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I wanted to check a UHF linear amplifier’s capacity to ship a low distortion AM sign output. Sadly, the output distortion requirement was higher than the distortion limits of any business AM sign supply I might discover. It was essential to improvise an AM sign supply appropriate for the aim.
I wanted a supply of low distortion AM to feed the amplifier’s enter however each sign generator I might discover (Hewlett-Packard, Fluke, Anritsu, and so on.) had AM output distortion specs that had been worse than the distortion requirement that the amplifier itself was supposed to satisfy. To make a supply whose AM distortion was low sufficient for take a look at functions, I needed to gin one thing up alongside the next strains proven in Determine 1.
Determine 1 Block diagram for an improvised AM sign supply. Supply: John Dunn
The plan was to take a double balanced, ring diode mixer and inject a service and a modulating audio sign into that to create a double-sideband (DSB) mixer output sign with a suppressed service. I wanted to comply with that with a bought business amplifier to make up for mixer loss. I then added the DSB sign to the unique service to create the required AM sign having the service and the 2 AM sidebands.
This labored simply advantageous.
As an example the sign processing rules nevertheless, a SPICE mannequin can be utilized the place the double balanced mixer is represented by a four-quadrant multiplier (Determine 2 and Determine 3).
Determine 2 SPICE simulation of the designed AM sign supply delivering 50% modulation. Supply: John Dunn
Determine 3 SPICE simulation of the designed AM sign supply delivering 100% modulation. Supply: John Dunn
One factor to remember is that you really want zero section shift between the unique service sign and the reinjected service sign on the summing level. With no section shift in any respect, we are able to broaden the time scale (proven in Determine 4) and have a look at the properly even timing of the zero crossings of the AM waveform which is what we wish.
Determine 4 Even zero crossing timing of the AM waveform with no section shift. Supply: John Dunn
Nevertheless, as proven in Determine 5, If the reinserted service is section shifted with respect to the unique service, we get some negative effects.
Determine 5 Zero crossing timing with 90° section shift with respect to the unique service displaying negative effects. Supply: John Dunn
The section shift of 90° in Determine 5 is after all fairly excessive, however for demonstration functions, going to that excessive makes it straightforward to see how badness arises.
Determine 5 exhibits that the zero crossings are not timed the identical which suggests that there’s now undesirable section modulation (proven in Determine 6) of the sign output and degradation of the amplitude modulation impact we’d been looking for.
Determine 6 Undesirable envelope degradation of the sign output with 90° section shift. Supply: John Dunn
After we return to our earlier time scale, we additionally see with the 90° section shift that the AM envelope has been degraded as nicely. Placing our section angle again to zero levels once more, we see very good AM sign technology all the way in which as much as 100% modulation (Determine 7).
Determine 7 AM sign supply delivering 100% modulation with 0° section shift. Supply: John Dunn
Hold the section, child.
John Dunn is an electronics marketing consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York College (MSEE).
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