A problem for the spice boys
Posted: Fri Feb 15, 2008 5:35 pm
First of all let me say that I do not have or use Spice programs, so if what I am asking is too time consuming and troublesome, there are definately no hard feelings if you choose to ignore this post.
PROBLEM #1:
I have to filter and match a 100 MHZ HCMOS clock oscillator to a 50 0hm termination at a level of 0 DBM. Harmonic content should be <25 DBMc at the term. point. The oscillators maximum rise/fall time is 4 nSec. At 100Mhz the waveshape is a slight triangular shape with a moderate degree of truncation and 5volts P-P (+18 DBM) , exactly as one would expect from this "half can " module.
I did a rough calculation on third harmonic content (this is the dominant one) with this wave shape and came up with approx.
20% of the fundamental. A quick scope check some what confirmed this.
PROBLEM #2:
My calculator died half way thru a proving analysis of the circuit design i came up with, and this is where I need help in confirming what I have.
WHERE I AM AT NOW:
I am capacitively coupling (no reactance here) the clock oscillator into a series 200 ohm resistor. The other end of this resistor feeds an L-C Pi network and its purpose is to clean up the wave form and match it to a 50 ohm term.
PI filter input-15 pf cap (100 ohm Xc) from resistor to grd. - 150 nH inductor* (100 ohm XL) [this is my calculated value and also my problem area hence the asterisk], connected from cap./resistor node to another 15 pf cap of which the other end is grounded. The 50 ohm term. is across the last cap. Since this is wired right now on a ground plane in rather sloppy bread board fashion, there is a lot more lead lengths than would be in its final form (possibly 100 nH intermixed). The thing I do not understand at this point is that the PI network looks like it is still calling for more inductance in spite of all the extra added parasitic inductance of the sloppy circuit.
At this point, a very properly terminated scope check shows a fairly clean out put of + 4 DBM. The PI network is calculated to be a lo 'Q' network (maybe 10) resonating at 100 MHz and when optimized will probaly raise the level at the term point and require some adjustment of the series resistor to obtain the level desired here.
So if any one out there has a Spice program that can easily work out this type of circuit, I would certainly appreciate your input, espcially in regards to the inductor value as the other components seem to be more or less doing their job at this point. BTW the finished circuit will have no more than 1/16" lead lengths per component and calculated to be less than 15 nH total, so does not need to be entered into any answers at this point. The HCMOS out put impedance is assumed to be zero ohms out put so PI network input impedance is taken as 200 ohm.
PS - I did not intend for this post to be so long but felt I needed to supply all info I could in order to be more helpful.
PROBLEM #1:
I have to filter and match a 100 MHZ HCMOS clock oscillator to a 50 0hm termination at a level of 0 DBM. Harmonic content should be <25 DBMc at the term. point. The oscillators maximum rise/fall time is 4 nSec. At 100Mhz the waveshape is a slight triangular shape with a moderate degree of truncation and 5volts P-P (+18 DBM) , exactly as one would expect from this "half can " module.
I did a rough calculation on third harmonic content (this is the dominant one) with this wave shape and came up with approx.
20% of the fundamental. A quick scope check some what confirmed this.
PROBLEM #2:
My calculator died half way thru a proving analysis of the circuit design i came up with, and this is where I need help in confirming what I have.
WHERE I AM AT NOW:
I am capacitively coupling (no reactance here) the clock oscillator into a series 200 ohm resistor. The other end of this resistor feeds an L-C Pi network and its purpose is to clean up the wave form and match it to a 50 ohm term.
PI filter input-15 pf cap (100 ohm Xc) from resistor to grd. - 150 nH inductor* (100 ohm XL) [this is my calculated value and also my problem area hence the asterisk], connected from cap./resistor node to another 15 pf cap of which the other end is grounded. The 50 ohm term. is across the last cap. Since this is wired right now on a ground plane in rather sloppy bread board fashion, there is a lot more lead lengths than would be in its final form (possibly 100 nH intermixed). The thing I do not understand at this point is that the PI network looks like it is still calling for more inductance in spite of all the extra added parasitic inductance of the sloppy circuit.
At this point, a very properly terminated scope check shows a fairly clean out put of + 4 DBM. The PI network is calculated to be a lo 'Q' network (maybe 10) resonating at 100 MHz and when optimized will probaly raise the level at the term point and require some adjustment of the series resistor to obtain the level desired here.
So if any one out there has a Spice program that can easily work out this type of circuit, I would certainly appreciate your input, espcially in regards to the inductor value as the other components seem to be more or less doing their job at this point. BTW the finished circuit will have no more than 1/16" lead lengths per component and calculated to be less than 15 nH total, so does not need to be entered into any answers at this point. The HCMOS out put impedance is assumed to be zero ohms out put so PI network input impedance is taken as 200 ohm.
PS - I did not intend for this post to be so long but felt I needed to supply all info I could in order to be more helpful.