Ramp generator II

Ramp source and control knobs

@2013/10/6

Alan's lab present a good discrete  Discrete Sawtooth Oscillators,  after carefully study Alan's circuits, i change a little bit, get more suitable to use as oscillator for a fully functional ramp signal source.

 Compare to opamp sawtooth generator,  this one has a wide sweep range:Vee-1.5 to Vcc-1.5. this make it  no need to amplify it to full rail sweep. The  555 sawthooth generator  sweep from 1/3 VCC to 2/3 VCC.   The negative output for X axis  is really good feature, which enable the DC couple to oscilloscope, eliminate AC couple distortion( AC couple use a coax, lack of compensation, coax capacitance leading to a bad sawtooth). 

Testing And Debugging

in general speaking, this circuit works very well at a 'special' quiescent  work point.  see the picture , it's good enough. 

change T2,T3 to a configurable Unijunction Transistor may help a lot:

Now come to signal process part. U2, the LM358 work as a buffer,  then  a divider to given a +- 5V output for 1V/div  of oscilloscope. U3 make a span control buffer, nothing special.

U1 is the centering control. to get better precise control, use a buffer for centring pot, but for fine tune, there is no buffer.  U1 have another task: adding +9V to the sawtooth wave, convert it from (-8V +8v) to (0-16V). 

wired thing happen while positive rail go up above 15V, the lm358 seem's introduce high frequency spur oscillating,  add C5, C14 could filter it out. 

following photo show the effect of AC coupled and DC couple:

right photo is ,  distortion obviously,

More @2013/9/10

add an offset resistor to the centre, now output from 1V to 16.5V. most of  varactors linearity is bad at 0-0.5V.

 Power supply Board and  Case /Panel

Most of control function is in the ramp board, i finished it. so it's time to got a case put all of them in it and design a panel.  i utilized a box of a old bench power, the extra award, a powerful AC transformer.

let's us look the great part, pictures of the  HW:

Why i need another cases for this

this case is not suitable for sweeper, it's internal space is relatively small, it's a cube, but we prefer it's be a flat cube, not so high.  and all my other instrument is another shape, i.e, this one: Couter: EP frequncy counter.  I finally get a similar, and even better cases for hold sweeper signal source and even could integrated the Power Meter.

Full Ramp Signal Schematic

sawtooth generator include T1 T2 T3.

1.  C4 must be a low leak capacitor, the green capacitor works very well.

2.  The SYMMETRIC pot actually is not very useful actually, if R1+ SYMMETRIC > 2k  the output get severely distortion. to make it fully symmetric, use a adjustable negative rail supply. [NOTE: negative power rail not fully address the problem]

3.  SPEED control work fine, but don't try to get it too fast, or the output peak will impacted by the speed. suggest 3.9k at least under +- 9V supply.

4.  D2,T1 is a constant current source. but it is not a normal type constant source,  if you use 2 1n4148, the current flow through T1 will be always constant leading to stop oscillating. saw-tooth oscillation utilize such a character: current source is not always provide current to C4. if you have 2 1n4148, the T2,T3 might always keep saturation make it stop oscillating.