Standard Communications mt930b, Service Manual

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MT930B Service Manual Page 5-1
CHAPTER 5
PRINCIPLES OF OPERATION
This chapter discusses the principles of operation for each MT930B module and
circuit. diagrams in Chapter 9 must be used to follow the discussion. Figures 9-1
through 9-3 are especially useful in understanding overall receiver operation.
1ST CONVERTER
The MT930B receiver is equipped with four RF output terminal systems. It is
possible to select one of the systems for using relays L101, L102, and L103.
When a 950 to 2150 MHz RF input signal is selected, the signal is input to front
end module A101 via a bias-tee circuit. A tracking filter, RF automatic gain con-
trol (AGC) amplifier, frequency converter, and PLL circuit are built into A101 so
an IF signal converted to 479.5 MHz can be output. After passing through helical
bandpass filter (BPF) F101, the IF signal is sent to the IF AGC amplifier. Gain
reduction for the IF AGC is performed by PIN diode D105. When the RF input
level is low, only the RF AGC amplifier in A101 operates, and gain reduction is
not performed by the IF AGC amplifier. When the RF input level is high, both
AGCs perform gain reduction.
2ND CONVERTER
The converted 479.5 MHz IF signal is input via J401. After the IF signal passes
through the low-pass filter (LPF), it is input to Q401–an IC used for frequency
conversion. The most important function of Q401 is to convert the 479.5 MHz IF
signal to 70 MHz. Q401 also includes a spectrum reversal and AFC functions.
The spectrum reversal function is used to reverse video polarity. A local oscillator
(LO) is built into Q401. The spectrum of the 70 MHz IF signal is reversed when a
409.5 MHz signal is used as the LO signal. Spectrum reversal is not performed
when a C band signal is used.
The LO frequency changes in the range of
±
2 MHz when an automatic fre-
quency control (AFC) function is used. The 70 MHz signal passes through LPF
and buffer amplifier Q403 to J402. The LO signal , after passing through Q403 is
input to PLL IC QL01. The comparison frequency used by QL01 is 50 kHz. An
error signal is fed back to the LO via LPF Q402.
70 MHZ BPF
70 MHz bandpass filters (BPF)s are inserted between J202 and J203. The inser-
tion loss in this interval is designed to be about 0 dB. It is possible to select six
bandwidths (BW)s: 36, 27, 25, 22, 18, or 16 MHz. Two types of filters are pro-
vided for the different bandwidths. The 25, 22, 18, and 16 MHz bandwidths use
surface acoustical wave (SAW) filters, while the 36 and 27 MHz bandwidths use
inductance/capacitance (LC) filters. The reason for using different filters is that
optimal characteristics can be obtained for both C band and Ku band.