Scope Probe Enhancing Performance of Circuit

...and then the circuit gave you the middle finger?

There are lots of situations in instrument design where you want to avoid some singular point, such as exactly zero phase error or exactly 100% duty cycle. In my business, for instance, making a truly dark-field optical measurement will lose you many decibels of S/N compared to a *nearly* dark-field measurement.

The dead band on a 4046 is one of those minor warts that make design more interesting...like the weak pulldowns on some bipolar single-supply op amp outputs. It's easily fixed with one resistor, as you point out, and once that's done, you still have all the other virtues of the 4046, such as that very nice wide range VCO.

Metal-gate 4046s have about a 1000:1 range, but all the HC ones I've tried crap out on the low end, limiting them to a range of at a bit more than 10:1. Are there any quick 4046s nowadays that preserve the nice wide range?

Cheers,

Phil Hobbs

I'm not sure about the VCO, but I use a '9046 in place of a '4046 for those rare occasions when I need a stand-alone analog phase detector. It has a charge pump (i.e. current source and sink) output, and avoids most of the pitfalls of the 4046 PD2 design.

Unfortunately, it's only (readily) available in HCT, so I have to find a +5V rail someone in amongst all low voltage stuff (most of my last board was 1.2V, 1.5V and 2.5V). I used caps to couple 2.5V LVCMOS outputs to the PD inputs on the '9046.

Regards, Allan

If it's a PFD you want you can roll your own... 1 pkg dual-D, 1 pkg quad-2-IN-NAND

...Jim Thompson

I made a phase detector with a single 4001.. not the fastest nor the widest range (maybe 5-355 degrees) but it works for me.

Tim

-- Deep Fryer: a very philosophical monk. Website:

A PFD is an edge-matcher.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

Your board construction is crap and when you alligator clip the probe GND lead, you are joining the otherwise open connection between top and bottom GND planes.

My "2.5V LVCMOS" source is actually a large FPGA, so I could put the PFD logic in that, but I still need the external charge pump (or at least external CMOS buffers (to avoid the noisy Voh and Vol of the FPGA) followed by an opamp integrator with differential inputs).

The '9046 works and gives me the minimum IC count, so the design is already optimal in some sense.

It gave 8ps RMS jitter, from memory. (That was three of them in series. I had difficultly measuring just one.)

Regards, Allan

Sometimes its more than just capacitance. I have a portable TV set that receives perfectly in a marginal reception area if I touch the chassis. I wish I had a good RF equivalent circuit for the human body.

Do you have a capacitor between your isolated ground plane and your supply input ground? A cap in the 1nF - 0.01uf range is often connected between the isolated ground output of a switching supply and the ground of the input side.

Ah ha, i get it, clock distribution systems.