LT Spice For Radio Amateurs: PART 6

So anyone who is not living in the 1850’s would have noticed a few years ago a revolution in signal generation with the advent of cheap micro controllers and modules in the way of the Arduino AD9850 DDS VFO. Now cheap and highly accurate signal sources were available to the masses, but they did have some limitations. Firstly the DDS output has a 200 ohms impedance, secondly its output was low, in around 300mv p-p and thirdly its output gain was not uniform across the entire frequency range and if it was being used as a local oscillator feeding a diode ring mixer the output needed to be amplified and buffered.

Alas poor Yorick, the internet came to everyone’s rescue with some eager beaver grabbing a handful of parts and some popcorn transistors and knocking up a buffer amplifier to add to the DDS to bring its output up to ear destroying levels, well -7dbm needed to drive the input of a diode mixer. But, was it any good? We now have enough peanuts in the brain box to test this thing and see if it was really all that up to scratch.

First lets run a transient analysis and take a look at the waveform. Its not very sinusoidal now is it. It is clipping hard on the negative rail and well when any form of clipping happens we make fart noises AKA harmonics. So here is a new trick to add to your LT Spice bag of tricks, we can look at the FFT output and see the harmonic content.

RIGHT CLICK the waveform window, select VIEW->FFT and you will get a nice frequency analysis of the harmonic content, we can see that the 2nd and 3rd harmonic are about -20db down on the fundamental. This might be important in your design and if this was the FFT display of a final amplifier you should be hitting panic buttons because the Law is generally the 2nd harmonic needs to be -50db down on the fundamental. Check with your local authority to be sure to be sure because a clean signal is a nice signal. However, with a DDS buffer, this may or may not be a problem in your design.

Next we can perform an AC Analysis on buffer amp and see its gain over a range of frequencies. We can see there is a large rolloff in gain from about 10Mhz and onward and by 28Mhz we have lost almost -3db gain. Now this might be enough to stop your diode ring mixer from turning on if your -7dbm signal is now more like -10dbm, the mixer will not mix, and that might suck bad for your circuit.

So now you know enough to make simulations on amplifiers and buffer stages to see if they actually do what you want them to and if you are really cleaver you will now be thinking of ways of optimizing this circuit to make it work better, like by rebiasing Q1 by adding a low value resistor from emitter to ground so that its not clipping the negative rail, by adding a low pass filter maybe to clean up the harmonics and by optimizing component values to get a flatter gain response. What you do will depend on your actual needs and implementation. Either way have fun.


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