My Minima Progresses

Well,

Following a whistle-stop tour of the Middle East, I am back and have done some more with my Minima. After much deliberation and head scratching, I have decided to box the linear amplifier and Low Pass Filters separately. This way I can re-use the amp later for other projects should I decide to do so.

So here's the finished Minima:

You will see that I have used a DIN socket to take the BCD band selection information from the processor to the rear panel, this will allow me to automatically switch the LPFs in the linear as the radio tunes. I have also added a simple PTT line to the rear panel to switch the linear into TX.

Here's the linear and filters in a state of incompleteness:

So, the BCD output is configured in accordance with this table:

I've included 6 low pass filters with the frequencies listed in the column TX LPF above. So as the radio tunes when the frequency passes one of the thresholds the output binary value is set on the three lines I have attached to the DIN socket. The only thing to note is that the 19MHZ TX filters is engaged with BOTH the 15MHz and 30MHz TX filter. This has meant I have to use a simple diode matrix on the output of the 4028 CMOS chip to enable the same filter on two different outputs. So in the linear box we have the 4028 CMOS BCD converter plus a DIL package with darlington pairs to physically switch the relays.

If you need a schematic of the linear control wiring that takes the BCD signals from the processor and switches the LPFs email me and I will create one.

I've made a few modifications to the software, including the logic needed for my LPF switching in the linear. Rather than explain all the details I have simply uploaded my code here:

http://www.qsl.net/g/g0mgx//files/Minima_MJ_Final.ino

If you were to use the most excellent utility Beyond Compare to create a difference listing between my version and the original, you would be able to see the changes very clearly.

So, on with the linear....

Good, egh?

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?