12. Meters

Definitions

Diagrams

Circuit diagrams

Formula

Notes

Different Types

  1. Moving coil meter
    • Used for direct current / voltage only
    • For AC the pointer will vibrate at 0
    • AC at low frequency
      • the current is rectified before it passes through the meter coil
      • many meters use a metal oxide full wave bridge rectifier
      • shunts & multipliers are then added to give various AC ranges
    • AC at high frequency
      • diode bridge will have too much self-capacitance
      • can be reduced by using special low capacitance diodes
      • better to use thermocouple meter, hot wire ammeter, oscilloscope
    • Thermocouple
      • a small voltage is generated when two different metals are joined together
      • Voltage generated is proportional to the temperature at the joint
      • Radio Frequency (RF) current to be measured is passed through a heater wire that is close to one junction
      • Thermo-couple meter is slow to react but will indicate RMS values, whatever the waveform
    • Hot wire ammeter
      • RF current is passed through a wire that is suspended between two fixed points
      • current warms up the wire and causes it to expand
      • expansion is used to mechanically move the pointer of a meter
      • wire is kept under tension by a little spring
      • as it relies on the heating effect, will indicate RMS that is independent of frequency
    • Cathode Ray Oscilloscope (CRO or “scope”)
      • similar to thermionic valve
      • In the CRT (cathode ray tube) the stream of electrons is focused (by focus anode) into a narrow beam that strikes a phosphor coated screen
      • point on the screen “hit” by these electrons gives off light
      • electric field is set up between the deflection plates
      • plates:
    • plates that deflect the beam vertically are called the “Y” plates
    • plates that deflect the beam horizontally are called the “X” plates - Absorption wave meter circuit types - Circuit 1
    • has the disadvantage that the tuned circuit (L & C) would have a low Q
    • they are damped by the diode and meter circuit - Circuit 2
    • overcomes this problem by having a second coil to couple the signal into the meter circuit
    • coils act like a step down transformer and matches the high impedance L & C to the low impedance meter circuit - supplied with several plug-in coils so that a wide range of frequencies are covered - dial, fitted to the tuning capacitor, may have all the ranges marked directly or, simply, a 0 to 100 logging scale - frequency meter of this simple type would often cover 1MHz to 100MHz in several ranges - absorption wavemeter gives a low accuracy measurement of frequency - it is adequate to ensure that a transmitter is selecting the correct oscillator harmonic - often used when adjusting a transmitter by ensuring that the correct harmonic or mixing product, is selected at each stage - Digital Frequency Meter - the counter actually counts the number of cycles (of the frequency to be measured) that reach it during the time that the gate is open - not practical to make a 1Hz oscillator (to control the gate) - low frequency is obtained by a crystal oscillator running at a much higher frequency, say 1MHz - 1MHz is then passed though a series of “divide by ten circuits” - gate could be open for 10sec, 1sec, 0.1sec - only as accurate as crystal controlled oscillator
    • very important to ensure its frequency does not drift
    • part of the circuit is usually enclosed in a temperature controlled compartment - DFM could be used for checking:
    • Oscillator (VFO or crystal)
    • Frequency output of each stage
    • Frequency of the final output of a transmitter, but not if the transmission is SSB or deeply modulated AM - A frequency counter should not be used to check that a transmitter is using the correct multiplication factor - should first be done with a simple absorption wavemeter

Modulation

Depth of Modulation

Method 1: modulation depth

Method 2: modulation depth

Dummy loads