I should have a 10Mhz square wave generated by CMOS logic. My 50Mhz scope shows overshoot, dampening or ringing on the peaks of the wave which should be ~5 Volt p-p square. How can I minimize this phenomenon? I looked at a 10Base T filter to "clean" this up. I use RG-174 coax to port this signal to my application but not shure the probe/scope or the coax is doing this. The application works ok otherwise.
The scope *may* not be telling the truth. I presume you're using a 10:1 probe which has been adjusted on the scope's calibration signal...right? Even so, a 50-MHz scope may not be an accurate judge of the squareness of a
10-MHz signal. Still, if it's mainly the scope's limitations, you'd expect it to look like it's been rounded, not ringing.
You don't mention what kind of CMOS logic. In general, it is incapable of driving coax. The filter will do no good without a load resistor on the output. To start out, remove the coax, and look at the CMOS chip with the 'scope. Make sure the probe is grounded somewhere within a couple of inches of the chip.
Instead of the 10Base T, you might want to try a 33 Ohm series resistor with something like 10 PF from the output side to ground. Depending on how much drive your CMOS has, putting a 47 Ohm resistor in series with the coax input might help if the coax is less than about a foot long.
Keep in mind, that in order to drive a 5V signal into 50 Ohm coax, and do it right, your CMOS device would have to drive 5/50 or 0.1 Amp. You would also need a 50 Ohm resistor across the output.
While you are at it, look at the VCC of the CMOS. If the chip does not have a bypass capacitor on VCC, add a .1 - .33 before bothering to look.
I will add, the 10Mhz comes off a pin (gate) of a 74AC86. It's referenced to ground through 7 inches of RG-174 to an input pin of a Xilinix XC9536 PLCC. The scope probe is Tek P6103B. The probe manual describes high and low frequency calibration. I forgot if the Hi trim tab was broken or if I didn't have the correct calibration generator but remember reducing the amplitude of the effect by the low freq. probe cal procedure.
I have done that. Put something like a 27 to 47 Ohm resistor in series between the output pin of the 74AC86 and the RG-174. Do not terminate the output of the 174. You should get some slowing down of the rise and fall times, but no ringing. Make sure the AC86 has a bypass cap on it; it will be drawing heavy current during transitions.
I think real, as based on measurements. I used both a 10X high impedance probe, and a 20X passive probe feeding a 50 Ohm 300 MHz 'scope. The latter probe only had about 0.8 PF of capacitive loading ( and 1K reistive loading).
Another thing that sometimes works is to terminate output side of the coax during transitions only. Load it with a 50 Ohm reistor in series with a 10 -
50 PF capacitor to ground. Obviously, you can't have a 50 Ohm DC load, or you won't get 5V out of the circuit, plus, you will dissipate a lot of power.
There will be no transmission line effects at your frequencies for a 7" section of RG-179 and termination is not necessary. Winging a C-distribution of 30p/ft gets you L=Cx50^2 or 75nh/ft for an equivalent circuit of: View in a fixed-width font such as Courier.
Using the fundamental result for a 2-pole critical damping R>2xsqrt(L/C) applied to the two sections and assuming driver gate output of ~20 ohms results in this: View in a fixed-width font such as Courier.
The input of the PLCC is located on the primary circuit application with 0.1 pin header for 10Mhz oscillator inputs. First prototype osc. used a 3.3V Maxim econoscillator through 7 inches RG-174. Second, a PLL with 74AC86 through 7 inch RGG-174. Although completely different oscillators, I observe the same ringing effect. What's in common is the coax and the scope probe.
I think my edge detector in the PLCC counted double off the rising edge ringing in the econoscillator 10Mhz because it's three volt p-p. The PLL 10Mhz is 5 v p-p.
Should I just use a "wire" between the primary appllication and oscillator, omitting the coax completely?
Where exactly are you placing the probe, its capacitance and GND lead inductance can have a significant effect at these frequencies? Another commonality is the PLCC input gate.
This could be anything like anomaly in the driver output under high frequency loading.
No- the coax is much better than a wire because it is phase stable. The SPICE sims show this is pretty well damped- going for a lower output impedance at frequency: View in a fixed-width font such as Courier.
In both oscillator prototypes, the probe hooks to the end of the 7 in of coax. Same ringing both cases. So it's measuring the effects of the coax? It's not connected to the PLCC during measurement. The calibrated 10Mhz seems to be doing it's job being quite stable over a
48 hour period.
It damps out in 1/2 the period and on both high and low peaks. I wondered if this was detrimental to my counting circuit. Largo