LT Spice For Radio Amateurs: PART 5

All radios are made up of some rather standard blocks we use over and over, we have filters, amplifiers, mixers, that is is, no rocket surgery required. A product detector is not someone searching for a product to buy, its just a mixer, a beat frequency oscillator is nothing more than an amplifier with positive feedback to make it go bonkers in a controlled manner and filters, well they just filter out the crap we do not want.

Sure this is an over simplification, but it is near enough to being the truth. I know for myself, once i got my head around the nomenclature of radio and started to see things for what they really are, that things started to make sense to me. So in the next few parts, lets take a look at some of the specific things we can do in LT Spice to get a better understanding of the performance of our circuit under test, starting with filters.

So this is a filter I have used in a number of projects before. I think we have all used someone else’s design without really knowing if that design is any good or not. So, now that we can used LT Spice, we can see for ourselves if someone else’s design is actually any good, or if their design is rubbish. And when we look around the net we see these kinds of blocks being copied over and over and this assume that its good, if might just be that everyone is copying the same bad design.

So if you are paying attention to the above schematic you will notice some things different. Previously we have performed Transient Analysis, and now we are doing something different, performing an AC Analysis which will allow us to sweep the circuit under test with a range of frequencies and get a Bode Plot at the end to look at.

We set up our voltage source with an AC Amplitude of 1 and of course 50R series resistance as the filter is 50 ohms impedance in and out.

Next click SIMULATE->Edit Simulation Command and select the AC Analysis tab. Change the type of sweep to Octave, and add in the other details, for the number of sample points, start and stop frequency. Click ok, then run the simulation and let the fun begin.

Now, if you click on the trace number n004 in my case, you can add upto 2 cursors per trace and move them about to to take some measurements. Cursor 1 is showing the center frequency of the filter, 7.150MHz and its at -6db, cursor 2 is at 12MHz and its -54db, 54 minus 6 is 48db DOWN on the bandpass. Which is good information to be able to work out, you can then tell how far attenuated the Image frequency of your mixing scheme is, the IF freq etc etc to see if your filter is good enough for your task.

Now we have only done this with a single Bandpass filter, you can also do the same with the Low Pass Filters on your transmitters and do the same math to work out how great the attenuation is of the 2nd and 3rd harmonics. A clean signal is a good signal. But I will leave that up to you to try. And now you know enough about how to use LT Spice to perform AC Analysis and to design, align and specify the properties of filters. Get out there and have fun, test your designs and make sure your homebrew radios present the cleanest signal they can.

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