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eHam.net Forum : VHFUHF : test vhf ssb linearity

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test vhf ssb linearity	Reply
by WB5PDW on September 4, 2006	Mail this to a friend!
I have a 100 watt homebrew vhf linear made from a GE masterII PA board. I shorted out the transistors and bias diodes by connecting it to cheap power supply. I have replaced the diodes and transistors and checked the static bias voltage. I get reports of slightly distorted audio. What test equipment do I need to check this amp?

RE: test vhf ssb linearity	Reply
by WB2WIK on September 4, 2006	Mail this to a friend!
I'm not sure what you're actually doing there, since the MASTRII started out life as a Class C FM amplifier. Sounds like you've modified it to add some biasing, in hopes of making it linear. What is the idling current (zero signal emitter or collector current)? What is it for each transistor, independently? What did you do to stabilize the operating point over temperature? If you want to check linearity, all you need is a wattmeter, dummy load and way to bypass the amplifier. Or you can use two wattmeters and a dummy load, and you don't need to bypass the amplifier. Measure power output vs. drive power starting at a very low level like 1-2 W output power, and increasing drive until the maximum output power is achieved. Plot output vs. input on a linear graph. It should be a straight line from min drive power up to a point where the line angle bends and is no longer straight. The power achieved while the line's still perfectly straight is linear. The power achieved after the line starts to bend isn't. I've never "linearized" a MASTRII amplifier so I'm not sure what it can achieve in linear operation; in Class C, it easily achieved 100W output power. In linear service, probably less. WB2WIK/6

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