2 Read & translate the text (orally) 20.1 – 20.3.4:

3 Find Russian equivalents:

side by side	the mixing process
the opposite process	the recovered channel
the required set of frequencies	sufficient selectivity
the lower order demodulating equipments	the lower order of translation far economic reasons
the drive	different bandwidth capability
odd harmonics of the carrier frequency	in the gaps between the speech signals
different transmission media	at a premium

4Find English equivalents:

частотное разделение каналов	выбирается
отметить период несущей	сдвиг частоты
подавлять	нужная (требуемая) полоса частот
соседние каналы	не желаемые продукты
развитие цифровых систем	проникновение
разделение каналов	перегрузка
конечные этапы модуляции	увеличение мощности канала

5 Answer the questions:

1. What principle is illustrated in fig 20.1?

2. What are channels C1 & C2 mixed with?

3. Why f1 &f2 separately generated at B?

4. What is the function of filters Fa1 &Fa2?

5. What information can the signal contain?

6. What transmission media are available?

7. What provides a 4/3 increase in channel capacity?

8. Where are groups and supergroups used?

PART 2 (20.4 – 20.6)

20.4 Frequency translation

The ring bridge modulator/demodulator typically provides the general features for frequency translation. With reference to Figure 20.7, a sinusoid carrier is supplied at a high level (+10dBm to +13 dBm) sufficient to forward bias the diodes Di and D2 or D3 and D4 depending on the polarity of the carrier. The signal path, also through the diodes, is thus inverted on alternate half cycles of the carrier.

The current flowing, i (t), due to the carrier signal is close to a square wave due to the clipping action of the diodes and by Fourier analysis is given by Equation 20.1, where k^ k₃, k₅ etc. are circuit constants.

If the input signal is represented by i . (t) then the resulting modulated waveform at the output is given by Equation 20.2. i.e. the output frequency spectra is formed by the upper and lower side­bands about the carrier frequency and odd harmonics of the carrier frequency.

Note that neither the carrier frequency, carrier frequency har­monics, nor the original baseband signal is present at the output, i.e. the modulator is 'balanced'.

20.4.1 Ring bridge modulator/demodulator design considerations

20.4.1.1 Carrier compression.

With this type of modulator the ratio of the change in carrier power to the change in signal loss of the modulator, known as 'carrier compression', is approximately 10:1 so that accurate level stability of the carrier is not required.

20.4.1.2 Carrier and signal suppression

This is also known as carrier 'leak' and signal 'leak'. Perfect balance of modulator referred to above is not possible in practice. The best balance that can be achieved in a manufacturing environment is in the order of 30dB.

There will therefore be some unwanted products at the output of the modulator that will have to be removed by filtering before presentation to the combined path. Filter Fa (see Figure 20.1) requirements are shown i n Table 20.1. The requirement is higher for the signal leak and the upper sideband as these products will give rise to crosstalk.