Follow up to the USB - TTL UART

Docklight running

In the post dated 31 July 2013 about the USB-TTL UART interface, I described how I tested the interface using my PIC development board. A much simpler method requiring only the circuitry with the FT232RL chip is a loopback test arranged by connecting together the TTL TX and RX lines. Using the serial comms analysis software called "Docklight" a complete sequence can be set up before sending. I also configured Docklight to repeat the transmit sequence every 5 seconds; in image above TX data red, RX data green, timestamp and comments yellow.

Reflow soldering - Part (1) Oven trials

Reflow soldering is a way to solder all surface mount components onto a circuit board simultaneously. I would find this very useful, and as I don't want to buy an industrial reflow soldering system, I considered a DIY approach.
The first step was to choose a mini electric convection oven to provide the necessary heat. A new Adler model AD6003, 1000 watt, 9 litre oven was purchased for only $35.

Adler AD6003 "toaster-oven", cover removed

The guarantee was immediately invalidated by removing its cover to expose the thermostat and mechanical 60min timer/bell, and then disabling both of them. The next task was to check that the temperature would at least reach the solder reflow point of about 217C for Pb/Sn solder. With the oven switched to maximum power, ( upper and lower heating elements ), this temperature could be exceeded by a sufficient margin. To obtain accurate measurements a thermocouple, temperature data logger and a laptop running a control program written in Visual Basic were used.
After adding some electronics to operate a solid state relay connected to the heating elements, several plots of temperature versus time during heating up at different power duty cycles were obtained. Ultimately, the aim is to achieve, to a close approximation, the idealised heating and cooling profile shown below.

Idealised thermal profile

This project is still a 'work-in-progress'. A Part (2) will be posted later.

Analog Devices AD9850 frequency synthesiser

AD9850 evaluation board, 45mm x 26mm

Connected to MYDEV2 PIC MSSP module for programming

SINA and QP outputs

Another visit to an online auction site and another electronics purchase. This time I spent $9 on an evaluation board for the AD9850 frequency synthesiser chip. Surely the 125MHz 'can' oscillator and the chip itself are individually worth more than that. However it was made in China.
I mounted it on a larger piece of experimenter board and connected its programming inputs to a microcontroller PIC18F4550 on my MYDEV2 PIC development board. Before the AD9850 will produce an output signal it has to be programmed.
So I wrote a few lines of code to use the PIC's Master Synchronous Serial Peripheral ,( MSSP ), interface module to send the 40 bits of frequency, phase and control data to the AD9850.
The resultant output signals are a sine wave ( CH1 yellow trace ) of 1.04V peak-peak directly from the chip's digital-analogue convertor, ( DAC ), and a variable pulse-width square wave ( CH2 blue trace ) of 5V peak-peak via the chip's comparator for use as an external clock.
I have intentionally allowed plenty of space on the experimenter board to fit a dedicated PIC later; probably the PIC18F14K22 as I already have one.
The AD9850 will be a useful signal source and clock generator upto about 40MHz.

USB - TTL UART

FT232RL board, USB left, TTL right

My descriptor embedded in the FT232RL

Serial comms setup

I had been looking for a usb interface solution for a piece of home-made gear when an old friend, who is a professional electronics engineer, reminded me of FTDI's range of interface chips. He even gave me one to play with; the 28 pin FT232RL. It's a USB-TTL UART device. So I did some experiments to practise using it.
First I made a circuit to use it as a usb powered device. After installing the drivers on a pc, I could see another usb(com) port had been detected. Then with a few more components added to the circuit I used it in my intended application as a self-powered usb device.
I programmed the user area of the chip's eeprom with my application data and some configuration options, e.g., allocating a couple of pins as outputs for LEDs. I was pleased to see that the same data I had just programmed in now appeared in the usb connection properties window.
Confirmation that the circuit was functioning correctly came when the red and green LEDs flashed in response to sent and received data between the PIC EUSART on MYDEV2 development board and the host pc running serial communications software.

