ADVANCEDTEMPLATE.MZT

ADVANCED LESSON 71

LEARNING OBJECTIVES and NOTES
Measurements - Meters
10a.1 Understand the use of multiplier resistors in analogue voltmeters, shunts in ammeters and the effect of the test meter on the circuit under test. There are two types of meter in common use by radio amateurs: 1. Digital meters that produce their output on a digital display. These can be bought for £5-£10 from many retail stores. In addition to measuring current and potential difference (volts) they also measure resistance and frequently include inputs for testing transistors. The advantage of these is that the voltage is easy to read often producing the average of a number of readings; they are relatively cheap, they have a high input impedance which is not affected by the circuit they are attached to. 2. Analogue meters are based on a moving coil meter which moves a needle across a calibrated scale. These vary in price according to the size of the meter, the input impedance, the meter bearing used and the inclusion of a mirror behind the scale. The advantage of these is that for voltage and current readings they do not require any battery power; they enable the user to see more rapid changes in voltage and current. In addition the position of a meter gives the user instant information about the approximate voltage from a distance. A digital meter has to be read a close quarters to work out the voltage. We will concentrate on the analogue meter, although the digital meter uses a similar technique to create different ranges of voltage and current. All meters are rated to draw a certain current at full scale deflection (FSD) of the needle ie when the needle is as far over as will go. This value is very important as it has to be taken into account when adding multiplier resistors and shunt resistors to measure a specific voltage or current. Common meters have a FSD between 50uA and 1mA. they also have resistance so a 50uA meter is about 3.5kOhms and a 1mA meter is about 200 Ohms. Meters are normally used for DC voltage / current although a diode can be added to the circuit to convert AC to DC. Voltage multiplier resistors To measure voltage a resistor is placed in series with the meter. The voltage of this resistance is calculated for Ohms Law: For example to have a maximum reading of 20 volts using a 1mA meter the resistor would be: R=V/I R = 20/1 R= 20kOhms However the meter has a resistance of 200 Ohms so: 20,000 Ohms - 200 Ohms = 19,800 Ohms This is not a standard value so could be made up from an 18k resistor plus a 2.5k potentiometer. The potentiometer could then be adjusted to give a FSD at 20 Volts. A switch would be used to place a different multiplier resistor for each range required. Typical ranges might be 200mV, 2V, 20V, 200V and 1000V. The current rating of the resistor would need to match the current through the circuit. Current shunt resistors To measure current a resistor is placed across the meter. Most of the current will flow through the resistor, but a small amount will pass through the meter. Lets use the same meter as above ie a 1mA FSD with a resistance of 200 Ohms. the formula is: Rs = Rm / (Is / Im) Where Rs is the shunt resistance, Rm is the meter resistance, Is is the shunt current and Im is the meter current. For a 1mA meter, with a 200 Ohms resistance with a FSD of 2A. First of all if 1mA flows through the meter then 2000 - 1 = 1999 mA flows through the shunt resistor Rs = 200/ (1.999A / 1mA) Rs = 200 / 1999 Rs = 0.1 Ohms Resistors of this small size are often made out of a length of resistance wire, cut to length or a length of resistive metal cut and filed to achieve the desired resistance. Effects of the meter on the circuit under test We do not want the voltage or current to change because we have attached a meter across it. The higher the resistance of the meter circuit, the less effect on the circuit. A cheap digital voltmeter usually has a higher resistance than a cheap analogue meter. One can also build or buy a FET voltmeter. These use a FET with a high impedance between the circuit under test and the meter. For measuring lower currents a 50uA meter is more accurate than the 1mA meter used in our examples.
Measurements - Frequency Checking
10b.1 Recall the uses and limitations of absorption wavemeters, heterodyne wavemeters, crystal calibrators, digital frequency counters and standard frequency transmissions. A variety of tools are used for checking the frequency of oscillators, transmitters, receivers etc. here is a list of what they do and their limitations: Absorption wavemeters - the pick up antenna is placed close to the equipment. The variable capacitor and variable inductor are adjusted so that the meter shows a maximum reading. The tuned circuit scale is used to read off the frequency. This can also be used to check for harmonics, spurious and parasitic oscillations from a transmitter. Limitations - the device is not powered and depends on the power radiated by the equipment. This may not be strong enough to be read on the meter. Calibrating the tuned circuit may not provide an accurate enough reading to be used to measure the exact frequency. Heterodyne wavemeter - This generates a low power signal from a tuned circuit which is calibrated. When picked up by a receiver, the frequency on the heterodyne wavemeter can be used to calibrate the receiver. Unlike the absorption wavemeter this needs to be powered. Limitations - frequency readout is not particularly accurate - to within a few kHz., but slightly better than the passive absorption wavemeter Crystal calibrators - When I obtained my licence in 1970 the crystal calibrator was my main piece of frequency calibrating equipment. It consisted of a 500KHz crystal in a valve oscillator which generated harmonics at 500 KHz intervals up to the 28MHz band. It was particularly useful for checking that some part of of each amateur band was on frequency. For example 2MHz for top band 3.5 MHz for 80m 7.0 MHz for 40m 14 MHz for 20m 21 MHz for 15m 28, 29 and 30 MHz for 10m Its accuracy depended on trimming the crystal for 500KHz. This was done using one of the standard frequency transmissions. See below. Also, it only enabled the checking of frequencies that were multiples of 500kHz. Digital frequency counters - These consist of an electronic gate that is open for a set period of time. The number of sinewaves passing through the gate is counted, frequency is calculated and displayed on a readout. Most modern TX/RXs use one of these to display frequency. Limitations - the timing circuits are crystal controlled. The crystal has to be correctly set so that the gate is open for exactly the time period required. Standard frequency transmissions - These are transmitted on various frequencies on the LW and SW bands. Standard frequencies include 2.5 MHz, 5.0 MHz, 10MHz and 20MHz. They have a variety of formats, but usually the tones change for a few seconds at minute intervals. To calibrate a receiver switch to AM and adjust to zero beat. This means if you move the frequency dial up or down you will hear a tone. When zero beat the oscillator and transmission are on exactly the same frequency, so no audio is produced. Limitations - the reception of Standard Frequencies depends on ionospheric conditions. Sometimes you can't hear them! If this is the case try using the Radio 4 transmission on 198 kHZ.