Tuesday, July 30, 2019

introduction to electronics part 7 a side trip into RF part 2

 The velocity of a transmission line is less than air so we have to take the dielectric and its thickness into consideration. For now just be aware a 3cm wavelength signal may see a 2cm path in your wiring as a wavelength.
The velocity for some common coax. So why would a radio engineer want to know these things?
Here is an example. Notice the 1/12 wave line being two different cable would not be the same length. The cable must be adjusted to suite the velocity factor.

introduction to electronics part 6 a side trip into RF

I would like to explore the ground plane just a little. It seems the ground is considered as a fixed reference at all points. In low frequency circuits we can ignore the ground plane but in RF we must consider strays effects including how the ground acts. As the frequency increase the wavelength decreases and the circuit layout becomes more critical. First let's look at a couple of datasheet references.
Note pins 4 and 8 go to ground. Someone ask why didn't they just tie them together inside the IC? That was a very good question. Now a look at the recommended hookup.
My friend connected 4, 7, and 8 at the chip. The manufacturer has pin 4 going way over there and connecting to the input circuit while 7 and 8 are connected with the output. Why? Consider a length of wire suspended in space will have inductance and capacitance. This is where to sage advice "keep your leads short and close to the ground plane." came from. The ground plane connecting the input and outpoint grounds will effect the circuit. A signal flowing between the two points will have a phase shift (time delay) simply because the distance between the two points represents a fraction of a wave length. So as frequency increases the strays become more prominent. More on this later. (don't want to start chasing rabbits.)
What if I was wiring this circuit and one component lead was to short but the other was long enough. Why not do this? As it says we have a common current path for two signals.
If C2 is bypassing R1 and we make that connection we add L1 into our circuit. C1 is not bypassing R2 it is forming a series circuit with L1 that could introduce instability. At some frequency C1 and L1 will be at resonance and possibly cause oscillations.
So when I connect the circuit as shown doesn't it introduce two coils into the circuit?  Even if they couple together the effect is weaker. Maybe more on that later.
It's a lot in a short time but I hope this is enough to show the ground plane is not universal.
In a future post we may look into a circuit that uses this info to inject inductors into the design.

introduction to electronics part 5 coils continued

mutual - self inductance
Inductance in a transformer

Inductance is a very different animal than resistance or capacitance. To coils may add or subtract when placed in series. This will be explored more later. For now the above post should be enough.

Monday, July 29, 2019

introduction to electronics part 5 coils in DC circuits.

As you can see when I close the switch current doesn't build immediately. 

I added a resistor to limit the current. It looks a lot like the capacitor curve, only in reverse. With the capacitor current starts at max and decays. with the coil current builds with time.

When I raise the resistor value it reduces the charge time.
TC = L/R remember raising the resistor with the capacitor increased the TC.
TC = R*C

The coil opposes a change in current. How? Why?
That will be the subject of the next post.

introduction to electronics part 4 - capacitor response to DC

We know a 1 farad capacitor will charge to 1 volt with 1 coulomb of electrons on its plates. How long will it take to place this high number of electrons on the plates?
In this circuit I assume NO resistance. As you can see it would take 1000's of amps to charge the capacitor instantly. In the real world this cannot happen we will have some resistance in the circuit.
With 1 ohm resistance the charge is limited too 1 amp. Notice the instant the circuit is closed the charge starts dropping. As the capacitor charges it acts as a battery with an opposing charge being inserted in the circuit. This decay in signal is at a log rate. More on that later.
Changing R1 to 10 ohms limits the current to 100ma and the charge time is extended. Note the curve has the same shape. This time is prodictable.
I changed the values to more realistic values. It follows the same curve but is not easy to see with the time base of the scan.
The time constant (TC) is found with the formulas.
TC = R*C
TC = 100ms
We assume the capacitor is fully charged in 5 TCs. At .5 seconds it is charged for our analysis purposes. In reality as it continues to charge beyond 5 TCs but we won't look at that for now.

The main point is the capacitor will accept a charge in a period of time defined by its value and circuit resistance. It passes current as it charges and then blocks DC.

introduction to electronics - part 3 - capacitors

Capacitor are made by placing an insulator between two conductive plates. The area of the plates, distance they are apart and the dielectric constant of the insulator determine the capacitance. The unit of measure is the farad. A capacitor that has a one volt potential between the plates with one coulomb of electrons charge is one farad. A farad is a very large unit we will usually see micro (10^-6), nano (10^-9) or pico (10^-12) farad components. 
When capacitors are placed in parallel the plate area is combined and the capacitance is added.

When two capacitors are placed in series the charge is limited by the smaller plate area and the outside plates are farther apart than either of the originals so the total capacitance will always be LESS than the small.

Capacitor in series add as resistors in parallel.
Capacitor in parallel add as resistors in series.

Saturday, July 27, 2019

introduction to electronics - part 2 - DC circuits

A DC circuit will consist of a power source (battery) and  resistor. The battery unit is volt and resistor unit is ohm.

The circuit can have more than 1 resistor or more than one in series or parallel. The resistors are made with a material that has a certain resistance per unit of cross section and length.
Adding resistors in series as in (a) is the same as increasing the length of one resistor. We simple add the resistor values. If each resistor is 1 ohm the four in series will be 4 ohms.
Adding resistors in parallel as in (b) is the same as increasing the cross section of 1 resistor. This lowers the resistance. If the value of each resistor is 1 ohm the four in parallel will be 1/4 ohm.
You should be able to solve any resistor network using these formulas.
Divide the circuit into sections and solve each. If you want more info you can go to resistorguide.com.

intro to electronics - part 1 - the terms

I plan to introduce the subject as if it was a day one student being taught. This may to basic for some but a good review has never hurt me. So we begin.

