951KHz -27dB, and 3rd harmonic 1426.5KHz -47.8dB. This is a useful amount of extra attenuation. So I built it for use with my MF amplifier, ( posted 6 December 2015 ), to provide additional suppression of harmonics in the transmitted signal. It is connected externally in-line with the amplifier output.  

Schmidt trigger input for the dual band amplifier

50% +DUT drive signal on the gate of one of the four MOSFETs

Schmidt trigger circuit installed

I have improved the amplifier efficiency by increasing the duty cycle ( +DUT ) of the drive signal on the MOSFETs' gates from 43% to 50% with the addition of a Schmidt trigger first stage. After experimenting with several CMOS NAND and inverter logic chips, ( i.e., CD4093BE, CD74HCT132E, SN74HC14N, SN74HCT14N ), which already have Schmidt trigger inputs, I found I could only obtain the 50% +DUT I wanted by making my own Schmidt trigger circuit using a quad 2-input NOR gate chip ( CD4001BCN or HEF4001BP ). I eventually decided to retain the older CD4001BCN in the circuit as the low pulse rate doesn't really justify using the newer HEF4001BP for this application. I built the circuit on a tiny piece of pad board and fitted it above the main driver circuit board. ( See also post dated 6 December 2015 ).

Dual band transmitter power amplifier for the LF ( 2190m ) and MF ( 630m ) bands

home made enclosure 25 x 24.5 x 12cm

band module for the 2190m band is shown installed

I've recently finished building another amplifier for my transmitting setup for the 2190m/136KHz and 630m/475KHz bands. It is a switching amplifier design based on Class 'E' topology, using four IRF640N  MOSFETs in parallel. To allow operation on both bands I constructed the output tuning and matching circuit for each band as a removable module; changing the frequency band of operation just requires installing the appropriate module. I have also fitted a RF voltage sensed automatic antenna changeover circuit.
In use the amplifier runs only slightly warm. Each MOSFET is mounted on a separate 4.4degC/W heat-sink, two cooling fans are running and there is ample ventilation. So my work on the thermal aspects of the design was worth the effort. By using 4 MOSFETs in parallel there is very little heat to be dissipated anyway as their combined 'on-resistance' is extremely low. Amplifier efficiency is about 83%.
I was very pleased to get a reception report of my signal on the 630m band from Bantry, south-west Ireland, ( distance 2085 kms ), as well as reports from Greece and Spain.
My future plans are to paint the front panel of the enclosure, and make a hinged top cover.
I can provide the circuit diagram on request by email.

Improvements to the 475KHz amplifier

Note the new, larger heat-sink and its position

Automatic antenna changeover circuit

I've made a couple of changes to the amplifier since I first reported on this project on 22 September 2014.
The mosfet has now been fitted to a larger ( 4.5degC/W ) heat-sink for better cooling, and repositioned to make device replacement much quicker and easier than before, should it again be necessary !
I've also added a circuit for 'rf voltage sensed' automatic antenna changeover switching which is a much more convenient arrangement than a manually operated  coax switch which I was using.

By the way, my 'countries worked' total on the 475KHz / 630m band has now risen to 14.