Midland Alan48 ExcelMulti Circuit Diagram

Description Of Frequency Determining And Stabilizing Circuit Introduction

[MHz] (CH.40)

The frequencies for transmitter and receiver local frequencies are all derived a signal 4.5 [MHz]
crystal by means of a phase locked loop. The first local oscillator frequencies are 16.27 [MHz]
(CH.1) to 16.71 [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 doublers circuit Q301, L301, L302
which doublers the frequency to generate 26.965 [MHz] (CH.1) to 27.405 [MHz] (CH.40).

To Transmitter

~ 13.7025 [MHz] (CH.40) ~27.405
The VCO operating frequency for the receiver is 16.27 [MHz] (CH.1) to 16.71 [MHz] (CH.40) as

the first local oscillator, injected through the buffer amp Q506 into the first FET balanced mixer
Q107, Q108

Descriptions Of Each Block

1. Introduction

X-TAL (X401)
VCO, Varicap Diode (D503,504)
IC101 is a MCU that includes LCD driver and all of control circuit.
The PLL IC (IC6) of PLL block,

2. Reference Frequency

Q503 Q504

VCO

13.4825[MHz] (CH.1) 26.965 [MHz] (CH.1)

The synthesizer is implemented with the following

Components : PLL IC (IC6)

The Q503, L404, C511, C512, C513, C507,Varicap diode D503 and D504 are clap oscillator
circuit to operate as a VCO.
The Q505 is a switching transistor to turn on or turn off the tuning capacitor in the VCO
oscillator tank circuit for transmitter or receiver. The Q506 works as a buffer amp for RX
local frequencies (16 [MHz]) and TX carrier generating frequencies (13 [MHz]).

The crystal, X401(4.5 [MHz]) and other components at pin1 and 16 of IC6 can make a
reference frequency oscillator with internal amplifier.

2-1. VCO

The Q503 and surrounding parts are consisting a clap oscillator work as a VCO of IC6 with
appropriate control voltage on D503 and D504, the can be oscillate over the required range
of 13.4825[MHz] to 16.71 [MHz].

2-2. Programmable Divider And PLL Data Control

The MCU(IC101) and PLL IC (IC6) communicate by serial data for PLL IC data setting.
The programmable inputs for each channels set by MCU (IC101). Each input signal to
control the PLL IC is done with the provided key input pins 20 and 21 of IC101.

Q301

Doubler

2-3. 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 RC low pass filter
output signal of IC6 is filtered and fed to varicap D503 and 504 control the VCO frequency.

TRANSMITTER / RECEIVER BUFFER AMP

Output signal of Q503 is fed into the buffer AMP Q506, L401 to increase the strength of TX
carrier frequency and 1st local frequencies.

Transmitter Doublers
The output signal of Q506, L401 goes to an amplifier with tuning circuit Q301, L301, L302
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 ~ 16.710 [MHz] for receiver. To comply above range of VCO, the
tuning Capacitance should switched for transmission or reception.
The tuning circuit consists with L401, C507, C512, C513, C522, when the VCO is working
as a receiver Q505 becomes turn off. So, L401 AND D503,504 makes tuning function.
When transmitting Q505 becomes on. So, L401 and the parallel capacitance of C522 and
D503,504 make turning function.

Receiver Local Oscillator outputs

1ST mixer : The secondary output signals of L401 is injected to the source of 1ST mixer
Q107,Q108 in the 1ST IF mixer section.

2ND mixer : The output of 10.24 [MHz] oscillator circuit with X1 is injected into the IF

module (IC102) . Incoming IF signal and 10.24 [MHz] signal are mixed inside
the IF IC to extract 2ND IF signal 455 [MHz]. FM and AM signals are recovered
with envelope detector.

Frequency Stability
Let : Fo = Crystal Oscillator Frequency
Fr = Phase Detector Reference Frequency
Fvco = VCO Frequency
Ft = Transmit Frequency
Then : Fr = Fo/1800
And under locked conditions : Fr = Fvco / N
Where , “N” is the programmable divider ratio.
Then : Fvco = N X Fr

From which it can be seen, the percentage error in Ft is the same as percentage error in
Fo.
The stability of the crystal oscillator is determined primarily by the itself and having 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.