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12. Meters

Definitions

  • Cathode Ray Oscilloscope / CRO / scope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time
  • Absorption wave meter is a simple electronic instrument used to measure the frequency of radio waves
  • SWR / VSWR (voltage standing wave ratio) meter measures the standing wave ratio in a transmission line. The meter can be used to indicate the degree of mismatch between a transmission line and its load (usually a radio antenna), or evaluate the effectiveness of impedance matching efforts.

Diagrams

  • Modulation

Circuit diagrams

  • Absorption wave meter circuit 1

  • Absorption wave meter circuit 2

Formula

  • Average DC of peak AC = 0.637 * peak AC
  • Depth of modulation = (A - B) / (A + B) where A is maximum envelope height, B is minimum envelope height

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

  • Modulate the transmitter with a sine wave
  • Connect the transmitter to a suitable dummy load and connect the CRO Y plates across it

Method 1: modulation depth

  • apply the CRO's own timebase to its X plates
  • modulation envelope will be displayed
  • shows a high frequency carrier wave whose amplitude follows the shape of the modulating signal
  • maximum (A) and the minimum (B) dimensions are them measured
  • CRO display does not need to be calibrated as it is just the ratio of A and B that is used in the depth of modulation calculation

Method 2: modulation depth

  • time-base is disconnected from the X plates of the CRO
  • replaced by a proportion of the audio modulating signal
  • This will give a trapezium display on the CRO screen

Dummy loads

  • When setting up (tuning up) or testing a transmitter, it is important that minimum interference is caused to other stations
  • most of this work should be done with the transmitter not connected to the aerial
  • Aerial must be disconnected and power must be suitably absorbed
  • dummy load must be of an impedance to match the transmitter output usually 50Ω / 75Ω for handling ~100W
  • construct a 50Ω dummy load from 10 X 500Ω in parallel, with each resistor rated at 10W
  • you can use a lower rated resistor if they are immersed in oil