Sailor RM2042 Technical Manual page 70

5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS
The crystal X3 is constructed to work at the 7th. overtone and is used in a series resonance mode.
Unfortunately the crystal has also a parallel resonance frequency, which is located only 4 to 5 kHz above
the wanted series resonance. This parallel resonance frequency is determined by the static capacitance
C and is effecting the phase response of the crystal in an unwanted maner, which is lowering the tracking
o
range. To overcome this problem, the crystal is parallel connected with the inductor L9, which partly is
eliminating the static capacitance.
The oscillator is build-up around the bipolar NPN transistor Q5, which has a typical transition frequency
f of 5 GHz. The transistor is used in a commen base configuration, where the capacitor C79 is used to
t
ground the signal at the base terminale.
The oscillation is obtained by feeding back the collector signal to the emitter, where the crystal is used
as the feed back element. The adjustable coil L6 at the collector form a resonance circuit together with
the series connection of the capacitors C80, C81 and C82. The oscillation frequency can be adjusted by
tuning the resonance freguency of this tank circuit, which will increase or decrease the phase shift in the
open loop response.
The oscillator is followed by two buffers - an oscillator buffer and an LO buffer.
To minimize the capacitive loading of the oscillator, the output signal is taped across a relative large
capacitor of 56pF. The taped signal is buffered by the transistor Q6, which is working as an emitter
follower.
The output signal from Q6 is loaded with a resistor of 47 , which will camouflage the capacitive loading
by the input of the LO buffer and then stabilize the oscillator buffer at higher frequencies.
In the LO buffer, the signal is amplified to give an output level of 7 dBm into 50 . The LO buffer is build-
up around the transistor Q7, which is used in a commen emitter configuration. By means of the two
capacitors C89 and C90, the output impedance is matched to the mixer input of about 170 .
CRYSTAL FILTER AND FIRST IF BUFFER AMPLIFIER
The receiver adjacent channel selectivity is obtained by means of the crystal filter FL1 at the 1st. IF and
the ceramic filter FL2 at the 2nd. IF.
The input and output of the crystal filter is impedance matched to 3k, which is obtained by means of the
resistors R13, R14, R15 and R16.
From the 1st. mixer, the signal is led through the crystal filter to the input of the 2nd. IF buffer amplifier.
This amplifier is build-up around the dual gate MOSFET transistor Q4, which has a tuned drain circuit
consisting of the inductor L5 and the capacitors C29 and C31.
SECOND MIXER & LO, CERAMIC FILTER, FM-DETECTOR AND AF AMPLIFIER
The 2nd. mixer and LO, FM-detector and AF amplifier are all included in the integreted circuit U1, which
is of the type MC3372.
From the 1st. IF buffer amplifier, the signal is led to the 2nd. mixer, where it is mixed with LO2.
The second local oscillator frequency is crystal controlled and is tuned to 14.85 MHz by the trimming
capacitor C34. The LO2 signal is generated by a build-in bipolar NPN transistor, which form a colpitts
oscillator by means of the crystal X1 and four external capacitors.
The output of the 2nd. mixer is led to the ceramic filter FL2, which is centered at 450 kHz. The 2nd. IF
signal is then amplified by the limiting IF amplifier, that approximately has a gain of 92 dB.
The signal is detected by the build-in quadrature detector, which use an external capacitor and ceramic
resonator as the 90° phase shift network.
After detection, the signal is amplified by the build-in AF amplifier, and the carrier component at 450 kHz
is removed by means of the resistor R38 and the capacitor C51. The output level from the following de-
emphasis filter is adjusted by the trimming resistor R66 to 250 mV with an input carrier modulated with
1 kHz to give a peak frequency deviation of 3 kHz.
As an extra facillity, the MC3372 has a level meter output, which in this design only is used to adjust the
front-end filters. The level meter output is formed as a current generator, that produce a DC-current
proportional to the carrier level measured in dBm. The current is transformed to a voltage by means of
the resistor R27 and is filtered by the capacitor C43.
AF FILTERS
The output signal from the AF amplifier inside U1 is led through the MOSFET switch U6 to the de-
emphasis filter, which is build-up around the operational amplifier U3.1. This filter is implemented as a
second order band pass filter with a center frequency at 950 Hz.
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RM2042
9341