Ramp generator I

2011/8/22

After experiment Simple Man's SA sweeper and Alan's constant current version ramp generator( lm358 versatile ramp generator)  it's time to built a worth able boxed unit for further experimental circuit to be construct: VFO, frequency sweeper, even a SA.

The final prototype  board seems not very clean, but smaller, All component is SMD, except POT to panel. 3  jumpers connect separated GND together, single layer board.

C3 is much important, don't use a 0.1uF, use a large one, Tantalum capacitor is prefer, i use a 47uF Tantalum capacitor, the yellow big capacitor in above image. small capacitance make the op amp oscillation.

another key component is C4, for linearity, the following 2 image show the linearity for 2 type capacitor. left wave form show a electrolytic which lead to distortion at changing start. after use a SMD(might be monolithic) CAP, linearity is much better, shown in right.  

The following photo is example of 2 sweep result (by Hartley sweep generator ),  10.7Mhz ceramic filter,   7 Mhz crystal.  The  pitfall is pretty obvious: X axis's  right end more bright than rest of it.

That is because scope AC couple the X sweep signal.  Then AC couple  severely distort the sawtooth wave by x1 probe, ramp raising speed reduce at high voltage.   X10 Probe works much better.

Alternatively,  we can  use DC couple.  X scan is hard to adjust , because the ramp output not start form 0V.  A better way to address this problem is using another lm358,  get a copy of  U3G$1, then everything gonna be good.