I tried and I tried. I tried to use Linux for my shack PC for ages, and it was always a battle. Sure, more often then not I got there, but a lot of time was wasted messing with computers rather than radios.
Well, I've completed my SoftRock Lite II for 40m. It may have taken me some six months to start building it, but it was pretty straight forward in the end. A couple of hours a night for a couple of nights and all was done. It was my first time doing SMD components, and I was surprised at the success I had. No need for all the procrastination. It's also the first receiver and the first amateur radio kit I've built – well, aside from a crystal radio kit. 😉
Of the various types of antennas I've built, I've never built a magnetic loop. It always seems like there's a bit of magic involved and relies on hard to get suitable air spaced variable capacitors.
Thanks to Dale (VK1DSH) I now have a PCB version of my oscillator. Further, thanks to his input it also now has a few additional items to the circuit. It has a diode on the +ve supply input to protect against reverse polarity, etc. as well as better use of capacitors for the voltage regulator and to provide better bypassing.
Last night I had a bit more of a play with my osciallator. I wanted to look at a few things:
- The RF on the supply rail;
- The use of an Rd capacitor in the oscillator circuit; and
For RF on the supply rail I was reminded by both Dale (VK1DSH) and Dimitris (VK1SV) about the use of a bypass capacitor on the Vcc pins for ICs. I say reminded as I remember reading about these when I was playing with microcontrollers briefly, but had completely forgotten.
Being a best practice, I decided to follow suit and add them. So the slighly revised schematic is now – note C6 and C7:
On the use of Rd in oscillator circuit, maybe it’d help if I first show what I’m talking about. In the below is a red box where often an Rd resistor is added:
In the datasheet for the 74HC4060N they suggest trying a 2k2 resistor, but for my crystal this stopped oscillation. Indeed, testing last night showed 1k5 stopped oscillation and only once I went to 1k did it kick off. However, I found at 1k the waveform was a distorted sine wave and only once I got it down around 100R did things resume a reasonable sine wave. So in the end, I decided I’d do what many others before me have, and continue to leave it out.
But, an interesting exercise none the less, and first real good opportunity to play with my new resistance wheel.
Finally, I also wanted to see if this thing was stable. Not an overly detailed test, just something indicative. So, I hooked it up to the scope and ran it for just shy of two hours on the bench. In that time, the frequency counter on my ‘scope did not budge from 137.500kHz. So, at least to an accuracy of 1Hz I can say it seems pretty damn stable. 😉
Recently I was frustrated with the amount of time it took to get some simple 2.205 MHz and 2.185 MHz crystals from Futurlec – in the end it took some 8 weeks to only get a small number of what I was after (and only with a bit of poking and prodding). This is not uncommon with Futurlec as you’ll find if you search the ‘net, but the thing is there are very few suppliers of such options and these were needed as I was trying to build something like the VK1SV MEPT transmitter for 2200m. Indeed, to build a crystal based oscillator for 2200m there are very few options, so that made it more frustrating.
- 74HC4060N 14-stage binary ripple counter with internal oscillator
- 74HC4017N decade counter
- 22 MHz Crystal
The idea here is simple, the 74HC4060N first provides a 22 MHz Inverter Oscillator with the 22 MHz crystal. This is then used as it’s input for which we derive the ouput on the Q3 pin to achieve a divide by 16. This giving us a frequency of 1.375 MHz. This is then feed into the 74HC4017N so that we can further divide it by 10 which gives us the final frequency of 137.5 kHz. (A total operation of 22 MHz divided by 160.)
The only other bit of the circuit is the power supply. This is contoled with a 78L05 to provide 5V regulated as required by the 74HC chips. This should be noted, as these are not the plain CMOS chips (CD series) as these would not be able to work at the 22MHz clock frequency – normally only workable up to 12 MHz, but in some cases only 1 MHz. As a result, they require a Vcc of 2 to 6 volts.
Going back to the 22 MHz oscillator in the circuit, a couple of notes. The 4 ~ 40pf trimmer capacitor with the 100pF capacitor are to provide the required crystal load capacitance: CL = (C3 x C4) / (C3 + C4) + Cs. With those values – and using an arbitrary stray capacitance (Cs) of 5pF – it will cover a CL range of 8pF to 38pF – that should cover most crystals (mine required 18pF). Also of note in the inverter oscillator circuit is the missing Rd resistor (or Rs depending on your reference) to limit current to the crystal. With most circuits I see online this is ommited, but I think it could be interesting to try and figure out what this value shold be – as there are benefits. Initially I tried with 2k2 and that was far too large and meant no oscillation. But from calculations I guess you could try 75R, but I’m yet to experiment.
The other item to note is that I have observed there is a significant 22 MHz signal on the +ve rail (almost 1.5 Vpp). The above mentioned Rd resistor could help, but more so a decoupling network should ideally be added to the supply rail. Something I might consider if I have time to play with this circuit a bit more.
Anyway, even with these considerations this circuit works. Further, it also seems stable – although I’ve only run it for a few minutes and with no load. Oh, and don’t forget, the output from this is a square wave – so use appropriately. Here’s the output on my scope:
All the parts you require can be fetched from element14, however if for some reason you struggle with acquiring the 22 MHz crystal there’s good news on that. Another reliable local supplier stocks them: Mini-Kits. And if needs be, I see that Futurlec have a 22 MHz can oscillator for just under $2.
Another option on the crystal front, is to consider using an 11 MHz crystal. But then the circuit is slightly different as you need to divide by 80. You could do this with two 74HC4017N chips (the first could divde by 8, the second by 10), however you’d also need to build the 22 MHz oscillator on it’s own to feed into it. But something to consider.
Anyway, here’s some photos from my resulting prototype:
Finally, I’d like to send a thanks out to Dimitris (VK1SV). It was only a month or so back I had no idea about how you go about frequency division. I knew basically about frequency multipliers, but hadn’t even heard of frequency division. With his kind introduction to this theory I was able to produce the above and understand how it worked. Further, doing so also made me spend time to better understand inverter oscillators.
Thanks Dimitris! I may just get on 2200m one day soon.
The week-end just past saw the running of the annual Remembrance Day Contest. I was only aware it was on a couple of days before hand, but decided I'd at least throw out a few calls from home.