# Transmitters

## Modulation methods

### Amplitude modulation

It can be shown that the effective power in a carrier wave modulated to a depth of 100% by a sinusoidal modulating signal (ie a single pure tone) is 1.5 times the unmodulated carrier power. Thus to fully modulate the carrier, the power in it must be increased by 50%.

This extra power, which is in fact 'added' to the HT supply to the transmitter PA stage, must be supplied by the 'modulator', which is a fairly high-power audio-frequency amplifier. To fully modulate a transmitter operating at an input of 150W to the PA stage would require an AF power of 75W.

Fig 4.13. Amplitude modulation using transformer coupling to PA stage

The output of the modulator is coupled into the HT supply to the PA by the modulation transformer as shown in outline in Fig 4.13. The ratio of this transformer is determined by the output impedance of the modulator and the modulating impedance of the PA stage.

This is the most effective method of amplitude modulation as the PA stage operates in Class C, giving the highest efficiency. The disadvantage is that large and expensive modulating equipment is necessary for full-power operation, ie as indicated above the basic power required would be 75W, to which must be added an allowance to cover losses in the modulation transformer, so that the design aim should be 100W or so.

### Frequency modulation

Frequency modulation is modulation of a carrier wave created by the variation of the carrier frequency by the modulating voltage. The variation (deviation) is positive and negative from the actual carrier frequency (ie the centre frequency).

Fig 3.18. Typical variable-capacitance diode application

FM may be achieved by the direct action on the tuned circuit of the fundamental oscillator by a varactor diode. This is shown in outline in Fig 3.18. The diode would be biased so that the variation in capacitance upwards and downwards by the modulating voltage is both equal and linear.

It must be remembered that when the final frequency of an FM transmitter is obtained by multiplication, the deviation of the fundamental is also multiplied.

At VHF, crystal control is common, eg at 144MHz multiplication of x18 is typical. For a final deviation of 2.5kHz, the fundamental oscillator has to be deviated by less than 200Hz. The frequency of an 8MHz crystal oscillator can be pulled by this small amount by means of a varactor.

### Single sideband

The single-sideband (SSB) transmitter performs two distinct functions. These are:

(a) suppression of the carrier wave; and
(b) elimination of one sideband.

The carrier is suppressed by feeding the output of the carrier frequency oscillator and the modulating voltage into a circuit known as a 'balanced modulator'. This is a form of bridge circuit which, when correctly balanced, will cause the RF input lobe suppressed, so only the two sidebands appear at the output. The unwanted sideband is removed by a bandpass filter about 2.7kHz wide, shape factor 2-2.5. If for any reason suppression is not 100%, the possibility of some unwanted radiation occurring outside the band must be borne in mind if one is operating right on the band edge.

This combination, shown in block form in Fig 4.14, is called the 'sideband generator'. The single sideband so produced is in fact a band of frequencies corresponding to one sideband of an AM system. It can be produced at a fairly low frequency, typically 455kHz, in which case the frequency band is 455-458kHz. Alternatively the sideband can be generated in the megahertz region, eg at 9MHz, in which case the frequency band would be 9-9.003MHz.

From consideration of a typical band of frequencies, it is clear that multiplication cannot be used to cover several bands. For example. suppose the sideband is generated at 3.5MHz. The frequency band is then 3.5-3.503MHz. If this is doubled it becomes 7-7.006MHz, ie the width of the sideband is also doubled. This is obviously not acceptable and hence frequency translation to different bands must be done by the process of frequency mixing as mentioned in Chapter 2.

Fig 4.14. Block diagram of SSB generator

Fig 4.15 is a block diagram of a mixer-type exciter in which the outputs of a VFO and a crystal oscillator are mixed to produce an output in the required amateur band.

The VFO will typically cover a range of 500kHz; its actual frequency (order of 3-8MHz) and the crystal frequencies must be chosen so that the wanted and unwanted products in the output of the mixer are as far apart in frequency as possible. The mixer is followed by a tuned amplifier having coupled tuned circuits in the output to improve the rejection of the unwanted product.

