Simplifying the VFO

The last few days have seen an annoying failure of the (clone) Arduino NANO that I was using to control the "Two DDS" RF generator system for the new SSB rig.

I ordered a new NANO and received it promptly (having paid an extra 20 pence for the privilege of first-class post) and got the system up-and-running again.

However, I also got some ATmega328 micro-controllers in 28 pin DIP packages, so I decided to rip out the Arduino and replace it with just the micro...

First, I had to program the chip - so I put it into a simple configuration emulating the circuit of my Wotduino and programmed via a USB to Serial device. The programming task was trivial, as the chip had been supplied with the Arduino bootloader already in place.

Once the device was programmed, I could pop it into the RF Generator system with only a minimal amount of re-wiring - in fact, all that was needed was a 16MHz xtal, and a couple of passives.

Notice how the ATmega328 in the DIP package is nearly as big as the Nano !

Everything was working fine...

so I was able to get on with some more operating.

I enjoyed a nice QSO with Nicola, iz1lei, on 20m, who was in Cuneo near Torino (my best dx to date with the new rig) and I had a great contact on 40m with the gb3rs station at the National Radio Centre, Bletchley Park, operated by Henry, m0zae...

Henry gave me what he emphasized to be a genuine 5 and 9 report which delighted me - this little rig really does work!

...-.- de m0xpd

BITX Developments

Things have been moving on quite nicely with the little SSB rig over the past few days (when the pressures of work allow me to steal a few moments for leisure projects!).

First, I've switched away from the initial "development" configuration of a stack of Kanga/m0xpd DDS shields for the RF generation (BFO and VFO) to just the DDS modules themselves. They are mounted, at the moment, on a breadboard - along with an Arduino NANO and a little PSU.

The idea behind the NANO is that - eventually - I'll be moving to just an AVR microcontroller on a board with the DDS modules and the Nano puts us closer to that end-point in terms of physical size! All the parts will be available from Kanga UK if anybody wishes to follow in my footsteps (and the code will be published as freeware - see below).

Here's the RF generator in its original embodiment...

The digital RF generation scheme allows easy switch not only between bands but also between LSB and USB modes - requiring a change in the BFO frequency. To test this, I made up a pair of plug-in filter modules for 20m...

Both the low-pass and band-pass filters are built to standard G-QRP "recipes"; the LPF is described here, whilst the BPF is in SPRAT (v154, p25).

The 20m conversion was a piece of cake - just plug in the filters and modify the code to switch over all the frequencies required in the transition between lower- and upper-sideband operation.

The next development added a relay-based change-over from receive to transmit (I'd used a switch in the original build last week as a temporary measure). The switch remains as one controlling input to the relay system - but I also have a parallel input for a second (/third/fourth) switch (the Rx/Tx selection is a logical "OR" of any parallel switches) AND a TTL level input, such that (one day) a micro can control the whole system...

Also visible in the picture above is a new power supply for the 5V elements (the DDS modules, the Arduino and the display) - all implemented by a 7805 regulator on the main heatsink.

Given the additional Rx/Tx switching flexibility, I added a push-to-talk switch on my toy "fist" microphone...

Why - this is almost like a "real" radio HI HI.

Next, attention turned back to the RF generator system - I knocked up a mounting for the rotary encoder to make tuning a little easier...

The two push-buttons allowed me to cobble together a new version of the "Kanga Enhanced VFO System" suitable for this new Double DDS scheme with band-change via menu, automatic selection of LSB / USB as the 10MHz boundary is crossed, Rx/Tx detection (via an input to the Arduino from the power switching on the main board) which adds Receive Incremental Tuning, etc...

Here's a close-up of the display...

I'm gong to be working up the code into something suitable for publication - so please be patient!

Here's the entire rig smeared across the bench in my habitual style...

I've had fun building it but - I must confess - I've also enjoyed operating !!

Lots of FB QSOs in the UK and into Echo India, best dx so far into Germany, but the first prize in terms of operator satisfaction goes to my QSO with the RNARS London Group's gb2rn station on HMS Belfast...

