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oscilloscopes used for radio analysis should have:
can show waveforms of radio frequency (RF) signals, letting engineers inspect modulation characteristics (AM, FM, PM), distortion, envelope, and signal integrity
by calculating the period and frequency of signals, oscilloscopes help verify oscillator performance and monitor RF carrier stability
with FFT capabilities, modern oscilloscopes allow users to analyze spectra of RF pulses and carriers, similar to what spectrum analyzers do. This is vital for examining modulation processes, harmonics, and identifying sources of interference
can trace signals through radio circuits, aid in gain measurements, detect faults, and help isolate problems in oscillators, RF amplifiers, mixers, detectors, and audio stages
time-domain features enable precise measurement and triggering on radar and pulsed radio signals. Oscilloscopes can analyze pulse width, repetition rates, and envelope variations critical for radar and communication systems
checking if a local oscillator in a radio receiver is working by probing for expected oscillation waveforms
analyzing and measuring signal modulation to assess transmission quality and antenna performance
comparing input and output waveforms for power gain measurements across amplifier stages
monitoring pulsed RF signals in radar systems, evaluating modulation strategy and signature via time-domain and frequency-domain displays
measuring transmitter radio output power:
WARNING: ensure the voltage output will not exceed the max. voltage input for the scope otherwise it may get destroyed!
if measuring a transmitter radio power output, need to use a T-adapter with a 50 Ohm dummy load on the dummy side of the T to avoid reflected power back into the radio and potentially damaging it.
Power in W = (rms voltage)2/actual resistance of the dummy resister
power in W = 10(dBm/10) x 10-3
NB. for a AM radio transmitter such as CB radio, the un-modulated carrier wave power is usually 30-60% of the max AM modulated power output of the radio
a 4W radio may give ~11Vrms / 2.4W when unmodulated and ~14Vrms / 4W at peak modulation (loudest sound being sent)
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tuning an antenna
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impedance of antenna using his circuit = 50 Ohm x measured voltage / (input voltage - measure voltage)
where input voltage has a zero phase component as this is the reference signal from a signal generator
and measured voltage is a complex number = voltage + phase shift in nsec time delay (time delay as compared with input voltage signal)
measure length and impedance of coax
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uses speed of signal in coax based on a typical coax cable velocity factor (VF) of 0.66 (range 0.6-0.85) of speed of light in free space of 11.8“ or 30cm/nsec (this VF is related to the dielectric constant) which gives 7.79”/nsec speed in coax
using a T on the scope input with one side coming from the signal generator giving a square wave pulse and the other side connected to your open ended coax
the signal on the scope will have a step before the voltage continues to go up from the reflected pulse coming back from the end of the coax
the twice the length of the coax is then simply the width of this round trip step in nsec x 7.79“/nsec - so this needs to be halved to get length of coax
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you can measure the impedance of the coax by attaching a variable resistor to its end and adjust the resistance until the scope shows loss of the step displayed above and then the impedance is that resistor value as measured by an ohm meter