Crown FM150, User Manual

Page: 48 / 110

4.3 RF Exciter Circuit
This circuit is also known as the Frequency Synthesizer. The Frequency Synthesizer part of
the motherboard is no longer a separate module as was the case on older transmitters.The
entire component side of the motherboard is a ground plane. Frequency selector switches
located on the front panel of the transmitter establish the operating frequency. The VCO
(voltage-controlled oscillator) circuitry is inside an aluminum case.
Illustration 6-6 and accompanying schematics can be used as reference in this
discussion.
VCO1 operates at the synthesizer output frequency of 87 MHz to 108 MHz.
The frequency is controlled by a voltage applied to pin 8 of the VCO.
A sample of the RF comes from A2 and is fed to the PLL chip U13.
U13 is a phase-locked-loop frequency synthesizer IC. The 10.24 MHz from the
crystal oscillator is divided to 10 kHz. Internal programmable dividers divide the
87 - 108 MHz RF to 10 kHz. Differences between the two signals produce error signals at
pins 7 and 8 of U14.
Illustration 4–2 Motherboard (Exciter Section)
Frequency selector switches are read by shift registers U17 and U18. Data from the shift
registers is read by U16 which then programs the PLL (Phase Lock Loop) IC U13.
U14B is a differential amplifier and filter for the error signal. Audio that is out of phase with
that appearing on the error voltage is introduced by U14A., allowing for greater loop band-
width with less degradation of the low frequency audio response.
Lock and unlock status signals are available at the outputs of U15E and U15F respectively.
Modulation is introduced to the VCO though R72 and R122.
Exciter Section
4-6 FM30/FM150/FM300 User’s Manual