Cherokee CM10 / +
Documentation Project
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THEORY OF OPERATION

DESCRIPTION OF FREQUENCY DETERMINING AND STABILING CIRCUIT

INTRODUCTION

The frequencies for transmitter and receiver operation are Derived from a single 4.5 (MHz] crystal by means of a phase Locked loop.

The first local oscillator frequencies are 16.270 MHz (CH.1) to 16.710 MHz (CH.40). The second local frequency is fixed at 10.240 MHz TO GENERATE SECOND IF 455 (KHz). During transmit, the VCO of the PLL operates 13.4825 MHz (CH.1) to 13.7025 MHz (CH.40). The VCO frequency goes to the doubler circuit Q107, T9, T10 which doubles the frequency to generate 26.965 MHz (CH.40).

VCO Q101 TO doubler Q16 to transmitter
13.4825 MHz(CH.1) 26.96500 MHz (CH.1)
13.7025 MHz (CH .40)    27.60125[MHz](CH .40)

The VCO operating frequency for the receiver is 16.270 MHz (CH.1) to 16.710 MHz (CH.40) as the first local oscillator is injected through the buffer amp Q10 into the first TR mixer Q4

DESCRIPTION OF EACH BLOCK

INTRODUCTION

The synthesizer is implemented with the following components: PLL IC (IC901), X-TAL (XT901), VCO, VARICAP DIODE (D101) IC901 is a CMOS LSI that includes most of PLL block and LCD driver, The Q101, T100, C126, C125, C108, C112, C111, VARICAP diode D101 FORM a CLAPP oscillator circuit to operate as a VCO of the IC901. Q104 is a switching transistor to connect or disconnect the Tuning Capacitor in the VCO oscillator tank circuit for transmitter or Receiver. Q107 works as a buffer amp for RX local frequencies 16 MHz and TX carrier generating frequencies 13 MHz.

REFERENCE FREQUENCY

The crystal, XT901 4.500 MHz and other components at pin.1 and 80 of IC901 form a reference frequency oscillator with internal Amplifier.

VCO

Q101 and surrounding parts form a CLAPP oscillator which Works as a VCO for the CPU. With appropriate control voltage on D101, the VCO can oscillate over the required range of 13.4825 MHz TO 16.7100 MHz.

PROGRAMMABLE DIVIDER AND ITS CONTROL

The programmable inputs for each channel are set from inside The IC, each input signal to control the PLL IC is done with the provided key matrix input pins T1 (pin.15) to T5 (pin.18) and K0 (pin.10) to K3 (pin.14). For each key matrix input, an internal code from the EPROM provides the appropriate binary control of the programmable divider for that channel.

Since the binary number is required to change during transmit and receive, an additional bit is required at pin 66 of IC901 to allow the ROM to recognize the status is TX or RX. During transmit, the push to talk switch makes pin.66 of PLL IC high. The programmable divider output is fed to the phase detector for comparing with the 2.5 KHz reference frequency inside IC5.

PHASE DETECTOR AND VCO CONTROL

The phase detector is a digital phase comparator which compares the phase of the reference signal with programmable divider output square waves and develops a series of pulses whose dc level depends on the phase error of each signal. The phase detector pulse output is fed to an active low pass filter, and fed to VARICAP D101 to control the VCO frequency.

TRANSMITTER / RECEIVER BUFFER AMP

Output signal of Q101 is fed into the buffer amp Q107 to increase the strength of TX carrier frequency and 1st local frequencies. 

TRANSMITTER DOUBLER

The output signal of Q107 goes to an amplifier with tuning circuit Q601, T601, T602 which doubles incoming 13 MHz signals.

SWITCHING OF TUNING CAPACITOR IN VCO

The VCO circuit must tune with a wide range of frequencies 13.4825-13.7025 MHz for transmitter and 16.27 - 16.71 MHz for receiver. To comply with above range of VCO, the tuning capacitance is switched for transmission or reception. The tuning circuit consists of T100, C126, C125, C112 when the VCO is working as a receiver, Q104 is turned off. T100 AND C113 make up tuning function when transmitting Q104 turns on. T100 and the parallel capacitance of C125 and C126 make up the tuning function.

RECEIVER LOCAL OSCILLATOR OUTPUTS

1st mixer: the secondary output signal of T503 is injected to the source of 1st mixer Q502, Q505 in the 1st IF mixer section.
2nd mixer: the output OF 10.24 MHz oscillator circuit with XT201 is injected into the IF module IC201.
The incoming if signal and 10.24 MHz Signal are mixed inside the IF IC to extract 2nd IF signal 455KHz.

FREQUENCY STABILITY

LET:

Fo=   CRYSTAL OSCILLATOR FREQUENCY
Fr=   PHASE DETECTOR REFERENCE FREQUENCY
Fvco= VCO FREQUENCY
Ft=   TRANSMIT FREQUENCY

THEN Fr=Fo/2048

AND UNDER LOCKED CONDISTIONS: Fr=Fvco/N
WHERE, "N" is the divide ratio of programmable divider.
THEN : Fvco=N x Fr

From which it can be seen, the percentage error in Ft is the same as the percentage error in Fo. The stability of the crystal oscillator is determined primarily by the crystal itself and associated passive components of the oscillator. The choice of crystal and components is such that the required frequency stability is maintained over the required voltage and temperature range.

 DESCRIPTION OF OTHER CIRCUITS

TRANSMITTER

RF AMPLIFICATION

The output of doubler amp Q601 is fed through tuning circuit T601 and T602 to the base of pre driver amp Q602. The output is then supplied through capacitor C634 to RF driver amp Q602. The output of Q18 is split with tuning circuit L607, L608, C616 and goes to the base of final RF amp Q603. The output of Q603 is supplied to the antenna through L-C tuning circuit.

CIRCUIT FOR SUPPRESSION OF SPURIOUS RADIATION

The tuning circuit between the output of final amp Q603 and antenna is a 4-STAGE "PI" NETWORK. T604, C607, C620, L604, C609, C605, C606, L605 serves as a spurious radiation suppressor. This network also serves to match the impedance between TX power amp Q20 and the antenna.

MAXIMUM MODULATION CONTROL

Modulation signals are filtered with RC network and goes to the audio amp IC401 to produce a nominal signal level to achieve wanted modulation. To control incoming audio signal, Q402, 0405, and corresponding ALC circuit control the audio output level. Adjustment of RV-401 shall not exceed ±90% modulation with 1.0KHz], 20 dB up from 50% MOD level input.

RECEIVER

(1) RF AMP Q501 amplifies the RF signal from antenna. The amplified RF signal is fed through tuning coils T501, T502,T503,T504 TO THE GATE OF MIXER Q502, Q505. Also VCO frequency is lower by 10.695MHz than the frequency of each channel supplied to mixer.

(2) The source of 10.695MHz frequency is passed to tuning coil T503 and fed through MCF501 TO Q201.

(3) The 455KHz frequency signal produced from IC201 by mixing the output signal of FL1 10.695MHz filter and the 10.240MHz frequency from crystal oscillator XT201.

(4) Detected audio signal is fed through pin NO 13 OF IC201, TO LOW frequency audio amp for driving the speaker.

Disclaimer: Although the greatest care has been taken while compiling these documents,

we cannot guarantee that the instructions will work on every radio presented.