AM DEMODULATION

AM DEMODULATION

Amplitude modulation refers to any method of modulating an electromagnetic carrier frequency by varying its amplitude in accordance with the message intelligence that is to be transmitted. This is accomplished by heterodyning the intelligence frequency with the carrier frequency. The vector summation of the carrier, sum, and difference frequencies causes the modulation envelope to vary in amplitude at the intelligence frequency, as discussed in chapter 1. In this section we will discuss several circuits that can be used to recover this intelligence from the variations in the modulation envelope. DIODE DETECTORS The detection of AM signals ordinarily is accomplished by means of a diode rectifier, which may be either a vacuum tube or a semiconductor diode. The basic detector circuit is shown in its simplest form in view (A) of figure 3-5. Views (B), (C), and (D) show the circuit waveforms. The demodulator must meet three requirements: (1) It must be sensitive to the type of modulation applied at the input, (2) it must be nonlinear, and (3) it must provide filtering. Remember that the AM waveform appears like the diagram of view (B) and the amplitude variations of the peaks represent the original audio signal, but no modulating signal frequencies exist in this waveform. The waveform contains only three rf frequencies: (1) the carrier frequency, (2) the sum frequency, and (3) the difference frequency. The modulating intelligence is contained in the difference between these frequencies. The vector addition of these frequencies provides the modulation envelope which approximates the original modulating waveform. It is this modulation envelope that the DIODE DETECTORS use to reproduce the original modulating frequencies.






FIG 3-5A






FIG 3-5B




FIG 3-5C






FIG 3-5D




Series-Diode Detector Let’s analyze the operation of the circuit shown in view (A) of figure 3-5. This circuit is the basic type of diode receiver and is known as a SERIES-DIODE DETECTOR. The circuit consists of an antenna, a tuned LC tank circuit, a semiconductor diode detector, and a headset which is bypassed by capacitor C2. The antenna receives the transmitted rf energy and feeds it to the tuned tank circuit. This tank circuit (L1 and C1) selects which rf signal will be detected. As the tank resonates at the selected frequency, the wave shape in view (B) is developed across the tank circuit. Because the semiconductor is a nonlinear device, it conducts in only one direction. This eliminates the negative portion of the rf carrier and produces the signal shown in view (C). The current in the circuit must be smoothed before the headphones can reproduce the af intelligence. This action is achieved by C2 which acts as a filter to 3-8 provide an output that is proportional to the peak rf pulses. The filter offers a low impedance to rf and a relatively high impedance to af. (Filters were discussed in NEETS, Module 9, Introduction to Wave- Generation and Wave-Shaping Circuits.) This action causes C2 to develop the waveform in view (D). This varying af voltage is applied to the headset which then reproduces the original modulating frequency. This circuit is called a series-diode detector (sometimes referred to as a VOLTAGE-DIODE DETECTOR) because the semiconductor diode is in series with both the input voltage and the load impedance. Voltages in the circuit cause an output voltage to develop across the load impedance that is proportional to the input voltage peaks of the modulation envelope.

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VIZIO L32 HDTV, No Backlight, Repaired

A VIZIO L32 HDTV came into the shop that would power on, the standby/power LED would go from amber to green and TV had audio but no picture. The back lights were not coming on, not even briefly. I opened the TV and powered it on. Next I checked all my secondary voltages on the SMPS to see if they were all present and within tolerance. Immediately it was apparent one of my voltages was missing, and it was the voltage to the inverter board. I disconnected the cable that connects the SMPS board to the inverter board to see if the voltage returned with the inverter board disconnected, so that I could verify it was the SMPS and not the inverter board that was failing. After disconnecting the cable and turning the TV back on the secondary output was still measuring 0V. I removed the power supply and checked the corresponding components for that output on the SMPS. I found the .027uF 630V MKP "bootstrap" capacitor location C9 was swollen and obviously bad. I check the rest of the surrounding components, for instance secondary diodes and filter capacitors and the Power MOSFETs on the primary side that are connected to the bootstrap capacitor. Everything was fine so I replaced the MKP capacitor with a .022uF 630V MKP from another board I had and put the TV back together. Upon turning the TV on it powered up the backlights came on and the TV is working great.

Bootstrap Capacitor Defined:

A N-MOSFET/IGBT needs a significantly positive charge (VGS > Vth) applied to the gate in order to turn on. Using only N-channel MOSFET/IGBT devices is a common cost reduction method due largely to die size reduction (there are other benefits as well). However, using nMOS devices means that a voltage higher than the power rail supply (V+) is needed in order to saturate the transistor and thus avoid significant heat loss.

A bootstrap capacitor is connected from the supply rail (V+) to the output voltage. If the capacitor is polarized then the orientation of the capacitor is as follows: Anode(marked with ‘+’)→(V+) and Cathode (marked with ‘-’)→Output. In other words, the capacitor should be between the output (source of an N-MOSFET) and (V+). Usually the source terminal of the N-MOSFET is connected to the cathode of a recirculation diode allowing for efficient management of stored energy in the typically inductive load (See Flyback diode). Due to the charge storage characteristics of a capacitor, the bootstrap voltage will rise above (V+) providing the needed gate drive voltage.

