Tait TP9100 - Frequency Synthesizer and Control Loops

44 Description TP9100 Service Manual
© Tait Electronics Limited May 2005
2.6.2 Frequency Synthesizer
Introduction As shown in Figure 2.6 , the frequency synthesizer consists of two main parts:
■ FCL
■ RF PLL, comprising RF PLL device, loop filter, VCO, and VCO output
switch
Frequency Control
Loop
The FCL consists of the following:
■ TCXO
■ mixer
■ loop filter
■ VCXO
■ frequency control block
The FCL provides the reference frequency for the RF PLL. The FCL
generates a high-stability reference frequency that can be both modulated and
offset in fine resolution steps.
RF PLL ■ The RF PLL consists of the following:
■ RF PLL device
■ loop filter
■ VCO
■ VCO output switch
The RF PLL has fast-locking capability but coarse frequency resolution.
This combination of control loops creates improved frequency generation
and acquisition capabilities.
Note that patents are pending for several aspects of the synthesizer design.
Operation of
Control Loop
The RF PLL is a conventional integer-N-type design with frequency
resolution of 25kHz.
In transmit mode, the loop locks to the transmit frequency, whereas in receive
mode, it locks to the receive frequency minus the first IF frequency.
Initially, the VCO generates an unregulated frequency in the required range.
This is fed to the PLL device (ADF4111) and divided down by a programmed
ratio to approximately 25kHz. The reference frequency input from the FCL is
also divided down to approximately 25kHz. The phase of the two signals is
compared and the error translated into a DC voltage by a programmable charge
pump and dual-bandwidth loop filter. This DC signal is used to control the
VCO frequency and reduce the initial error. The loop eventually settles to a
point that minimizes the phase error between divided down reference and
VCO frequencies. The net result is that the loop “locks” to a programmed
multiple of the reference frequency.