More details of my low power transmitting setup for longwave

Since the posts on 22nd February and 8th May, I have received requests to post more information on the setup I used for my low power test transmissions on the longwave 2190m band.

The circuit schematic and pcb artwork for the AF amplifier are shown above; click on the images to expand them. The original size of the artwork is 70 x 100mm. The pcb is single-sided; top component layer, bottom copper layer. Anyone wishing to copy my pcb design might need to modify the tracks connecting T1, depending on the actual transformer which is available and the windings used.

A +18V dc power supply can be used for greater output power. I didn't try this only because I don't have a convenient way of providing that voltage, and also the fan is a 12V unit.

My very low power transmissions on longwave

Last night I made successful radio test transmissions on 137.7KHz, 2190m band, using only 3.5W transmitter power. My signal was received, ( screen capture below ), at a distance of 17Km. The signal strength suggests that 2-way communication at this power level would be possible over a much longer distance. The vertical streaks are probably static crashes as a thunder storm was active in the vicinity.

My setup was my own-design PIC controlled DDS and the TDA2030 AF amplifier featured on 22 February. 

It is unfortunate that amateur radio activity on the 2190m band is so low, as it is possible to enjoy communicating on this band with a minimal setup, as I have just shown.  

Cheap watts on 70MHz

A visit to an on-line auction site on 22nd October 2012 resulted in my purchasing, ( for about $18 ), an ex-commercial equipment radio frequency linear amplifier covering the band 60MHz - 80MHz, and probably capable of producing an output power of at least 100 watts. Very useful, I thought, for the amateur 70MHz VHF band.
I have no clue about the manufacturer. With the amplifier, however, came a much simplified hand-drawn circuit schematic with some notes in Russian.

To make it operational I mounted it inside a box, provided 24V, 12V and nominal 6V supplies, status LEDs, antenna changeover switching and a 7-pole Chebychev low-pass filter on the output.

 How smart it looks and it works too. What a bargain !

Experimental low power amplifier for 2190m longwave

I salvaged some potentially useful parts from a faulty pc power supply, e.g. bridge rectifier, schottky diodes, heatsink, fan, chokes, transformers. The 12V-0-12V, 5V-0-5V output transformer typically operates near 40KHz. I thought of using it for the output matching transformer in a low power transmit amplifier for the 136KHz, 2190m longwave band.
My design is based on the very cheap, ( half a $ ), TDA2030 class AB audio amplifier ic, which has a bandwidth of 140KHz.
The circuit is experimental. I was curious to find out if such an amplifier would be useful for 136KHz, despite using some untypical, possibly 'unsuitable', components.

I built the amplifier on a home-made printed circuit board, 70 x 100mm. The ex-pc transformer, ( yellow & black ), is on the left. The TDA2030 is mounted on the ex-pc heatsink. ( Pcb artwork and the circuit schematic are available from me on request ).

Fitting the circuit board inside the old pc power supply box, ( cover not shown ), with its original 12V fan, and adding a LED, rf and dc connectors, completed the construction.

For testing, I powered the amplifier from a +13.6Vdc power supply and connected the input to my frequency synthesiser tuned to 137.8KHz. With the input attenuation set to minimum, and the output terminated in a 50 Ohm load, the measured voltage gain was 41.75dB. Output power was 3.5W.
I could now either connect the amplifier directly to my longwave antenna and make some very low power test transmissions, or use it as an intermediate amplifier stage in a much more powerful transmitter, yet to be built.

Chirp-Hell on longwave

On 14th November 2012 I reported success with the tests on the work-bench of the improved ssb phasing exciter for my 2190m longwave transmitter. Soon after that I installed it inside the transmitter enclosure. Since then I had been waiting for an opportunity to test it using full transmitter power into my antenna in a real 'on-air' situation, with a more distant receiver. 

So early this morning at about 1.00am I carried out transmission tests with Jacek, SQ5BPF, in Warsaw. My signal was quite readable on his grabber; screen captures above. The noisy band conditions at his location are also very evident.