"OK, I'll hand it back to you, from MIKE ZERO X-RAY PAPA DELTA"

Rising to the challenge

Bill Meara, n2cqr, has been hugely supportive of my recent fun-and-games with digital systems - but he's also been goading me...

"I have encouraged Paul to take a break from the microcontrollers and build something discrete and analog... like a BITX!"

Soldersmoke Daily News, 28 Sept 2013

Well - I've taken Bill's advice...

Above you see the (IRF510-based) PA, the LPF and BPF filters (both arranged as plug-ins - see below) and some other lovely analog, discrete stuff. 

However, try as I might to please Bill, I couldn't resist sprinkling a little "digital" into the project...

Whilst I've taken direct inspiration from Farhan's words and music for most of the transceiver, I've put together a new digital "RF generator" system, which uses two DDS modules to generate BFO and VFO signals...

This approach - I argue - mixes the best of both worlds... 

You get all the fun of "conventional" radio homebrewing and you supplement it with the stability, controllability and simplicity of Direct Digital Synthesis for generating the RF signals. Exactly the "Occam's Microcontroller" thesis - but taken to the next level with SSB. 

Here's the main board in close-up...

The band-pass filter is the ordinary G-QRP circuit (SPRAT 154, p 25), which I had knocking around on the bench as a plug-in module I'd developed for the "Occam's" rigs. This was a 40m filter and it was this - more than anything else - which dictated that this will be a BITX 40. I preserved the "plug-in" format in the output low-pass filter, so now I just need to knock up different filters and adjust the "RF Generator" to change band (try doing that with analog RF generation).

I'm calling it an "RF Generator" for want of a better name - it is far more than a VFO, because it generates the BFO signal and automatically handles the "interlocking" between VFO, BFO and "dial frequency". This makes aligning the whole system a different kettle of fish than would be the case with old-fashioned (sorry!) analog oscillators.

The whole shebang reached operational status today - so I had some nice QSOs with Steve, m5stc, Brian, gw1uju and Ellis, who was operating the gm100rsgb station.

Brian put his finger on the button - "your audio sounds a bit stifled". He's right - my crystal filter (which was thrown together with zero thought) is too narrow. I'll re-work it and try to get my audio quality up where it ought to be.

I'm going to be publishing the Double DDS "RF Generator" code when I work it up into something worthy for your attention. Now, at least, I have an SSB platform with which to work - all thanks to Farhan and (of course) to Bill.

...-.- de MIKE ZERO X-RAY PAPA DELTA

Google Chrome debases...

There is, one supposes, somebody at Google who thinks they have their finger on the pulse. They might even believe their facile messages

are witty and useful.

I do not agree. "Aw, Snap!" means less than nothing to me. Worse, it is possessed of none of the attractions which might make me learn what it means and consider owning and using it.

Better the blunt clarity of IE10's "This page can't be displayed' or the simple explanation "Safari can't find the server".

Is there, I wonder, a Google Chrome regional preference for those who prefer to speak English (as opposed to some descending, homogenised, empty babble)?

I would ask Google, but fear I would not enjoy the answer's language.

...-.- de m0xpd

The Well-Tempered Beacon

After the disappointments of so-called "Real-Time" Clocks, I was pleased when the postman delivered a GPS module today...

I had ordered the module from Hans, g0upl, who offers them at a fantastic price (an important consideration for a cheapskate like me).

Thanks to the excellent TinyGPS library, the module was supremely easy to interface to my Arduino-based beacon. I deliberately used the Arduino MEGA with its lavish resources to exploit an additional hardware serial interface (not wanting to get tangled up with the "UART" used for connection with the PC during the development phase) but this will run on the basic Occam's Beacon hardware with a humble little UNO.

Here's the system "on the bench" (actually, on top of a Leslie speaker - the only vacant, nearby flat surface)...

Nearby, on the cill of a north-facing window, sits the little module, powered by 5V from the Arduino and sending back, in return, 9600 baud NMEA data...

The whole shebang works like a dream - just turn on the beacon and it starts in perfect time. No more clock watching and reset pressing. O frabjous day! Callooh! Callay!'

The beacon is doing its perfectly a tempo thing on 30m even as we speak. I'll post the code once I get it tidied up and ready for your enjoyment.

...-.- de m0xpd