2007-01-24
1 Watt DSB Transmitter
I combined the LO with a newly constructed single-balanced modulator and a microphone amplifier, and also with the Wee Willy power amplifier I've been playing with. The result is a well-behaved but rather inefficient 1 Watt DSB transmitter.
The microphone amplifier has an emitter-follower "final" stage to drive the low-impedance modulator. However, it has limited power it can deliver before distorting because I didn't want to stand excessive currents in it. Consequently the amount of RF from the LO it will let through to the TX power chain for amplification is AF-drive limited. Basically, the audio drive is too low to get the full power available out. It just peaks at 1 Watt, despite the amplifier being capable of a bit more than that. Ideally I want to be able to overdrive the TX power chain at maximum AF gain without AF distortion, but of course be able to back off the drive for actual use.
I did try an AF matching transformer but the results were not especially encouraging. I'd need to completely redesign the microphone amplifier to make proper use of it, and its 8 Ohms to 1 K impedance doesn't lend itself to the circuit I had in mind, not at this Vcc anyway.
A post-modulator RF amplifier would solve this "problem". But as the LO delivers +21 dBm this would be an extremely wasteful - a brute force fix. I may choose this option, but if I do, I will replace the LO with something less powerful, there is no need for anything much over 6 dBm if the AF amp can't make use of it.
The circuit as-is works pretty well. I plugged in my old Leson desk mic and recorded myself. It sounds quite nice, especially considering I set the HF roll-off to about 2.3 kHz in the mic amplifier. Then again, maybe I just goofed calculating the filter - I'll have to sweep it and check.
I'm seriously considering turning the +21 dBm LO into a CW beacon. It is just such a nice little circuit, it doesn't deserve to be put inside a larger rig, it should have a life of its own. A 2N3053 class-C amplifier would work quite nicely with it, putting out over 1 Watt, but I'll need to find TO-39 heatsinks first - so perhaps I'll use a BD-139 instead.
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Parent article: "2007 80m Homebrew Challenge".