Bryan G8DKK Outline AM and FM Waveforms FM Transmission Modulation Frequency Deviation Mic Gain Control LimiterLimiting amplifier Occupied Bandwidth FM Receivers Receiver bandwidth Filters ID: 1031590
Download Presentation The PPT/PDF document "Frequency Modulation (FM)" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
1. Frequency Modulation (FM)Bryan, G8DKK
2. OutlineAM and FM WaveformsFM TransmissionModulation FrequencyDeviationMic Gain ControlLimiter/Limiting amplifierOccupied BandwidthFM ReceiversReceiver bandwidth (Filters)Limiting amplifierSquelch – noise gateCapture effect
3. Amplitude Modulation (AM)RF CarrierAudio InputAM Signal
4. Frequency Modulation (FM)FM SignalRF CarrierAudio Input
5. WaveformsDemo of: Oscilloscope – Time Domain Spectrum Analyser – Frequency Domain Waveforms
6. FM Transmitter
7. Mod. Frequency RangeNormally taken to be300Hz to 3kHzThe average male voice has few frequencies, other than harmonics, above 2kHzFemale voices are pitched a little higherHarmonics at low frequencies e.g. 500Hz and lower fall within the normal range
8. FM DeviationDefined as:The frequency excursion of the carrier frequency due to the applied modulation.It can be specified as:PeakPeak to Peak (~2 x Peak)Mean (average)Modulation Meter
9. Controls in a FM Mobile
10. Occupied BandwidthThe total bandwidth used by a FM transmitterHopefully, less than the allocated channel width!In the 2m bandplan channel spacing is 12.5kHzA tiny bit of maths:Carson’s Rule: for mod index less than 1FM Bandwidth = 2 x Fmod + 2 x FdevExample: Fm = 2kHz and Fd = 3kHzBW = 4 + 6 kHz = 10kHzDemonstration with a Modulation Meter and a SDR
11. FM Receiver
12. FM ReceiverReceiver IF filterDetermines the maximum bandwidth that the receiver will accept without distortion.
13. FM Receiver
14. FM Receiver Off TuneWhat is the effect of a FM Rx being off-tune?Or a FM transmitter?In a 12.5kHz system a 1kHz offsetSounds like a little “splatter”The outer sidebands on one side are being rejected by the filter slopeSolutions:Use RIT?? Offset the VFO tuning?
15. FM Receiver block diagramRF AmpMixerIF FilterIF AmpLimiting AmpDemodSwitchAF AmpLOSquelch Control
16. Squelch (Noise) GateWhen there is no signal at the receiver input the limiting amplifier is driven to its maximum output.A separate detector detects this condition and drives an amplifier that has a threshold control – usually a front panel control labelled Squelch.The output from this amplifier is used to drive a switch that switches off the AF output amplifier.A received signal “quiets” the receiver and disables the switch.Demo with signal generator and FT60
17. Capture EffectKnown as “FM capture effect”:Where a second signal in the same channel that may be only 2 or 3dB stronger will eliminate the original signal.Demo with a signal generator.
18. Key PointsThe relationship between:Deviation & mic gainDeviation, mod frequency and signal bandwidthReceiver and/or Transmitter tuningSquelch operationSignals on the same frequency
19. Digital ModulationThere are many forms of digital modulation employing forms of FM or PMFSK, PSKBPSK, QPSKThe FM part tends to “blur” a bit as the complexity increases e.g. MSK, GMSK etc.Maybe a subject for another time
20. EndQuestions?