I like the KISS approach to QRP and much prefer a crystal oscillator based rig. The possible need for a go anywhere MEPT when it comes to future activity on 28MHz spurred me to try a modular system using my si570 synthesizer for my Softrock SDR.
Not actually a DDS the si570 tends to get lumped in with DDS’s when talking about them so if I use the term don’t panic.
ABOVE The Softrock si570 Synthesizer (DDS) Module.
It is not possible to FSK the Softrock si570 oscillator without a computer connected, no good for me, the computer consumption will far outstrip that of the transmitter.
What it does do is generate any frequency up to 260MHz in 1Hz steps without the computer connected. The computer is used merely to set the frequency via USB it is then disconnected.
I am pretty bored with plain old QRSS FSK Morse anyway, so often it is impossible to identify ones own MEPT in the presence of QSB on a grabber despite it being on the correct frequency, no wonder reports do not come flooding in.
Far more identifiable is a simple unique ident or a whole load of different modes which no one else is duplicating and the addition of a burst of WSPR ident every so often.
ABOVE The Modular KISS DDS on 28.300MHz.
The solution to applying modulation is to modulate the Oscillator of a Mixer, I used the SA612. In this way the setting up of the deviation for accurate WSPR steps is done only once.
ABOVE The SA612 Mixer/Modulator Module.
Just one Mixer/Modulator Module is used to cover 1.8MHz to 144Mhz with a c. 9MHz IF Frequency. Any old Xtal frequency will do in the range as the Output Frequency is merely I.F. Frequency + DDS Frequency. For 1.8 and 3.5 MHz it is – DDS due to the lower frequency limits of the si570. 7.0 and 10 MHz would benefit from a 3-4 MHz XTal, VHF could use 33MHz the Crystal is plug in and so fully flexible.
I have tested the Mixer over the full 1.8MHz to 144MHz range, only the 28MHz HiQ Filter/Preamp has been constructed as I write.
ABOVE The HiQ Filter/Preamp Module for 28MHz.
One HiQ Filter/Preamp Module will be required per band. The transistors in the Preamp stage are so cheap they may as well be duplicated for each band. Thus for VHF bands higher frequency devices can be used.
Regarding the Power Amplifier. A Broad Band PA could be used, this would require a BPF for each band and some form of switching. Just like the Preamp the devices are so cheap that including the PA device on the BPF Module is a reasonable choice thus avoiding any changeover other than plugging in the Module.
ABOVE PA/BPF Module 28MHz.
Spot the PA device, it is so small, a surface mount 74HC240 for 50mW Output into 50 Ohms, you can’t tell it is there. It is the same arrangement that I have run on 10MHz 24/7 for a year.
ABOVE The precision timed PIC Modulator Module.
This Module produces 5 WSPR based frequency shifts c. 1.4Hz steps to produce any modulation mode that can be achieved with 5 units. I still like this system as the 5th unit is ‘free’ produced by a simple little trick I used. The De-Multiplexer Enable puts the 4 outputs into an all high state, another set level pot gives me another bit (unit) of information to play with.
ABOVE The Modules connected together on test on 28MHz.
This system allows me to experiment with any individual module and try new ideas for an on going MEPT project for all bands up to 2m. Modules will slide into a standard slotted plastic box for use as an MEPT for a particular band.
There are no plans for a posh enclosure, that would put a halt on its experimental status.
No Digital read out, that would use current that donates nothing to the signal. The ‘compromise’ of using a stand alone DDS has meant more than a doubling of the MEPT power consumption
No switches and knobs etc. Modules are connected together as needed. Now where can I get some nice little sockets to fit the square gold pins I use on the boards, I have run out.