WEBVTT NOTE Modulation, Overmodulation, and Link Budgets 1 00:00:00.000 --> 00:00:13.397 One carrier, three things you can change. A sine-wave carrier has exactly three properties available to modulate, and each gives its own family of modes. 2 00:00:13.397 --> 00:00:30.471 Change this, instantaneous power level; Get this, amplitude modulation. Change this, instantaneous frequency; Get this, frequency modulation. Change this, phase angle; Get this, phase modulation. 3 00:00:30.471 --> 00:00:42.117 The pool asks each of these directly, in those words. Worth having verbatim. And one implementation detail asked as its own question: 4 00:00:42.117 --> 00:00:51.517 Question. What emission is produced by a reactance modulator connected to a transmitter RF amplifier stage? 5 00:00:52.417 --> 00:00:55.697 Answer. Phase modulation. 6 00:00:56.297 --> 00:01:16.975 A reactance modulator varies a reactance in the oscillator or amplifier circuit, which shifts phase. FM and PM are closely related, FM is the integral of PM, and this is why the pool routinely pairs them as "FM or PM". 7 00:01:16.975 --> 00:01:18.777 Digital modulation. 8 00:01:18.777 --> 00:01:25.314 Question. How is direct binary FSK modulation generated? 9 00:01:26.214 --> 00:01:33.266 Answer. By changing an oscillator's frequency directly with a digital control signal. 10 00:01:33.866 --> 00:01:42.506 Frequency shift keying in its simplest form: two frequencies, one per bit. The two are named: 11 00:01:42.506 --> 00:01:51.542 Question. How are the two separate frequencies of a Frequency Shift Keyed (FSK) signal identified? 12 00:01:52.442 --> 00:01:56.096 Answer. Mark and space. 13 00:01:56.696 --> 00:02:02.136 Question. What type of modulation is used by FT8? 14 00:02:03.036 --> 00:02:07.226 Answer. 8-tone frequency shift keying. 15 00:02:07.826 --> 00:02:14.168 Eight tones rather than two, so each symbol carries three bits. 16 00:02:14.168 --> 00:02:19.208 Question. What is QPSK modulation? 17 00:02:20.108 --> 00:02:34.078 Answer. Modulation in which digital data is transmitted using 0-, 90-, 180- and 270-degrees phase shift to represent pairs of bits. 18 00:02:34.678 --> 00:02:41.773 Quadrature phase shift keying: four phase states, two bits per symbol. 19 00:02:41.773 --> 00:03:03.969 And QPSK31, the pool answers "all these choices are correct", is used for keyboard-to-keyboard on HF, has a 31 Hz bandwidth, and its forward error correction is exactly what distinguishes it from plain PSK31. Bandwidth. 20 00:03:03.969 --> 00:03:14.510 There is a diagram here. Horizontal bars on a logarithmic frequency axis from 100 hertz to 10 megahertz. 21 00:03:14.510 --> 00:03:36.706 PSK31 occupies about 31 hertz, FT8 about 50 hertz, CW about 150 hertz, single sideband voice about 3 kilohertz, FM voice on VHF repeaters between 10 and 15 kilohertz, and analogue fast-scan television about 6 megahertz. 22 00:03:36.706 --> 00:03:43.146 Question. Which of the following phone emissions uses the narrowest bandwidth? 23 00:03:44.046 --> 00:03:47.286 Answer. Single sideband. 24 00:03:47.886 --> 00:04:08.696 Among voice modes, SSB. It is AM with the carrier and one sideband removed, so it occupies a third of the space and puts all the power into information. Overmodulation. Three questions circling the same failure. 25 00:04:08.696 --> 00:04:14.143 Question. What is the modulation envelope of an AM signal? 26 00:04:15.043 --> 00:04:21.820 Answer. The waveform created by connecting the peak values of the modulated signal. 27 00:04:22.420 --> 00:04:34.780 The envelope is the shape traced by the carrier's peaks, the audio waveform riding on the RF. On an oscilloscope it is the outline you actually see. 28 00:04:34.780 --> 00:04:42.620 Question. What is meant by the term "flat-topping," when referring to an amplitude-modulated phone signal? 29 00:04:43.520 --> 00:04:50.240 Answer. Signal distortion caused by excessive drive or speech levels. 30 00:04:50.840 --> 00:05:03.470 Drive the transmitter past its linear range and the envelope's peaks are clipped flat instead of rounded. The distortion that follows is not confined to your channel: 31 00:05:03.470 --> 00:05:08.983 Question. Which of the following is an effect of overmodulation? 32 00:05:09.883 --> 00:05:13.203 Answer. Excessive bandwidth. 33 00:05:13.803 --> 00:05:30.191 Clipping generates harmonics of the audio, and those appear as sidebands well outside your intended 3 kHz. This is splatter, and it is what the neighbours three kilohertz away hear. 34 00:05:30.191 --> 00:05:47.123 The chain to remember: too much drive → flat-topping → distortion → harmonics → excessive bandwidth → interference. One cause, and the last link is what other people notice. Link budgets. 35 00:05:47.123 --> 00:05:50.843 Question. What is a link budget? 36 00:05:51.743 --> 00:06:00.440 Answer. The sum of transmit power and antenna gains minus system losses as seen at the receiver. 37 00:06:01.040 --> 00:06:21.730 Everything in decibels, added up along the path: P received equals P tx plus G tx minus L path minus L lines plus G rx This is the decibel arithmetic from the foundations track, put to work. 38 00:06:21.730 --> 00:06:29.680 Gains are positive, losses negative, and the whole path becomes addition. 39 00:06:29.680 --> 00:06:33.360 Question. What is link margin? 40 00:06:34.260 --> 00:06:43.585 Answer. The difference between received power level and minimum required signal level at the input to the receiver. 41 00:06:44.185 --> 00:06:59.166 The budget tells you what arrives. The margin tells you how much more than you need, and therefore how much fading the link survives before it fails. 42 00:06:59.166 --> 00:07:06.807 A link with 3 dB of margin drops out constantly; one with 20 dB is reliable. 43 00:07:06.807 --> 00:07:24.905 Worked, in the pool's own style: 10 W (+40 dBm) transmit, 10 dBi antenna, 3 dB cable loss, 136 dB path loss, and a receiver needing −90 dBm: Received = 40 + 10 − 3 − 136 = −89 dBm. 44 00:07:24.905 --> 00:07:42.802 Against a −90 dBm requirement, the margin is 1 dB, technically a link, practically not one. Check yourself. Which modulation changes the instantaneous power level of the carrier? 45 00:07:42.802 --> 00:08:01.905 A station reports you are 5 kHz wide on SSB. What is happening and what is the fix? Your link budget gives −85 dBm and the receiver needs −95 dBm. What is the margin, and is the link usable? 46 00:08:07.905 --> 00:08:27.556 Amplitude modulation.. Overmodulation, excessive drive or speech level, causing flat-topping, distortion, and the harmonics that widen the signal. Reduce microphone gain and drive. 10 dB of margin. 47 00:08:27.556 --> 00:08:32.145 Usable, and enough to survive ordinary fading.