I was transmitting on 136.9KHz upper sideband, modulating with 800-810Hz chirp-hellschreiber audio tones. Transmission speed was either 5 or 10 secs/character; the vertical markers are 1min apart. We then completed a chirp-hell to qrss1 cross-mode contact; quite obscure, so it's probably the first time ever it has been done !

New antenna for 80 metres goes up

Surprisingly, the weather recently has been suitable for me to work on antennas. So even this late in the year I had an opportunity, not to be missed, to erect a new antenna for the 80m band; a base-loaded vertical wire element supported by a Spiderbeam heavy duty 12m long telescopic fibre-glass pole. The pictures show the antenna base with loading and matching coil inside the storage container, and the antenna's location.
I have just made my first contact on the 80m band using this antenna with a radio amateur in southern Spain, 2463kms away. My transmitter output power was only 35 watts. The antenna's performance could be worse !

Improved phasing exciter

I have completed some improvements to the single-sideband phasing exciter, ( post 25 January 2010 ), for my longwave transmitter, just in time for use during the good propagation conditions on the 2190m band over the winter.
The passive phase shift network was not producing accurate 0 and 90 degree phase shifted AF. In fact it was well outside specification; perhaps not surprising as I bought it in 1978. So I replaced it with an active circuit based on a dual op-amp. I optimised the unwanted ( lower ) sideband suppression at 800Hz. There is little point anyway in achieving exact quadrature audio channels over the entire speech band as the modes in use on longwave are extremely narrow-band.
I added a tuned class-A post-mixer transistor amplifier stage.
Upper sideband is now selected automatically as I have disabled the sideband switching facility; until such time when I see a lower sideband signal on the band.
The top picture is my signal, received on 137.7KHz, from just the exciter sitting on the workbench, ( lower pic ), when transmitting using chirped multi-tone Hellschreiber mode. As can be seen, the signal occupies only about 5Hz of band !

PC-to-PIC serial comms

I have been programming the EUSART module of a Microchip PIC18F4550 microcontroller on my MYDEV2 development board for RS-232 serial  communications with a pc running a terminal application; in my case 'HyperTerminal'. I have configured the connection for 1200 baud asynchronous operation. Although this is one of the slowest of the standard com port baud rates, however in conjunction with the 8MHz PIC clock it gives the lowest baud rate error of only 0.1%.

The connection is called PC-PIC_EUSART. Pressing any key 'wakes up' the EUSART and starts the sequence. The above capture of the Hyperterminal screen shows, for example, the steps taken to change the dfcw dot-dash frequency shift to 4Hz. All seems to be error-free !

Now that I have the connection working, I am considering incorporating serial comms functionality into my PIC controlled frequency synthesiser, so that values, e.g., relating to priority band and dfcw shift, which are held in PIC data eeprom, can be changed quickly in real time without having to modify, build and reload the entire source code file.

PC-to-transceiver audio interface

Sometimes there are quick and easy construction projects on my long 'to do' list, e.g., a pc sound-system to transceiver interface. I have one already, ( just visible in the picture posted on 14th July 2012 ); but with the arrival in the shack of pc number 5 another interface would be convenient. The existing one incorporates transceiver mic switching, mic gain and speaker. The one I have just completed is basic, containing only the bare essentials of the two line isolation transformers.
The transformers provide the path for audio without allowing any dc connection which is the preferable situation when connecting together these two valuable pieces of equipment; radio transceiver and computer.
I obtain these transformers from discarded telephones and modem cards. Using temporary connections, the windings and taps which give the best results on both transmit and receive can be identified. In the finished interface the transformers are orientated for minimum interaction.
Currently I have two spare transformers, sufficient for another interface; one from a telephone, and the other yet to be removed from a modem.

Trap and match

I have completed the 'L-match' network and re-checked the resonant frequency of the trap, 1821KHz; both are now ready to be installed with the new antenna, ( last posting ).
I made a very important improvement to the water-proofing of the original trap, ( posting dated 8th April 2010 ), by fitting end-caps. These are translucent polythene and originally the tops from curry paste jars. I was delighted to discover that they are a tight clip-on fit on the 90mm diameter former. I had carried out beforehand the traditional microwave-oven loss test on the material which passed. I will now look for similar jar tops for my other coax traps.
I must hurry to finish the antenna as a Canadian radio amateur is eager to contact me on the 160m band.

