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5 . C I R C U I T D E S C R I P T I O N
Refer to the annexed blockdiagram for the circuit composition of the receiver. This
receiver is composed of receiving, local signal generator, panel and power supply sections.
5 . 1 R E C E I V I N G S E C T I O N
The receiving system is an up-conversion type double-superheterodyne with a first
intermediate frequency of 70.455 MHz and a second intermediate frequency of 455 kHz.
A signal frequency of 90 kHz to 29.9999 MHz is supplied to the RF input filter after
passing through the BK relay and the input protection circuit. The signal thus supplied
passes through the 35 MHz LPF and is supplied to one of the seven filter circuits, depending
on the operating frequency.The signal is then fed into the lst Mix. in whichthereceiving
signal is mixed with the first local signal of 70.545 to 100.4549 MHz and is thereby
converted into the first IF of 70.455 MHz. The converted output passes through the crystal
filter with a center frequency of 70.455 MHz and a bandwidth of l2kHz, and then is
amplified by the lst IF AMP. Then the siganl is supplied to the 2nd IF in which the
amplified signal is mixed with the 2nd local frequency signal of 70 MHz and is thereby
converted into a 2nd IF of 455 kHz. The converted output passes through one selected out
of three filters with bandwidth of 6 kHz, 3 kHz and 0.5 kHz and the filtered signal is
amplified by the 455 kHz IF AMP.
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The amplifed receiving signal is supplied to the AGC circuit and demodulator circuit
respectively. The AGC voltage for automatic gain control is applied to the lst IF AMP
(70.455 kHz) and 2nd IF AMP (455 kHz.). In case of DSB detection, the demodulator
circuit takes out the carrier component from the receiving frequency signal and operates as
a synchronous detector. In case of other than DSB detection, the BFO signal supplied from
the BFO circuit and the demodulator circuit operates as a product detector.
The demodulated output is supplied to AF AMP and LINE AMP respectively. AF AMP
is a AF power amplifier for speaker and headphones. The gain is adjusted by IC for
electronic volume placed on the fbrmer stage of AF AMP according to the DC voltage
supplied fiom AF GAIN controller on the front panel.
The LINE AMP output is supplied to [BRE- LINEI connector on the rear panel and
used for connecting to the tape recorder, FAX, FSK demodulator, etc. The LINE AMP
output is approx. 0 dBm which is not affected by the AF GAIN controller on the front
panel.
The BK circuit is provitled for protecting from burning due to the inducation of own
station's transmitting radio wave. This BK circuit is controlled by the external +BK and -BK
(keying signal). When -BK line becomes 0V (keyed), the BK circuit operates for grounding
the antenna terminal. At the same time, since the AGC circuit is controlled, the gain of the
receiver is reduced and the speaker output does not appear (muting condition).
5 . 2 L O C A L S I G A N L G E N E R A T O R S E C T I O N
The local signal generator section is composed of the lst local oscillator,2nd local
oscillator and BFO circuit.
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