Ericsson LPE-200 manual Transmitter, Transmit Frequency Generation, TX/Converter/Modulator N150

AE/LZB 119 1902 R1A

TRANSMITTER

Transmit Frequency Generation

The main VCO, in the synthesizer circuit, is programmed to generate the 1st LO receive injection frequency (1011.2 to 1016.2 MHz). This carrier frequency is mixed with a transmit offset frequency of 115.2 MHz to generate the carrier and two side band frequencies. The carrier and upper side band are suppressed and only the lower side band is transmitted. For example: the carrier frequency of 1015.2 MHz minus the offset frequency of

115.2MHz is equal to the transmitted lower side band frequency of 900 MHz.

TX/Converter/Modulator (N150)

Transmit Offset

The LPE-200transmit offset frequency is 115.2 Mhz. This frequency is generated by a seven-times-multiplier circuit off the reference oscillator. The output of this VCO is applied to a Phase-Lock-Loop (PLL) where it is divided by 7 (115.2 ÷ 6 = 19.2 MHz) and compared to a reference frequency of 19.2 MHz. The 19.2 MHz reference frequency is generated by oscillator module U201 in the synthesizer circuit. A DC phase lock voltage, which is the difference of the two input frequencies, on N150, Pin 9 (PHSOUT) is applied to a loop filter. This loop filter consists of capacitors C151, C152 and C155 and resistors R153 and R154. The DC voltage can be metered at test point TP150 and should be 1-4 volts while transmitting and 0 volts while

receiving. The output of the filter connects to the tank circuit through resistors R155 and R160. The tank circuit for the VCO and consists of capacitors, C160, C161, and two variable capacitors in V180 and inductor L154. The loop filter and the tank circuit are tuned to 6x19.2 MHz or

115.2MHz. The tank circuit connects across N150, Pins 6 (TANK_1) and 7 (TANK_2). The DC voltage applied to the loop filter changes the capacitance of the variable capacitors within V180 of the tank circuit to maintain the VCO output of 115.2 MHz.

When the transmitter is keyed, transistor V152 conducts to increase the bandwidth for PLL acquisition.

Single Side Band Mixer

The main VCO frequency is amplified then passed through a 90-degrees phase shifting network to the SSB mixer. The 115.2 MHz offset frequency is also amplified and passed through a 90-degree phasing network to the SSB mixer. These two signals are summed together to produce the carrier frequency, an upper side band and a lower side band. The carrier and upper side band are suppressed by 40 dB. The lower side band passes at full amplitude. There are two outputs of the lower side band from the SSB, each 90- degrees out of phase. Each signal is connected to another mixer circuit where it is summed with the transmit modulation from HILLARY. Resistor R159 sets the gain for phase detection.

Low Pass Filters

There are two modulating inputs from HILLARY, one is the I input (MODI and MODI_B) and one is the Q input

Figure 4 - Frequency Generation

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