Straight up

Today I finished installing the vertical section of a new 'trapped inverted-L' antenna for the 160 metre and 2190 metre bands to the final height of 17.8m. It is constructed from 4m lengths of aluminium tubes of various diameters giving it a taper from 45mm diameter at the base to 26mm at the top.
I have been making ground-mounted vertical antenna elements for low frequencies for many years and in my experience 18 metres is about the maximum height for this form of construction using light material, as well as being at the limit of what one person can erect. Higher than this and the construction can quickly become uncontrollable during lifting, resulting in disaster. However, I still have about 2m in reserve should I feel bold enough one day to try to increase the height still further.
Prior to erecting the vertical part, I had already connected a 15.8m top wire. I now have to fit the trap and an additional 15m of wire to an anchor point on a 12m pole about 30m away.
The storage box just visible at the antenna base will contain the L-type 'L-C' matching network to 50 Ohm coax cable feedline for operation on the 160m band.
Thunderstorms are forecast here tomorrow; an early survival test !

Two generations

The early MKI ( lower ) and very recent MKII ( upper ) embedded control frequency synthesisers are both resting on top of my longwave transmitter. I will interface the MKII with the transmitter as that was always one of my intended applications. The MKI will now be used as an item of test equipment on the workbench to provide an lf signal source.

Poland now on the 4 metre band

A new band, 70.1 - 70.3MHz, was officially released to radio amateurs in Poland at 00:00CET today. I have been prepared for this moment for the last 5 years, and was ready and waiting. I made radio contacts using morse code, voice and data; the latter being in FSK441 mode using meteor-scatter with Enrico, callsign IK0BZY, in Italy at a distance of 1295 kms, as can be seen in the partial screen capture from WSJT9.

Keypad handler

Today I finished writing the PIC code in 'C' language for handling the operation of a typical matrix keypad, as mentioned in the previous posting. Characters 0-9, *, # can be keyed-in and displayed one at a time.
The purpose of this activity is to pave the way towards my ultimate goal which is to enter frequency tuning data via a keypad instead of a rotary encoder, in an alternative version of the embedded control frequency synthesiser. At this later stage, the * and # buttons will be assigned to 'clear/backspace' and 'enter'.
For now, however, further development has to be put on-hold.

Gradually moving forward

Having a lot of spare time recently has enabled me to progress with some unfinished PIC MCU related activities. For the first time I have been writing code in 'C' language and programming PIC MCUs using Microchip's MPLABX IDE and ICD3. So far I can write characters to a display, and also detect when any button on a particular row of a keypad has been pushed; all quite encouraging. But I still have much more programming to do in order to complete even the quite basic operation of keying-in and displaying numerical data.
I have also been experimenting with software, ( assembly language in this case ), to detect disconnecting the power source from a PIC microcontroller, ( PIC18LF4455 ). A variable low voltage power supply would be very useful here.
And finally, from a most unexpected source, I have obtained cut and engraved black plexi front and rear panels for the frequency synthesiser enclosure.

MyDev2 enhanced

Schematic - note Vcc connection depends on usb-powered or self-powered application

I have upgraded to version 3 my PIC MCU development platform, MyDev2, ( first posted on November 2nd, 2010 ), with the addition of pull-down resistors R25-R28 for the keypad rows connections now that I have decided on the method I will use in coding the operation of the keypad and the interfacing required to the microcontroller. I have already started writing the code in 'C' programming language, instead of the 'Assembly' language which I have used for all my PIC projects until now. The keypad will be a 'telephone' style 4 row x 3 column, 0-9, # and * type.
I have also fitted a new microcontroller type, Microchip part PIC18LF4455-I/P, which, as well as being a few cents cheaper than the original PIC18F4550 and PIC18F4685, has an extended operating voltage range, ( denoted by the 'L' in the part number ), down to +2V, as I want to experiment with coding the detection of switching off the power !