Hope you enjoyed this repair tip if you have any questions you can always email me john@preher-tech.com. Have a great day.

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Homemade ESR (Equivalent Series Resistance) Meter

Here is a great article I wanted to share with all of you. It will show you how to make your own ESR meter at home. This is a fun project and can also be another option for those who don't want to spend the money to purchase a professional grade ESR meter.


The Homemade Equivalent Series Resistance Meter



Electrolytic capacitors are by far the electronic parts that suffer aging soonest. If you have any electronic equipment that over the years has degraded its performance, developed quirks, sometimes ending in complete failure, the chances are good that one or more electrolytic capacitors inside it have degraded, causing the problem. Electrolytic capacitors age in several ways: They can become electrically leaky, causing a DC current through them that can make them blow up. They can shift in capacitance value. But the most common way they degrade, by far, is by unduly increasing their equivalent series resistance, which is the undesired internal resistance that appears in series with the wanted capacitance at a given frequency.
The ESR of an electrolytic capacitor is normally just a small fraction of an Ohm for a high capacitance, low voltage capacitor (such as a 1000µF, 16V cap), and can be as high as two or three Ohm for a low capacitance, high voltage cap (1uF, 450V). When the capacitor ages, this resistance increases, and it often does so in such a dramatic way that the equipment completely ceases to function or even blows up semiconductors. It's very common to find capacitors that have degraded to 100 times their normal resistance, while their capacitance remains fine! On a typical capacitance meter they will measure close to their correct values, but they are completely bad! This is where the ESR meter comes in: It measures the equivalent series resistance of the capacitor, almost independently of its capacitance.

An additional beauty of an ESR meter is that in almost all cases it can check capacitors while they are in the circuit! This is so because a good capacitor would measure almost like a short circuit, and so any other parts connected in parallel will have minimal influence on the measurement. These are the features that make an ESR meter an irreplaceable tool for troubleshooting electronic equipment.

The design presented here works by applying a 50kHz, 200mV square wave to the capacitor under test, in series with a 10 Ohm resistor. The AC voltage appearing across that resistor is measured and displayed on a meter. So the whole thing is nothing else than a simple ohmmeter that uses ultrasonic AC for measurement instead of the usual DC used by every common ohmmeter. Since the Ac voltage used is so low, it does not make semiconductor junctions enter conduction, which further helps to make this meter suitable for checking capacitors mounted in a circuit.

Here is the schematic, which you can click on to get a larger version for printing.

One section of a dual low power operational amplifier is used as a square wave oscillator. A small ferrite core transformer is used to step down the voltage and provide the necessary low impedance output. A 10 Ohm resistor loads the output to absorb inductive spikes from the transformer, which could cause a false reading for low value capacitors. The other section of the op amp amplifies the signal that gets through the capacitor being tested, and its output is rectified and applied to a 50µA galvanometer through a calibration potentiometer. A small 5 Volt regulator maintains the supply constant while the instrument is being powered from anything between about 7 and 15 Volt. I power the meter from the 13.8V bus which I have in my workshop, but if you prefer, you can use a 9V battery instead, connected through a switch. The power consumption of this circuit is so low that a 9V battery should last at least 100 hours.

Building this ESR meter is simple and straightforward. I assembled the circuit on a scrap piece of project board, and used a small plastic box to install the board and the meter. The only part that could pose problems to inexperienced builders is the transformer. I made mine using an Amidon ferrite core, type EA-77-188, which is a tiny double-E core having a cross section of 22mm2, and external dimensions of about 19x16x5mm total. I used the nylon bobbin that Amidon delivers with it, wound a primary winding consisting of 400 turns of AWG #36 wire, and as secondary I wound 20 turns of AWG #26 wire. If you have a larger or smaller core, you can adjust the turn numbers in inverse proportion to the cross section area. The wire size isn't critical - the gauges I used are about 3 or 4 numbers thicker than necessary, while at the same time this bobbin has room for wire at least two numbers thicker than the ones I used. Thus, you can choose from about 6 different wire gauges for each winding, with negligible impact on the performance.

Considering that the transformer is so uncritical (because it runs at very low power), feel free to use any small ferrite core you have on hand, as long as it has no air gap. Dead PC power supplies and old monitors or TVs are great sources for such cores. Do not use an iron core, because it would probably have far too much loss at 50kHz.

The test leads are soldered into the circuit, and fixed in place using hot melt glue. Soldering them is much preferable over using any sort of connectors, because this meter easily detects resistance as low as 0.1 Ohm, and a connector can easily vary its resistance more than that! By the way, this set of test leads was bought as standard tester replacement leads, for very little money.

The meter is a reasonably good one rated at 50µA full scale, which I had on hand. If you find a cheap VU meter that works well, you can use it, of course. If you prefer to use a 100µA meter, change R11 to 50k. I used a trimpot for R11, but you might want to use a panel-mount potentiometer instead, which would allow fine adjusting the full-scale point if your meter happens to be unstable. If you use a cheap meter I would recommend this.




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