Fig 4.15. Block diagram of mixer-type VFO

This exciter followed by a conventional PA is a preferred, although complex, alternative to the frequency multiplier circuit for the following reasons:

(a) the absence of internally generated frequencies which may be radiated and so cause interference to other services; and
(b) the overall frequency stability is improved because it is constant and not dependent on the band in use.

For the reasons discussed earlier it must be used in the SSB transmitter.

Depending upon the frequency at which the sideband is generated, one or more mixing processes may be required to reach the final frequency and to introduce the output of a VFO.

Fig 4.16. Block diagram of SSB transmitter

Fig 4.16 is a block diagram of an SSB transmitter in which the SSB is generated at a high frequency, say 9MHz, hence only one mixing process is needed to translate the SSB generation frequency to the output frequency. This diagram is a combination of Figs 4.14 and 4.15.

The PA of the SSB transmitter has to amplify a modulated RF signal without distortion. It therefore operates in Class B rather than Class C and is known as a 'linear amplifier' as the relationship between the output and input is linear.

By convention the lower sideband is transmitted at radio frequencies below 10MHz and the upper sideband above 10MHz. The sideband required is selected by switching the crystal in the sideband generator (see Fig 4.14).

For a number of reasons, the sideband transmitter is combined with the appropriate receiver giving rise to the 'transceiver'. This combination is discussed in Chapter 5.

The modulation systems described above are utilised for the transmission of telephony. The UK licence also permits the use of other more specialised modes of transmission such as radio teletype (RTTY), high-definition television (HDTV), slow-scan television (SSTV), Packet etc.

These modes are achieved by basically similar forms of modulation. Their use is quite small compared with telegraphy and telephony; they are not included in the RAE syllabus and hence are not covered in this manual. Included below are some sound samples, of the various modes, plus QSO samples to give a taste of what can be heard. PLEASE NOTE, some of these files are quite large (2.5Mb), and could take some time to download if you are not using this course on a CD ROM.

The files are available in Windows' .WAV format or MP3. Press for WAV format, press for MP3

CW (morse) QSO | AMTOR
FM QSO | Helscriber
SSB (voice) tuned high in frequency | PSK31
SSB (voice) tuned low in frequency | RTTY
SSB (voice) tuned in properly | Packet Radio
| SSTV

### Transmitter power level

The UK amateur licence now defines maximum power on all bands and modes in terms of output rather than input. The actual levels are quoted in 'dBW', ie so many decibels above one watt.

Powers in watts and dBW are:

26dBW = 400W | 16dBW = 40W
22dBW = 160W | 15dBW = 32W
20dBW = 100W | 9dBW = 8W

In a CW, AM or FM transmitter it is considerably easier to measure input power than output power. This is because input power is the product of the direct voltage applied to the output stage and the direct current drawn by the output stage. This current does not vary.

The inputs corresponding to the maximum carrier powers now permitted for modes other than SSB (note that there is no change as regards SSB) are shown in Table 4.2. They assume an output stage efficiency of 66 and 55% for output stages operating in Class C and Class B respectively.

Table 4.2. inputs and maximum permitted carrier powers

Output | Input
| Class C | Class B
26dBW | (400W) | 600W | 720W
20dBW | (100W) | 150W | 180W
16dBW | (40W) | 60W | 72W
9dBW | (8W) | 12W | 14W

### Derivation of the SSB power level

In an AM transmitter, the maximum output power permitted (in the absence of modulation) is 100W (20dBW).

When the transmitter is 100% modulated, the amplitude of the peaks of the modulation envelope is twice that of the unmodulated carrier wave (see Fig 4.12). As we are considering a voltage waveform, and since power equals V2/R (R is the value of the load resistor and V is the unmodulated carrier amplitude), the output power of each cycle of RF energy at the peaks of modulation is (2 V)2/R (because V is the unmodulated carrier amplitude and hence the modulated earner amplitude is 2V). This is equal to 4V2 /R or four times the unmodulated carrier power. Hence the power at the peak of the modulation envelope or the 'peak envelope power' is 4 x 100 or 400W.

This is the maximum power permitted by the UK amateur licence.

Back | Index | Next