JBoT - A linear for Minima

Well,

In an attempt to finish up my Minima tranciever, I have today been making the linear amplifier to follow the gubbins we have so far.

The Author has used this:

http://www.hfsignals.org/index.php/File:Qrp-linear.png

and I saw no reason not to follow suit.

Here's what the linear looks like right now:

Mine is built with 2N3053s (because I had some). I had a spot of bother getting the correct output from T2, this should produce 2 equal amplitude but out of phase signals. I eventually managed to make something that seemed to work, the output of T2 looks like this:

and the output of the final transformer after the 4 finals looks like this into a dummy load:

So, I make 56V peak to peak about 7.8 watts, so I suspect I am driving the amplifier a bit hard with my sig gen and attenuator set up. I also think I have a bit of a tendency for the finals to oscillate.

So, we will have to see. The next bit I need to do is understand the band selection outputs from the Minima software - I am sure these are designed to allow me to switch LPFs after the linear, which I need to do as the linear will have to be followed by suitable filters to remove any harmonic content.

Good progress though, egh?  

Minima - Remaking the 15MHz LPF

Well,

Now that I have some life in my Minima, I need to remake the 15MHz Low Pass Filer. You may remember the sweep I did originally:

There is a bit too much attenuation at the 14-15MHz frequency range - this will result in a very deaf radio on the 20M band!

So I used the QUCS tool to design an 11th Order filter with a cut off at 15MHz, I then changed the capacitor values to "real world" numbers and ended up with this:

I then made the filter and after winding each toroid I measured the value and carefully spaced the turns to get as close to the required value as I could.

The physical construction looks like this - all the toroids are T50-6s:

and when I sweep the filter on the Spectrum Analyser I see this:

So this looks much more like what we need - I will use this and replace the one in the radio right now.

Good, egh?

My Minima is Alive and Well!

Well,

Following a quick trip to Stockholm, I have resumed my building and testing of my Minima transceiver. I like to take things in a kind of build a bit, test a bit way and have been stepping through the RX and checking as I go.

I didn't actually find anything amiss, so I connected a wire antenna and tuned down to the 40M band.

Here's a quick video of some of the things I heard:

Just fantastic, egh?

And at that point - it ground to a halt!

Well,

Here's how my Minima is looking on the bench right now:

I've left just enough room between the front mounted processor and associated parts and the boards on the base to fit a vertical RF screen if I find I need it:

And here is the back of the case with a reset switch, DC inlet, antenna socket and connector for the USB serial adaptor to connect.

Unfortunately I have to return to working for a living tomorrow and that means Stockholm this week, so progress will now have to halt for a while.

I hope to have enough room on the base of the case to the rear for the PA and also the right hand wall will be used for the output filters. I have made the RS232 interface and that's on the back wall of the case.

I will finish this when I return....

Watch this space.

Minima Processor Board

Well,

Following on from my case building yesterday, I have been assembling a board to contain the processor and associated components. I have simply made this using stripboard so that it will mount on the back of the LCD display:

And this board fits inside the front panel like this:

I included a 5V regulator on the board and also a LM317 to regulate the 5V down to 3V3 for the 570 which I am proposing to mount on a small copper board next to the 1/4" jack socket. I also now need to build the RS232 interface as I am definitely going to want to be able to reprogram the processor without removing it from the socket.

Good, egh? 

Boxing to the Minima

Well,

Now that I have the beginnings of a working project, I need to start to think about how I am going to box the resultant transceiver. So today, to add to the mess on  my bench, I have created a box out of sheet aluminium and the front panel is loaded with the LCD, volume (with on/off switch), mic and headphone socket, the tuning control plus the function push button:

So the back of the front panel (if you get my drift) looks like this:

I have decided not to include the reset switch on the front panel as I will possibly mount that on the back, although I think power off and then on will have much the same effect!

I am thinking that the main board with the majority of the gubbins will fit on the bottom of the box, the LPFs and crystal filter plus KISS mixer on the rear panel vertically and I hope that will leave enough space for a PA plus some band pass filters.

The next job will be to mount the microprocessor and the 570 to the rear of the front panel - perhaps I will make something out of stripboard for the processor and mount it on the holes in the LCD.

We shall see.

Our two latest additions Elmo and Darley have been playing "who can run round the garden the fastest" today and are now just about fit to drop:

All good, egh?

Minima - Full Circuit TX - Take II

Well,

Following on from my last post, having looked at this spectrum plot:

We needed to understand what the RF peaks on the left were; there seems to be one at 5.something MHz and another at 8.something.

The first thing I did was throw the cable I was using to connect the audio generator to the mic amp in the bin - it was clearly a pile of dingos kidneys. I then connected the audio sig gen to the 'scope and set it for about 40mV peak to peak at 1.2 (ish) KHz.

I then repeated the sweep above:

So, this tells us a few things - primarily that these RF peaks are related to the audio and the drive level - I was probably overdriving the audio amp in the earlier scan.

Now, if I then focus in on the peak we can still see at 5.something MHz and exaggerate the problem by increasing the audio drive I see this:

Now, these multiple peaks look very suspiciously like they are spaced by the audio frequency to me, so I increased the audio sig gen frequency to 3.5KHz:

I can reduce the amplitude of these peaks by setting the balance pot on the bias for the KISS mixer - but this is about as low as they will go.

This will all mean something to those far cleverer than I, but I actually suspect that there isn't really a problem as long as the audio levels are sensible.

What do you recon, fun, egh?