toggle quoted messageShow quoted text
The uBitx as currently drawn gets by with just two output filters. For example, a 40m low pass filter also gets used when transmitting on 80m. For that to be legal, the second harmonic of your 80m signal had better be awfully low coming out of the IRF510's, since the 40m low pass filter isn't going to do anything about it.
That's possible to do with the push-pull IRF510 arrangement of the uBitx. But requires a spectrum analyzer, and you would have to keep PA voltage and modulation levels in check, and consider effects of aging and temperature around those IRF510's. Farhan has backed away from that strategy, the final design will likely have more output filters, enough to address the second harmonic at each freq of interest. Like every other rig built since about 1925 that had any sense of responsibility.
I applaud Farhan for trying, don't get me wrong. These rigs have more functionality per part than anything that has come before, and it is by trying stuff like this that you move in that direction. Unfortunately, you simply need more than the two output low pass filters shown to get good repeatable results across all those bands. If I scratch build a uBitx, the low pass filter will probably be a lump in the coax out to the antenna.
On Tue, Jul 25, 2017 at 07:33 am, Craig Wadsworth wrote:
If I understood Ashhar correctly, a spectrum analyzer will be needed to balance the uBitX's output amplifiers.