And even more.....

Well,

Following on from the basic DDS VFO I got going yesterday here:

http://g0mgx.blogspot.co.uk/2014/01/yet-more-ddsing.html

I've done a little bit more.

I started with the software package QUCS I mentioned back here:

http://g0mgx.blogspot.co.uk/2013/12/qucs-simulation-you-what.html

And entered the parameters for a 25MHz cut off low pass filter, the tool designed the filter for me and I edited it to enter real world component values:

I then simulated the filter and got this result:

So I made it, now, to get the inductors correct, I just use the mini ring core calculator and pick a known inductor, enter the inductance I want and it calculates the number of turns for me. As long as that isn't too few to be practical or too many to fit, I go with it:

And this is what it looks like built and attached to the output of my DDS:

The output now looks like this terminated in 50R at the 'scope:

and this is the view on the Spectrum Analyser:

So, all in all, looking much better than before. I even found the trimmer tool and tweaked the band pass filter I made from the designs on the G-QRP pages, it looks like this at the moment:

and here's the sweep of the filter:

Cat's not been helping much:

Good, though, egh?

QUCS Simulation - You What?

Well,

Been fiddling today with a new piece of software I have found called QUCS - the Quite Universal Circuit Simulator. It's a really neat open source project you can find here:

http://qucs.sourceforge.net/

There is lots and lots to this package, but my interest is in simulation of filter designs. Here, for example, is the design of a 9th Order Low Pass Chebyshev filter for a 4.2MHz target cut-off:

The package will design the filter for you based on your selected parameters but then you can tinker with the values to replace the theoretical calculated values of the components with real ones and then simulate the filter behaviour before and after. Here's the simulation of the filter above:

Now, I need to build the filter and see how close the simulation is to real world, but this looks like a very interesting package that I think will come in very handy indeed!

Good, egh?

Low Pass - Looks More Wobbly to me!

Well,

You may recall the basic ideas for a 4 and 6M linear that I discussed back here:

http://g0mgx.blogspot.co.uk/2013/11/now-that-smoke-has-settled.html

I've started to fiddle with the software and have most of the basics working. The software will just about control everything in the amplifier including cooling and alarms. The most important alarm will be the over drive - I suspect even with input attenuation I am going to find it very easy to overdrive the amp.

I started a few days ago to make the low pass filter. My first attempt looked kind of OK and used air cored inductors, but when I hooked it up to the spectrum analyser and tracking generator I didn't like what I saw at all:

The overall response wasn't very flat and at the target frequency I had quite a bit of attenuation. As a consequence and thinking it was my inductors that were suspect I ordered some T50-10 torroids as these would be good at 70MHz and also would create the required inductances with a reasonable number of turns.

The Low Pass Filter design has evolved a little bit and now looks like this in theory:

So after the T50-10s duly arrived a re-wound all the inductors only to find exactly the same result. Hmmm.

So, then I tried:

• Winding Air Cores with a smaller inside diameter
• Reducing the capacitor values and adding trimmers so I could fiddle with the filter
• Going for a curry

None of the above helped much, the results were still rather rubbish at my target frequency i.e. I had buckets of attenuation where I didn't want any.

Then I tried using some different test leads:

Conclusion? The test leads I was using are spectacularly rubbish at 70MHz - Aaarrrghhhh!!!!

I do have a slight bit of attenuation at my target frequency - the slope is starting just a little too soon, and I need to decrease the capacitors either side of the center inductor a tad.

So here's the filter as built right now:

So I can only conclude that the first attempt at the filter was fine, I just didn't know! The moral of this story is to always connect the test leads from the Tracking Generator output to the Spectrum Analyser input without the circuit under test in the way to ensure you have a flat line - do this first!

Here's the ham cat:

Annoying, egh?

A Pixie - Really?

Well,

Been fiddling recently with some very basic CW transceiver designs, most notably the classic QRP "Pixie" design.

This is a very basic CW transceiver comprising a Colpitts oscillator, a second stage which is an RF Amp in TX and a mixer in RX plus an Audio Amp. The basic schematic looks something like this:

I stole the image above from The free information Society:

http://www.freeinfosociety.com/electronics/schemview.php?id=1943

I've substituted the MPSH10 transistor in mine, but apart from that it's going to be the same circuit except for a different band.

To build one of these for a different band, it should be a case of changing the crystal frequency and also the output filter.I'd like to make one of these for the CW end of 20M; this led me to do some investigation into Low Pass output filters. Starting in the most excellent reference book Experimental Methods in RF Design, we can design a very simple Low Pass Filter like this:

Having constructed the filter above, it was clear that the roll off wasn't very sharp, so I did the maths for a 5 pole filter (above is a 3 pole). Here's the 5 poll filter constructed in a very experimental way:

I've hooked one side of the filter up to the Tracking Generator in the Spectrum Analyser and the other to the Analyser input, here's what I see:

Which means that when the basic oscillator in the Pixie is connected to this filter as its output low pass, we get this suppression of the output harmonics:

Which all looks rather good to me!

As an alternative to the "norm", here's a picture of Saki dog:

Fun, egh?