It is probably a layout or decoupling issue. You have a feedback path from the output to the input. It might be a power supply decoupling issue. Try driving the output stage from a seperate power supply as a quick test.
Howard
Quoting Mike <msmith@...>:
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Problem with my scratch-built uBitx. Can¡¯t get more than about 4W PEP on 40m, and 1W PEP on 20m without having what I think is the PA breaking into oscillation.
SYMPTOMS:
SSB: When I key the PTT (with MIC amp disabled),?I start seeing RF output at the antenna as I increase the drive level.?What¡¯s even more peculiar, the output frequency as shown on my scope is about 4.2mHz higher than my operating frequency.
CW: With increased drive, the frequency at the antenna as shown on the scope shifts about 4.2mhz higher *as soon as I let up on the key*. The amplitude also slightly rises, and there is more of a delay until the TX relay de-energizes. With key down, oscilloscope displays?correct frequency.
I realize the frequency measurement on oscilloscopes (in general) is not highly reliable, but should be in-the-ballpark. I can clearly observe this frequency shift with eyes alone.
*At lower drive levels ( 1 to 4 watts output, CW and SSB) things seem to work OK*. RF output looks about as clean as can be expected, and frequency measurement on the scope is approximate to operating frequency.
So, the ten-cent question: What the heck is?causing this? Has me baffled.
Side note: My?build is on double-sided board. Layout is virtually identical to the commercial ubitx kit.
-Mike
