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Frequency translation
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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.
Дата публикования: 2015-03-29; Прочитано: 184 | Нарушение авторского права страницы | Мы поможем в написании вашей работы!
