EFJohnson 7780 - page 67

CIRCUIT DESCRIPTION

4-3 November 1999

Part No. 001-7780-500

frequency is low,the VCO-derived input to the phase

detector is lower than the TCXO-derived input (or the

phase lags the TCXO-derived inpu t). Pin 15 of IC1

then pulses low and turns Q1 on. This charges theloop

filter capacitors which increases the VCO con trol

voltage. The VCO frequency then increases until the

phase detector inputsare again the same frequency and

phase. If the VCO frequency is too high, pin 16 pulses

high which discharges the loop filtercapacitors.

The loop filter is formed by R7-R9 and C4/C5.

This is a low-pass filter which controls synthesizer

stability and lock-up time andsuppresses the 6.25 kHz

reference frequency.

4.3 RECEIVER CIRCUIT DESCRIPTION

NOTE: A block diagram of the RF and logic units is

located on page 8-3.

4.3.1 RF AMPLIFIER (Q1), FIRST MIXER (Q2)

The receive signal from the antenna is fed

through low-pass filter FI7 and the anten na switch (see

Section 4.4.2) to bandpass filterFI1. This filter attenu-

ates the image and other unwanted frequencies and

also prevents the injectionsignal from being fed out to

the antenna.

The signal is then fed to RF amplifierQ 1.Imped-

ance matching with FI1 is provided by L32,C4, C138,

and R104. The RF amplifier is used to recover filter

losses and improve the sensitivity of the receiver.The

output of Q1 is fed to FI2 which is a bandpass filter

similar to FI1. Impedance matching is provided byL5

and C7.

From FI2 the signal is fed to gate 1 of first mixer

Q2.Thisisadual-gateMOSFETdevicewhichmixes

the receive frequency with the first inje ction frequency

to produce a first IF of 45.225 MHz (low-side injec-

tion is used). The injection frequencyis from the

synthesizer, and it is fed througha low-passfilter

consisting of C22-C24 and L12. This filter attenuates

harmonic frequencies presentin the injection signal. A

3-dB pad consisting of R9-R11provides the required

input level to the mixer, and L11 and C12 provide

matching.

Impedance matching on the input of Q2 is

provided by C11, L6-L8, and L33. Likewise,imped-

ance matching on the output isprovided by C15, L9,

L10, C18, C19, and R99. The first IF signal is then fed

to four-pole crystal filterFI3. This filter attenuates

wideband noise, adjacent channels,frequencies

resulting from intermodulation,and other frequencies

close to the receive channel.

Figure 4-1 Limiter/Mixer/Detector IC3

4.3.2 IF AMPLIFIER (Q2), LIMITER/DETECTOR

(IC3)

From crystal filter FI3, the IF sign al is fed to IF

amplifier Q2 on the IF board. Thisstage provides the

required input level to the next stage. The signal is

then fed to limiter/detector IC3. This device contains

second mixer and oscillator,limiter, detector, squelch,

and RSSI stages (see Figure 4-1).

The IF signal is fed in on pin 16 which is the

input of an internal mixer.The45.225 MHz first IF

signal is mixed with the 44.775 MHz second injection

signal to produce a second IF of 450kHz. The 44.775

injection signal is produced by mu ltiplying the TCXO

frequency by three. This multiplication is performed

by Q18 and other components.

The 450 kHz second IF is fed out of IC3on pin 3

and applied to ceramic filters FI6, FI5, and F I4. These

filters are used to attenuatewideband noise. A

switching network consisting of D13-D16 routesthe

IF signal through all three filter s with narrow band and

NPSPAC operation, and through only FI4 with wide-

band operation. This switching networkis controlled

by WIDE and NAR signals from Q19-Q21. The W/N

44.775 MHz

2nd Injection 1

Oscillator

Mixer

Limiter-Amp

Quad.

45.225

450 kHz 2nd IF MHz

1st IF

44.775

MHz

FI4, FI5,

Ceramic Filters

FI6

Audio

RSSI

Noise Det

Amp Out

Out

Detector