T.E.M. A07B500S, Manual

Page: 36 / 131

A07B500S manual
Pag. 36
5.2 SINTD board
SINTD board is placed at the rack centre, directly connected to the mother board from which it can
be quickly removed. It has the function of frequency synthesized oscillator (88–108MHz)
modulated by the audio composite signal.
The FET Q1 is the core of the board and oscillates at the set and controlled frequency. All the
techniques to obtain high performances in terms of noise and modulation linearity have been
adopted. Moreover for a decade T.E.M. already have been adopting these circuits solutions
(oscillators with coaxial line) for frequencies even till 3GHz for FM transmitters and audio links.
Eight varicaps DV1-DV8 modulate the oscillator being driven by the Q2 low output impedance
which reduces Nyquist this way wide band noise produced by the variable capacity diodes; at
1MHz between the carrier, the SSB noise is already better than –145dBc, in accordance to ETS-
ETSI-300-384. The Q3 transistor reduces the flicker-noise due to the power supply; the D3-D4
series doesn’t allow the Q1 saturation, while Q4 and Q5 uncouple the oscillator from the following
amplification stages. The U14 output has a power of 10dBm.
The Q6 transistor leads the oscillator signal into the prescaler of the PLL circuit (U4); this
integrated circuit performs all the frequency synthesis functions: it’s set by U1 ports through the
main microprocessor placed on the DLCD board. The reference frequency (10MHz) is produced by
Q7.
The error amplifier of the phase comparator internal to the PLL chip is composed by U13 and U2
and it has a closed loop cut frequency lower than one Hertz, so that the lowest frequencies of the
modulating stereophonic signal can maintain a separation higher than 50dB between the two
channels. The modulation, coming from the mother board and from the AUDIO-IN board, is simply
added to the VCO error voltage, no linearization has been provided to make the deviation constant
versus the output frequency; typically the deviation error is contained within 0,1 dB all over 20
MHz band.
The oscillator has been carefully shielded to avoid that close transmitters could induce spurious
frequencies on the output.