When the world was evolving towards our modern electronic wonder man kind had no knowledge of basic sciences. The alchemist was trying to turn lead to gold and doctors were bleeding their patients in an effort to cure them. Then the battery was invented. The battery was used to shock frog legs and make them jump! Electroplating was developed ...............

As the periodic table was developed man kind became aware of the characteristics of the elements and a new term was coined. The gram molecular weight which in a strange way becomes the basic for our world of electricity and electronics.................

To electroplate a gram molecular weight of a material requires a specific numbers of electrons. If we define this number of electrons as a coulomb it could be stated "when one gram molecular weight of XXXXX is deposited by the electroplating process  one coulomb of electrons flowed through the solution."..........

A quick search would reveal the number but the actual number is not important. We use the unit to define our other terms, that is why it helps to know what it is.

Ohm's law says when we apply 1 volt of potential across a 1 ohm resistor we have 1 ampere of current flow. One coulomb of electrons flowing through a circuit in 1 second is 1 ampere. One ampere (amp) of current produces 1 watt of power in a 1 ohm resistor. So the units were developed based on current flow and the potential to produce it through a specific value of resistance.

So we can use Ohm's law to find voltage (E), current (I) or resistance (R) in a circuit.

Friday, July 26, 2019

A closer look at the trumbone tuner

Let's look at it without the coil first. First question is at what frequency is 130 mm one quarter wavelength?
We can transpose the wavelength and frequency and we find the frequency is 300,000,000 / (.13 X 4 ) or roughly 577 Mhz.

If we look at  Zin with a 1/4 wavelength  transmission we line see the DUAL of Zl. (2-12-19 I posted a link to Mr Turners book on transistor theory) Mr Turner explains the theory of duals in his book. The simple statement is we see the opposite at the input and output. Some text say we see the conjagate.
In other words with the load end open we see a short. With the slider on the load end we see an open feed end. With a line shorter than 1/4 wavelength we see capacitance.
So any length will resonant at some frequency and represent a reactance at other frequencies, Below 1/4 wave length we see capacitance. Adding inductance will cancel the capacitance and make the line longer electrically.

The article from wiki says adding the inductor lengthen the antenna and lowered the resonant frequency. So Amie made the loop long enough to resonant at a lower frequency by adding tuning components (mainly the loading coil and then added the shorting strap to make it adjustable.

Here are some more variations. Notice the top right antenna has and inducter sticking off the side.
While Amie found an application in the broadcast band her system would be more common in microwave. When the wavelength is measured in centimeters lead lengths becomes more critical.
The recap:
A parallel resonant circuit is high impedance. A shorted 1/4 wavelength transmission line produces a high impedance on its input. This equals a parallel resonant circuit at that wave length. At other wavelengths the impedance is lower. Using the slide makes it adjustable.

Wednesday, July 24, 2019

Air band build - unique tuning

 Amie's TRB post

Sadly the link is broken
To much theory for TRB maybe?
I did copy the trombone tuner to the next post before it was removed.

A tip of the hat for you kid. That is clever indeed. It shows a firm grasp of the components, and an understanding of how they work together.

My friend made one based on this design.

A PDF of the radio he made.

We were discussing this coil

In his build he used a string around the coil adjusting screw.

Maybe this would be a good method?

The indicator mounts where the arrow 3 is. Instead of the drum mounted on the variable capacitor he wrapped the string around the slug screw on the coil. Could Amie's trombone be mounted with the slider attached at arrow 3? This would allow tuning from the front panel? The round drum could hold the indicator or the drum could be replaced with a spring (from a pen) and the indicator mounted on the vertical string. (arrow 4)

Food for thought.

Sunday, July 21, 2019

Using feed thru capacitors for isolation

feed thru capacitors

They are still being made but are mostly surface mount today. I saw some for several dollars each. The best price I've seen is around 3 for a dollar in larger quantities. The ones above have no pin installed. I was thinking about making something like them. Still considering the construction details. I'm storing this info until I get some shop time for the actual project. You might ask what are the good for?

The three terminal capacitor IS a feed thru capacitor. My low frequency application will be using them for isolation between stages. For example when feeding a DBM with a RF signal and a LO signal the stages must be isolated. The leads act as antennas radiating and receiving signals. To prevent the cross talk the stages will be separated by metal shields. The signal and supply leads will feed thru the shield using feed thru capacitors. I have a few on hand but intend to make some for this project. Next step will be to get the shielding and breadboard.

HINT: In microwave the vias have capacitance and act as feed thru capacitors.

QUESTION: could a via work in my circuit?

I will be exploring this next.

Friday, July 12, 2019

Simple AF amp with a kick

This amp will drive an earbud with the touch of a finger on the input. R5 and R6 are a 10k pot.
Top plot is input lower plot is the transformer secondary. Shielding the input and battery leads is necessary to prevent motor boating.

Simple FM detector

As simple as it can be stated: when the input is at the tank's resonant frequency the system is in balance as shown in (B). As it goes above or below resonance the circuit goes out of balance and produces an output. AM will pass through the detector.  Q1clips the peaks to prevent AM bleed through.