1
\$\begingroup\$


i want to make 2 to 4 level converter for 8-QAM modulation, i'm going to use two digital input to get +/- voltage level, i want the output approximately like this,,


when the input 00=-3 V , 01=-5 V , 10=+3 , 11=+5.


i already have one circuit reference, but i'm still confuse, how do i can get a - voltage ?, because the circuit only can make + voltage, take a look this circuit

schematic

simulate this circuit – Schematic created using CircuitLab

Any help would be appreciated :)

\$\endgroup\$
4
  • \$\begingroup\$ As a first look, I guess you need to use two comparators, one for each output (3v and 5v); this configuration wouldn't give a negative output.. it's a voltage follower. \$\endgroup\$
    – Siraj Muhammad
    Jun 1, 2013 at 10:26
  • \$\begingroup\$ Are you certain that the 00 and 01 levels are correct values. Should it not be 00 = -5 and 01 = -3V ? \$\endgroup\$
    – JIm Dearden
    Jun 1, 2013 at 11:25
  • 1
    \$\begingroup\$ @JImDearden: looks good to me -- most people use a gray code for QAM and PAM so that "small" amounts of noise ruin at most 1 bit, rather than every bit. \$\endgroup\$
    – davidcary
    Aug 1, 2013 at 18:50
  • \$\begingroup\$ @davidcary cheers david - just checking facts \$\endgroup\$
    – JIm Dearden
    Aug 1, 2013 at 18:55

1 Answer 1

Reset to default
1
\$\begingroup\$

Because there is some confusion about the weighting of your decode requirements I've shown a circuit below where this can be changed easily: -

enter image description here

First is the 2 to 4 line decoder. Whatever your two digital input pins are, it will result in one of the four pins on the output being connected to (say) +5V. The idea here is that you can set 00 to produce 0V, 01 to produce 1.667V, 10 to produce 3.333V and 11 to produce 5V. If there is any change of plan, change the resistor ratios to suit. MAX384 seems a suitable candidate although I'm sure a smaller device with just one 4:1 switch inside could be located.

They don't have to be monotonic - they can be in any order whatsoever. It's a lot easier if you use analogue switches as the decode element because then there is no inter-dependence between voltage levels produced i.e. only one switch is active at once and the three other series resistors have no bearing on the output voltage. Bear in mind that the on-resistance of the MAX384 is 100 ohms so choose resistor values that are in the tens of kohm to avoid significant errors.

Next is a unipolar to bipolar convertor made from a single op-amp. This converts (say) a range of 0 to 5V to -5V to +5V so if your unipolar voltages were 0V, 1.6667V, 3.3333V and 5V, the output would be -5V, -1.6667V, +1.6667V and +5V.

This has an identical voltage spacing between levels of 3.3333V.

Remember the individual levels can be in any order and don't have to be equally spaced - set the resistor ratios to suit your requirement. I've chosen monotonic equal spacing because it makes more sense to me but it needn't be monotonic and it needn't be equal spacing.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.