Earl's Corner

[Earl]

Hi All,

I have started building a pulser to test how narrow a pulse can be generated.  This will also serve as a test bed to see if the differential 12V output pulses can be boosted by means of an air-core autotransformer?  Because I wanted to minimize capacity and wasn't interested in winning any beauty contests, I decided to wire everything in the air by mounting the IC with its back to a copper PCB groundplane.  The ground, pin 7, is soldered directly to the groundplane while the Vdd, pin 14, is soldered to ground via a ceramic SMD capacitor.  Therefore two corners are solidly anchored and there are another pin or two also going to ground to give mechanical stability.  The picofarads start adding up with sockets and PCB dielectric material.  This is only experimental prototype, so I am not going to worry how it looks, it will end up inside a shielded cylinder anyway.  Didn't have enough time to test it, and am not sure if I have a 6V regulator, so will have to see what to use for power source.  Sure is tough on my old eyes.  Also will have to figure out a better way to hold things while soldering; at present I need at least three hands.

Regards, Earl

[Earl]

@Jason

here is the precision 90 degree phase shifting circuit.  It is more accurate than using cross-coupled F/F's since every stage is synchronously clocked and also there are no propagation delays such as when using cross-coupled F/F's.

The circuit scanned, with 8 stages is for a 22.5 degree phase shifter.  For 90 degrees less stages are necessary.  Adding the OPTIONAL circuitry below the blue line gives 180 degree shift in relation to the upper SR.

EDIT1:  Do you need only 0 and 90, or do you also need 180 and 270?

Regards, Earl

[Earl]

Jason,

here is how you use individual F/Fs instead of shift register.

Earl

[Earl]

Hi All,

here are the test results of my F/F pulser circuit.

Things turned out as I expected.

Next things to do:

try a coil of 3m wire connected to Q and Q/ outputs
and see what happens

hook-up to the 4426/4427 FET driver and see what happens
to the outputs.

Regards, Earl

[Earl]

Hi All,

Here is a scope shot showing both F/F outputs, Q and Q/, as well as the differential waveform.  As you can see the differential ouput has twice the voltage swing of a single-ended output, which is to be expected.

The scope has 300 MHz analog bandwidth and the scope probes are 500 MHz rated.

I have been brain-storming a lot and came up with another alternate idea of feeding the excitation coils.  Instead of my previous idea of using a parallel transmission line with a center-taped coil, my latest idea is to fed two coils with electrical and physical displacement of 180 degrees.  The coils would be end-fed and the open end would be connected to nowhere (except maybe to "the blob" under influence of a bias field).

Making a center-taped coil exactly defined both electrically and physically is difficult, the end-fed coil is much easier to wind, and now that I can use differential outputs, I feel much better.  I don't like nebulous grounds that are undefined - - using differential outputs with no relation to, nor necessity of, a ground pleases me.

Regards, Earl