### Speaking of tubes: Introducing the Grid Dip Meter

+Jonah Miller is currently working on a multi-part series of articles on how computers work.  In one of the installmetns he mentioned the good old triode vacuum tube with a cathode, plate, and grid.  That brings up the subject of today's post, the grid dip meter, (GDM).  If you wandered over here from the ham radio practice tests[1], and you want the shortest answer possible, then click here, or scroll down the page.  If you want more information, read on:

The grid Dip meter schematic is shown in picture 1.

The main purpose of the circuit, at least for amateur radio operators, is to determine the resonant frequency of some other circuit or device, (like an antenna).  Let's say you've built you're latest radio and you'd like to get all the power you can from the driving finals, into the antenna.  To do that, you'd like to make sure that the antenna resonates at the same frequency you'd like to send and receive at.  If the antenna resonates at a little bit lower frequency, you can always trim it's length, or if it oscillates at a little bit higher frequency, you can always make a longer one.  To find out which I need to do, I'll use my grid dip meter to determine the antenna's resonant frequency.

The basic grid dip meter's circuit consists of a Colpitts oscillator with the inductor, L1, of the LC feedback loop exposed and protruding from the device, (see picture 2), it's the coil on the far left edge of the picture.

To determine my antenna's resonant frequency, I'll attach a small coil to the middle of it, (I tend to only use dipoles because I'm lazy).  Next, I'll place the exposed coil of the grid dip meter near the coil attached to my antenna.  Then, I'll tune the oscillator's operating frequency until I see the reading on the meter on the front face of the apparatus, which is denoted by G in the schematic, dip, (hence the name).

So,why does the current being supplied to the grid of the Colpitt's oscillator dip?  At the resonant frequency of the device being measured, the magnetic field from the inductor begins to drive the coil on the device under test.  In this case, the antenna radiates that energy away.  The current that was driving the grid is now reduced in an amount related to the amount of energy that went into the device under test, (the antenna).  Pretty cool!

You want the two coils to be as loosely coupled as possible.  If they are tightly coupled, (located very close to one another), then the inductance in the coil attached to the device under test will effect  the resonant frequency of the GDM.  The end result will be that the resonant frequency won't be the actual resonant frequency of the device under test, it will be an odd munge of the device and the GDM circuit.  On the flip side, you don't want to couple too loosely either, or the grid current simply won't dip.

A Year and a Month Ago

Radiating Superconductors, Arduinos, and Data Acquisition

LabBook 2014_05_26 Superconductor Quenching Magnet Chilling Supply and Leak Detector Testing
http://copaseticflow.blogspot.com/2014/05/labbook-20140526-superconductor.html

References:

1.  http://copaseticflows.appspot.com/hamtest

2.  Jonah Miller's Post on Logic Circuits
http://www.thephysicsmill.com/2014/06/22/boolean-cirucit-logic/

### Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems, there seem to be fewer explanations of the procedure for deriving the conversion, so here goes!

What do we actually want?

To convert the Cartesian nabla

to the nabla for another coordinate system, say… cylindrical coordinates.

What we’ll need:

1. The Cartesian Nabla:

2. A set of equations relating the Cartesian coordinates to cylindrical coordinates:

3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system:

How to do it:

Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables.

The chain rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

### Lost Phone

We were incredibly lucky to have both been in university settings when our kids were born.  When No. 1 arrived, we were both still grad students.  Not long after No. 2 arrived, (about 10 days to be exact), mom-person defended her dissertation and gained the appellation prependage Dr.

While there are lots of perks attendant to grad school, not the least of them phenomenal health insurance, that’s not the one that’s come to mind for me just now.  The one I’m most grateful for at the moment with respect to our kids was the opportunities for sheer independence.  Most days, we’d meet for lunch on the quad of whatever university we were hanging out at at the time, (physics research requires a bit of travel), to eat lunch.  During those lunches, the kids could crawl, toddle, or jog off into the distance.  There were no roads, and therefore no cars.  And, I realize now with a certain wistful bliss I had no knowledge of at the time, there were also very few people at hand that new what a baby…

### Lab Book 2014_07_10 More NaI Characterization

Summary: Much more plunking around with the NaI detector and sources today.  A Pb shield was built to eliminate cosmic ray muons as well as potassium 40 radiation from the concreted building.  The spectra are much cleaner, but still don't have the count rates or distinctive peaks that are expected.
New to the experiment?  Scroll to the bottom to see background and get caught up.
Lab Book Threshold for the QVT is currently set at -1.49 volts.  Remember to divide this by 100 to get the actual threshold voltage. A new spectrum recording the lines of all three sources, Cs 137, Co 60, and Sr 90, was started at approximately 10:55. Took data for about an hour.
Started the Cs 137 only spectrum at about 11:55 AM

Here’s the no-source background from yesterday
In comparison, here’s the 3 source spectrum from this morning.

The three source spectrum shows peak structure not exhibited by the background alone. I forgot to take scope pictures of the Cs137 run. I do however, have the printout, and…