Electronic Experimentation (Spiralling Off . . .)

I wish now to relate something of the pleasure which can be experienced whilst carrying out simple experiments.

The Nutty Professor is at it again ! ! !

As many who experiment with home construction would no doubt relate, one thing quite often leads to another, which in turn leads to another, and so on (or to misquote a line from the "Commitments" movie - they go "spiralling off").

It's a wonderful thing on the whole, but one of the frustrations with it is that one never seems to get anything quite finished!

Readers of earlier blog entries will have spotted references to crystal controlled transmitters and receivers for use with the QRSS aspect of the amateur radio hobby.

Successful (one hesitates to say "the best" as this implies some sort of competition, and I don't think QRSS folk are particularly competitive) QRSS stations, whether simple or complex have to be well engineered.

Sloppy engineering practice soon becomes evident, manifesting itself in unreliability or in poor performance, and the poor performance I am thinking of in particular is that of frequency stability, or rather the lack of it.

My own home made efforts have exhibited acceptable, if not outstanding stability, and one quickly looks to ways of improving performance.

For some time now I have been pondering this issue, and a chance visit to a rather interesting web site produced a nicely written article explaining how to make a simple crystal heater
.
Thermostatic control of the temperature of the crystal used to produce the QRSS signal, or local oscillator in the case of a receiver seems to be a fairly obvious thing to do, and I had already looked at a commercial offering which offers high stability for a few pounds, and is a microcontroller based solution.

The heater to which I am referring though is a much simpler affair using a darlington transistor to modulate the flow of current through some low value resistors (the heating elements), with a thermistor to provide regulation and feedback.

This struck me as a brilliantly simple idea, so I resolved to build something similar if not quite identical.

With the exception of the thermistor, all parts came out of my junk box, in fact the darlington transistor was concocted from a small power transistor rescued from some long-forgotten project (the leads had been cut and had traces of old solder on them) and a low power Japanese 2SC something-or-other freshly removed from a scrap board which was probably a transistor radio or portable TV 20-odd years ago. Total cost of parts, probably quite a bit less than a pound.

The crystal being thermally controlled (heated) is physically but not electrically connected to a heated piece of thin tinplate (formerly part of a screening can also rescued from a scrap board).

When connected up to a power supply, my little heater worked first time (it is so simple it could hardly fail), and after some experimentation with the settings I had it heated up to just under 40 deg C, and was able to adjust the crystal FREQUENCY (the crystal was electrically connected to a completely separate test oscillator) by tweaking the temperature of the heater. The heater once it reached equilibrium maintained its set temperature perfectly.


My test set up involved monitoring the current drawn by the circuit, and the collector voltage of the transistor.

Pretty it ain't - interesting it certainly is!

Inspection of the circuit shows that they go in opposite directions - when the collector is high, the current drawn (and therefore the power being dissipated in the heater resistors) is low, and vice versa. If the surroundings change, or the circuit is heated up or cooled down deliberately the circuitry compensates for the changes very quickly thus restoring equilibrium.

One quickly realises that this little heater is really trying in its own small way to regulate the temperature of the
whole planet, and for it to be as economical as possible, it needs to be thermally insulated as much as possible from the outside world, and a little more thought reveals that in a way this little circuit is a miniature representation of, say, a domestic dwelling where the occupants try to maintain a comfortable temperature inside when the weather may be up to all sorts of tricks on the outside.

The heater schematic

Fascinating.

Also, as I have an interest in the application of microcontrollers, this application is also, as stated above, one where a microcontroller could be used, but this is where one needs to think about cost. Even buying new components, the unit cost of my little device would be no more than a pound, and the cheapest microcontrollers cost in the region of 75 pence per unit, so by the time you've bolted any additional components required, the simple version probably wins out, but only just. Die hard microcontroller enthusiasts would probably dispute that assertion!
Either way the mere consideration of one approach versus the other illustrates perfectly how experimentation can lead one through all sorts of interesting thought processes, which is really the whole point of me writing this note.

The Mad Mad World of Microcontrollers

It's been a while since my last blog, and that has been bugging me to the point where I feel I have to "tick the box" and "blog SOMETHING", so here it is.

The last entry was made shortly after I'd "discovered" the Arduino system, and this discovery has let me spiralling off into all sorts of strange directions, all associated with microcontrollers, one way or another.

In the meantime I've bought what I think is a rather good book called "The Quintessential PIC Microcontroller" by Sid Katzen which deals with the subject manner in a way which resonates with the way I approach things.   Recommending books is a dangerous business, rather like recommending restaurants or wines or other things which are extremely subjective in nature, so all I will say is that I think it's rather good and leave it at that.

I've also acquired a few bits and bobs associated with Atmel AVR microcontrollers (an Atmel AVR is at the heart of the Arduino), namely a "TinyUSB" programmer, a few low-end microcontrollers, an LCD panel, and I've just downloaded the AVR Studio IDE package from the Atmel web site.

Buying expensive text books apart (I only have a couple of microcontroller books, honestly!) the best way to "learn microcontroller" is by doing stuff, so that has now become the prime objective.

I should relate the story where my eyes were opened about the desirability of using microcontrollers.  This was when I was participating in a Radio Club project to scratchbuild a 70MHz transceiver (The Eden 9 Project) and had got to the point where working IF strips were emerging from our group of constructors.

The IF strip had been deliberately designed without an AGC system, but with the facility to add AGC later, and a prototype audio-derived AGC system was duly demonstrate.

Now some people are rather "sniffy" about audio-derived AGC - I may or may not have been one such sceptic, I couldn't possibly comment! - but this demonstration was remarkable!   Not only was the performance of the circuit so impressive, but the parts count (and cost) was so low!  

