80 Circuit Descriptions TM8100/TM8200 Service Manual© Tait Electronics Limited November 20073.4 Frequency Control LoopIntroduction The FCL is included in the block diagram of the frequency synthesizer(see Figure 3.5 and Figure 3.6).The FCL forms part of the frequency-synthesizer module. The basis of theFCL is a VCXO, which generates the reference frequency required by themain PLL of the synthesizer.Elements ofFCL Circuitry The FCL is a simple frequency-locked loop. The circuitry consists of thefollowing elements:■ VCXO (XL501, Q501, Q503)■ TCXO (XL500)■ buffer amplifier (IC500)■ mixer (IC501)■ low-pass filter (IC502, pins 5 to 7)■ modulator buffer amplifier (IC502, pins 1 to 3).The A4 band has additional circuitry:■ 2.612MHz mixer (IC506)■ amplifiers (Q504, IC509)■ TCXO divide by 4 (IC508)The TCXO supplies a reference frequency of 13.0000MHz (10.4MHz forthe A4 band), which is extremely stable, regardless of the temperature.The VCXO runs at a nominal frequency of 13.0120MHz, and is frequency-locked to the TCXO reference frequency.Circuit Operation The VCXO is mixed with the TCXO output to create a nominal difference(or offset) frequency of 12kHz SYN CDC FCL. In A4 band radios, there isadditional circuitry and the VCXO is mixed with the TCXO to produce2.612MHz. This is then mixed with 2.6MHz (TCXO divided by 4) toproduce the 12kHz SYN CDC FCL.The signal SYN CDC FCL is fed via the CODEC IC502 in the CODECcircuitry to the FPGA on the digital board. The FPGA detects the offsetfrequency, compares it with the programmed offset frequency, and outputsa corresponding feedback signal CDC VCXO MOD via IC205. The feedbacksignal is amplified and inverted by the modulator buffer amplifier and outputas the loop voltage for the VCXO. With this design the VCXO frequencycan be adjusted by very small precise amounts, and because the loop islocked, the VCXO inherits the temperature stability of the TCXO.Modulation The FCL modulation is implemented within the FPGA and appears at theoutput of IC205, and therefore on the VCXO loop voltage. Consequently,the VCXO is frequency modulated directly by the relevant modulationinformation. The latter may be the microphone audio, an audio tap-insignal, internal modem signals, or any combination of these.
