Rx Back End

AE/LZB 119 1902 R1A

Local Oscillator Buffer (V450)

The injection frequency (1011.2-1016.2 MHz) from the synthesizer circuit is applied to the base of Local Oscillator (LO) NPN buffer transistor V450 through capacitor C450 and inductor L450. This signal at the input of V450 is 0 dBM and is amplified to +4 dBm. This signal can be metered at TP450. The output of V450 is connected to the input of mixer Z402 (Pin 1).

Mixer (Z402)

The LO frequency and the RF signal are mixed to provide an Intermediate Frequency of 76.2 MHz (example: 1011.2 MHz - 935.851 MHz = 76.2 MHz). This signal on the output of Z450 (Pin 4) is applied to the base of IF AMP 1 NPN transistor V500 through filter capacitor C505, inductor L500 and coupling capacitor C504.

IF Amp 1 (V500)

IF AMP 1 amplifies the 76.2 IF signal +15 dB. The collector voltage for V500 is from the emitter of LO BUFFER amplifier V450 and can be metered at TP500 (3.8 V). The emitter voltage of V500 is used as feed back to control the gain of RF amplifier V400. The output from the collector of V500 is connected through coupling capacitor C502 to the input of band pass Filter Z500 (Pin 4).

IF Filter (Z500)

IF band pass Filter Z500 is a 4-pole crystal filter. The gain of Z500 is -3 dB and the center frequency is 76.2 MHz. The output of Z500 (Pin 6) is coupled to the emitter circuit of IF AMP 2 through a limiting zener diode circuit consisting of zener diodes V502-1 and V502-2 and coupling capacitor C508.

IF Amp 2 (V501)

The typical emitter voltage of V501 is 0.8 V. The typical base voltage is 1.6V and the typical collector voltage is 3.1 V. These voltage levels produce the proper bias for an input impedance of 250 ohms. This in turn loads the Z500 crystal filter properly. The 76.2 MHz IF output on the collector of V501 connects to the input of a 450 kHz IF chip (N551) located in the rear section of the LPE-200.

IF Chip (N551)

The IF input (76.2 MHz) to IF Chip N551 is connected through coupling capacitor C550 and inductor L550 to N551 Pin 1, (RF+). Internal to N551 (DIANE), this input signal is amplified and applied to an input of a second mixer circuit. A second LO synthesizer injection frequency

of 76.2 MHz is internally applied to another input of the mixer circuit. This results in a second IF of 450 kHz (76.65 MHz -76.2 MHz = 450 kHz) on the output of the mixer (N551, Pin 20, Mixer Out). This signal is then routed through a 450 kHz ceramic filter to set the channel bandwidth to 12.5 kHz. The output of the filter is routed to N551, Pin 18, IF In1. This input (-3 dB) to N551 is amplified and sent two places. The output (N551, Pin 16, IF Out 1) signal goes through capacitor C553 to the input of 450 kHz Ceramic Filter Z550, Pin 2. The output of Z550 on Pin 1 is connected through capacitor C552 to N551, Pin 14, IF In2. This input to N551 is further amplified and balanced outputs, RxIF and RxIF_B, are sent to HILLARY (D701). These two lines, one positive and the other negative, are used to cancel out any noise that might get on the line. These balance outputs are typically +3 dBm and can be metered at test points TP551 and TP552.

The Receiver Signal Strength Indicator (RSSI) circuitry internally monitors the RF signal strength. The RF level is represented by a DC voltage level proportional to signal strength. Feedback resistors R554 and R561 set the gain of the RSSI output on Pin 7. This signal becomes the FAST RSSI signal going to DEBBIE (D601). Feedback resistor R554 connects from the RSSI output on Pin 7 to Pin 8, RSSI Feedback. The RSSI signal can be metered at test point TP550.

2nd Local Oscillator

A 2nd LO circuit internal to N551 generates the 76.65 MHz injection frequency for the mixer circuit. The tank circuit for this oscillator consists of capacitors C556 through C560, variable capacitor V550 and inductor L551. This tank circuit connects to N551, Pin 3 (OSC Emitter) and Pin

4(OSC Base). A LO_VCTRL control voltage is applied through resistor R555 to V550 to adjust the capacity of the circuit and maintain the integrity of the injection frequency. The LO_VCTRL control voltage is a DC level generated in CHERYL and filtered by resistors R225 and R226 and capacitors C217, C218 and C219. This voltage (nominally 2.6 volts) comes from auxiliary synthesizer N204 and goes to the oscillator tank circuit. A +12 dB signal is then returned to a prescaler circuit in the synthesizer. The synthesizer provides a signal to the loop filter which outputs the control voltage.