2m Ver X2A Transmitter Discussion

[NZ0I]

The 2m transmitter now seems to be operational. I am measuring 1W into a 50-ohm dummy load. Rough signal purity (using FFT applied to signal by oscilloscope) measures the first harmonic -30dB below the carrier. An overmodulated AM signal, or a CW signal are currently possible. Some tweaking will be required, but a fairly clean AM signal can probably be achieved.

By adjusting the voltage applied to the final output transistor the power level can be varied between 100mW and 1W. The transmitter might be capable of doing a bit more than 1W, perhaps as high as 1.5W, but I notice the final transistor getting a bit warm after a long key down, so I haven't tried to push it higher than 1W.

The BLT50 transistor has been discontinued/obsoleted by NXP, so I have substituted the BLT81 instead. The BLT81 has more gain and a higher operating frequency than the BLT50, but runs at a lower voltage. For 1W output a collector voltage of 6.4V is required.

[NZ0I]

Interestingly, with zero volts supplied to the collector of the 2m final output transistor, there is still about 15 mW of power being delivered to the dummy load. That's power from the driver stage feeding through the final amplifier which is serving as an attenuator.