IEEE Consultants' Network of Long Island

Filter Pins, Part 1 - John Dunn, Consultant, Ambertec, P.E., P.C.

The physical structure of a single pin of a filter pin connector is represented as follows as a ferrite bead over the center pin with shunt capacitors to the shell on each end:    

 

Signal To Noise Ratio - John Dunn, Consultant, Ambertec, P.E., P.C.

The sketch below summarizes a basic model for signal to noise ratio (SNR) analysis:  

 

Note that the SNR gets worse and worse with each successive stage of signal processing. It never improves at the output of a later stage over what the SNR of the input to that later stage may be.

Therefore, you want the first stage's gain G1 to be as high as design considerations will permit. Lots of gain at G2 and later simply won't make up for a poorly designed G1.

A Sneak Noise Source - John Dunn, Consultant, Ambertec, P.E., P.C.

I once chose a TO-99 packaged, low-noise op-amp, the OPA2111 when that device was a product of Burr-Brown. Today, that device is being sold by Texas Instruments.

I got some really nice results with it, but I was just plain lucky to have avoided trouble with that part from a noise source I hadn't reckoned with, ambient light.   

The OPA2111 uses a pair of junction FETs for its differential input stage. The trademark for this feature  is now Difet ®.

According to a Burr-Brown tech-rep with whom I spoke way back then, those two FETs were quite large, occupying approximately 50% of the chip's area all by themselves. Being large, those FETs were highly sensitive to ambient light that could sneak into the package via the glass beads that held the eight leads of the TO-99 can.

Forgiveness - John Dunn, Consultant, Ambertec, P.E., P.C.

There is a really great application note AN-202 from Analog Devices that can be found on line at http://www.analog.com/static/imported-files/application_notes/135208865AN-202.pdf . Following the rail voltage bypassing example as shown in Figure 8 of that note, I set up a low-noise amplifier's op-amp rail voltage bypass capacitors as follows:

 

Audio Transformer Far-Field Cancellation - John Dunn, Consultant, Ambertec, P.E., P.C.

Two small audio transformers of very low power ratings had been pressed into service together in order to handle the required output power level of a particular audio amplifier. The arrangement worked well, but there was some 400 Hz interference getting into the intended signal.

The interference was found to be coming from a 400 Hz power transformer's stray magnetic field.

An approximately 10 dB reduction of that interference was obtained by reversing the phasing dots of both the primary and secondary windings of one of the two transformers, thereby achieving some measure of far-field cancellation without affecting performance with regard to the intended audio signals.

 

Inadvertent Waveguide - John Dunn, Consultant, Ambertec, P.E., P.C.

There was this amplifier constructed on a circuit board and mounted in a metal chassis which worked fine if the chassis cover wasn't attached, but which would go into parasitic oscillation at 1 GHz when the cover was put on.   

 
My assigned task was to fix it without changing anything. (My boss at the time may have suspected that I dabbled in the occult arts. I was never sure with that guy.)

Bypass Capacitor Boobytrap - John Dunn, Consultant, Ambertec, P.E., P.C.

We all know of the technique of paralleling capacitors of different capacitances to achieve wideband bypassing, but here is a look at a hazard of which to be aware.

Consider two layers of some circuit board, the following being sort of a typical case:  

  

Upper RMS Estimate of Noise Waveform - John Dunn, Consultant, Ambertec, P.E., P.C.

Imagine a noise voltage waveform of ±100 (name your units as mV or volts or whatever) and further imagine that this noise waveform has a flat probability distribution function (PDF) over its ±100 range and then hold that thought.

The PDF of a ±100 triangular waveform is also flat over its ±100 range and we recall from previous calculations (We are just so smart!) that the RMS of that triangular waveform of ±100 equals 100/sqrt(3).

Since the two functions, the noise and the triangular, have the same PDF, they also have the same RMS value.

A few examples of the RMS calculation bear this out: 

Radiated Susceptibility Problem and A Windom - John Dunn, Consultant, Ambertec, P.E., P.C.

You can fail a radiated susceptibility EMI test if the physical dimensions of the unit under test (UUT) are just right. Such was the case with an aircraft fuel gauge in which the fuel probe's dimensions had been designed  to match up with the aircraft's fuel tank.

At almost any irradiation frequency, everything worked okay, but not at 120 MHz.

 

It turned out that the probe's length was a half-wavelength at 120 MHz and worse still, there was a right-angle joint at the 37% of length point as shown above.

Dimensionally, this fuel probe assembly was a classic Windom antenna at 120 MHz and when it was irradiated at that frequency, the supporting electronics was thrown into all kinds of malfunctions.

None of the probe's dimensions could be changed because of fuel tank restrictions. However, since the fuel tank was metallic and therefore a good RF shield, a waiver was granted because there would never be any 120 MHz irradiation of the fuel probe in actual service.

How do you spell "relief"?