EFJohnson 7780 service manual Receiver Circuit Description, RF Amplifier Q1, First Mixer Q2

CIRCUIT DESCRIPTION

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 input). Pin 15 of IC1 then pulses low and turns Q1 on. This charges the loop filter capacitors which increases the VCO control voltage. The VCO frequency then increases until the phase detector inputs are again the same frequency and phase. If the VCO frequency is too high, pin 16 pulses high which discharges the loop filter capacitors.

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 and suppresses 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 antenna switch (see Section 4.4.2) to bandpass filter FI1. This filter attenu- ates the image and other unwanted frequencies and also prevents the injection signal from being fed out to the antenna.

The signal is then fed to RF amplifier Q1. 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 by L5 and C7.

From FI2 the signal is fed to gate 1 of first mixer Q2. This is a dual-gate MOSFET device which mixes the receive frequency with the first injection frequency to produce a first IF of 45.225 MHz (low-side injec- tion is used). The injection frequency is from the synthesizer, and it is fed through a low-pass filter consisting of C22-C24 and L12. This filter attenuates harmonic frequencies present in the injection signal. A 3-dB pad consisting of R9-R11 provides 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 is provided by C15, L9, L10, C18, C19, and R99. The first IF signal is then fed to four-pole crystal filter FI3. This filter attenuates wideband noise, adjacent channels, frequencies resulting from intermodulation, and other frequencies close to the receive channel.

44.775 MHz	1	Mixer		16
2nd Injection		44.775
	2	MHz		15
Ceramic Filters	3	Oscillator	45.225	14
		450 kHz 2nd IF
			MHz
FI4, FI5,
		Limiter-Amp	1st IF	13
FI6	4
			Noise Det
	5		RSSI	12
	6		IF	11
			Out	10
	7		Quad.
		Detector		9
	8		Audio
		Amp	Out
Figure 4-1		Limiter/Mixer/Detector IC3

4.3.2IF AMPLIFIER (Q2), LIMITER/DETECTOR (IC3)

From crystal filter FI3, the IF signal is fed to IF amplifier Q2 on the IF board. This stage 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.The 45.225 MHz first IF signal is mixed with the 44.775 MHz second injection signal to produce a second IF of 450 kHz. The 44.775 injection signal is produced by multiplying the TCXO frequency by three. This multiplication is performed by Q18 and other components.

The 450 kHz second IF is fed out of IC3 on pin 3 and applied to ceramic filters FI6, FI5, and FI4. These filters are used to attenuate wideband noise. A switching network consisting of D13-D16 routes the IF signal through all three filters with narrow band and NPSPAC operation, and through only FI4 with wide- band operation. This switching network is controlled by WIDE and NAR signals from Q19-Q21. The W/N

	November 1999
4-3	Part No. 001-7780-500