The Channel Pll - RF Technology Eclipse Series Operation And Maintenance Manual

5 CIRCUIT DESCRIPTION
The bias applied to varactor D600 is a combination of the potentials at two DAC
outputs (MOD_ADJ and MOD_ADJ_FINE), plus the modulating signal arriving at
MOD_IN (which is the same signal as MOD_OUT in Sheet 3).
The summing of these three voltages is performed by U607.
The Phase detector output of the PLL chip is then passed through the loop filter network
defined by C612, R618, C625, R617, C613, and C718 (see Sheet 7). L713 is used to
filter out any residual noise (outside of the audio bandwidth), including the phase
detector frequency, and/or any switch-mode noise from the dc voltage rails.
The loop filter signal is then fed as a control voltage to the Modulation VCO
(MOD_PLL_IN).
The output of the Modulation VCO is connected back to the PLL for phase detection via
signal path MOD_VCO_OUT.
The phase detector output is also buffered and attenuated for the analogue input of the
CPU. This is the function of U600, R622, and R625. In this way the CPU can monitor
the VCO bias to ensure that it is within specification (>0.5V, and < 4.5V).
The FoLD pin, of U602, can be used for many purposes. It can be connected to the
output of any of the 4 internal dividers, or be used as a LOCK-DETECT monitor, or as
a user programmable output pin. In this circuit it is used as a LOCK-DETECT output
when the frequency is being changed, but otherwise it is connected internally to the
unused reference divider of U602, to deliver a 400Hz pulse train to FoLD.
U602 is set up with a phase detector frequency of 20kHz.

5.6.2 The Channel PLL

U604 and the Channel VCO (see Sheet 7) form the Channel PLL. U604 acts as its own
crystal oscillator for its reference oscillator. X601 is a 5ppm, 12MHz, crystal. Its
resonant point is adjusted by the bias applied to varactor D601.
The bias applied to varactor D601 is adjusted by the CHAN_ADJ DAC output.
The Phase detector output of U604 is then passed through the loop filter network
defined by C622, R620, C626, R619, C623, and C725 (see Sheet 7). L718 is used to
filter out any residual noise (outside of the audio bandwidth), including the phase
detector frequency, and/or any switch-mode noise from the dc voltage rails.
The loop filter signal is then fed as a control voltage to the Channel VCO
(CHAN_PLL_IN).
The output of the Channel VCO is connected back to the PLL for phase detection via
signal path CHAN_VCO_OUT.
The phase detector output is also buffered and attenuated for the analogue input of the
CPU. This is the function of U608, R623, and R624. In this way the CPU can monitor
the VCO bias to ensure that it is within specification (>0.5V, and < 4.5V).
Page 22 RF Technology T50