Of course, as I am sure you have worked out already, there was a PIC microcontroller at the heart of this little add-on board, and when one stops to consider how the "all analogue" alternative might have been designed then it doesn't take long to realise that the alternative would be more complex, costly and less flexible.   With a microcontroller prototype, just changing a few numbers inside the embedded program is the way performance is tweaked.  To cap it all the PIC at the heart of this unit retails at around a pound at time. Good value, or what?

All I need now is the time, willpower and inspiration to get down and learn the ropes properly.

A New (Old) Direct Conversion Receiver

For a while now I've been participating in the "QRSS" aspect of the hobby, mainly by streaming my "grabber" or sometimes "grabbers" (I can run two simultaneously if I am feeling keen!).

Each of these grabbers ties up a radio and a PC.   As you may have realised I have quite a number of each of these commodities, a ridiculous number, if you were to ask my wife, but being brought up in post-rationing Yorkshire (though not of such Noble birth), I always felt that tying up a sophisticated multi-band multi-mode transceiver just to stream a Spectrum Laboratory grab onto the Internet was "overkill".

I've been pondering for some time knocking up a direct conversion receiver for, say, 10MHz which happens to be the most popular band with QRSS enthusiast.  I've built a few of DC receivers over the years.  They tend to be fairly "minimalist" devices, and yesterday it struck me in a flash that I have already got, gathering dust on a shelf, a 14MHz single-band receiver built back in the 1980s which would potentially fit the bill.

The receiver in question was dubbed the "RC-14", and was a project featured in Rad Com back in 1987, described as a "beginners receiver".   When built at the time it worked reasonably well, and I reworked it slightly to fit it into on of those ubiquitous die cast boxes as the one which came with the Cirkit (remember them?) kit was rather flimsy to say the least.  In my opinion, the nicest feature of that particular design by Steve Price GW4BWE was the AF stage which featured a nice two stage active low pass filter'  The performance of this filter meant that the radio sounded like a "proper" SSB receiver, when the received signal was strong and in the clear.

It hasn't had a great deal of use since then and it occurred to me yesterday that it would be relatively straightforward to "wavechange" it to the 30m band.

QRSS stations tend to operate around a single 200Hz channel either side of 10.140MHz, and just below the WSPR segment.   In  view of this, simply moving the built-in 14MHz VFO to 10.1MHz would probably not be the best thing to do.  Instead a fixed oscillator on the right frequency would fit the bill, or maybe a "VXO" to give a little more operational flexibility.

Now I haven't got a suitable 10.14MHz - ish crystal to hand at the moment, but as proof of concept I have temporarily modified the radio to accept an external oscillator, and used a synthesised signal generator as that external oscillator tuned to 10.1387MHz instead.   This puts the wanted frequency of 10.14 plus or minus right in the middle of the receiver pass band.  A slight tweak to the receiver's input tuned circuit was all that was needed to get the receiver up and running, and in this way I've been streaming this new (old) grabber receiver onto the Internet all afternoon with satisfactory results.

I am now wondering at the practicability of using this principle as the basis for a two or possibly three band dedicated QRSS receiver.  And so another project is born!

Voltage Probe Antenna - Part 2

This weekend saw a family visit (wife, grown-up children, and in-laws) trekking across the Pennines to the Sunderland Air Show, and a Jolly Good Day Out was had by all.
As with the Windermere Air Show we attended a couple of years ago, the only flying Avro Vulcan XH558 once again stole the show.  What a stunning aircraft, and what an impressive noise!

A fly-past with the bomb bay open

Today, in between watching the Hungarian Grand Prix and listening to the Second Test against India at Trent Bridge, I managed to get the prototype Voltage Probe Antenna completed and fired up for a quick test.

It certainly works, and in the brief period of time I had to test it before I packed up for the weekend, I had received amateur signals at good strength on all bands from 160m to 15m ,with the exception of 60m.

First VPA lash-up

I also had broadcast signals on Long Wave and Medium Wave at decent strength.  Next week I hope to carry out a more extensive evaluation, and naturally I will probably report back here.

This first version doesn't have the required circuitry to be powered from the output coax cable, this will follow in due course, no doubt.

A new project - the Voltage Probe Antenna

About this time last year I paid a visit to an old radio amateur friend whilst away on holiday, and he was enthusing at the time about a little "Active Aerial" he had built very quickly more or less from parts in his junk box.   This little aerial was about the size of a beer mat and was suspended from a piece of shelving, and was connected to his FT-817.   All kinds of stuff was being received, most impressive.

This was to a design by PA0RDT.

Moving forward to more recent times, in fact last weekend, I noticed from EMail traffic that our local radio club, the Eden Valley Radio Society were possibly embarking on another club project, and an "Active Aerial" was one of the contenders.   Ron, G4GXO (proprietor of Cumbria Designs) had come up with a circuit in part inspired by the PA0RDT design and it was mooted that this might be the basis of such a project.

Naturally my previous experience caused my ears to prick up, and after some correspondence with Ron I now have a new project - to build this little circuit and evaluate it.

In terms of complexity it is completely different from the "Eden9" project, in that an experienced constructor should be able to build it more or less as a "weekend project".   Looking at the modest parts requirement I realised that had all that was needed available and resolved to give it a go this weekend.

However, as is the way of the world, outside influences intervened, a major influence being the unexpectedly fine weather we were blessed with - cloudless blue skies and pleasant temperatures, so in the end I only got chance to get this project started, hopefully to be completed next weekend..

The aerial including the amplifier is all contained on the single piece of PCB - the photo below shows the project getting under way before I tidied the garden shed up to placate my XYL who insists on doing "things horticultural which I don't understand" in it whilst I am not around!