Reference to the block diagram and the following circuit description will provide you with a better understanding of the design of this receiver. The FRG-7700 is a superheterodyne receiver using upconversion to a first IF (Intermediate Frequency) of 48 MHz. Synthesized local oscillators are used for both the first and record mixers, providing a high degree of frequency stability.
The RF (Radio Frequency) signal from the antenna is fed through the defeatable RF attenuator to the MAIN Unit. The signal is passed through a low pass filter (fc = 30 MHz), consisting of L1001 C1001 C1002 and then through bandpass filters for the following ranges: 150 kHz - 1 MHz, 1- 2 MHz, 2 - 4 MHz, 4 - 8 MHz, 8 - 16 MHz. and 16 - 30 MHz. Selection of the filter to be used is provided by diode switches D1001 - D1002 (1SS53) which are driven by Q1013 (MC14555), Q1001 - Q1006 (2SA733), and Q1007 - Q1012 (2SA94SA-Q), according to the setting of the band switch.
The signal is then amplified by RF amplifier Q1014 (3SK73GR), a dual gate MOS FET with superior linearity and low noise figure. The amplified signal is then fed through buffer Q1015 (2SK125) to the first mixer, where the RF signal is mixed with the first local oscillator signal (48.055 - 78.055 MHz) delivered from the PLL Unit, resulting in a 48.055 MHz first IF. This up conversion technique provides superior image rejection characteristics when compared with conventional designs.
The first IF signal is amplified by Q1018 (2SK125) and fed through crystal filter XF101, which has a 20 kHz bandwidth at -6 dB, providing protection from in band intermodulation distortion while allowing sufficient bandwidth for effective noise blanking. The signal is then delivered to the second mixer, where the 48.055 MHz first IF signal is mixed with a 47.6 MHz local oscillator signal from the PLL Unit, producing a 455 kHz second IF signal.
The 455 kHz signal is passed through a ceramic filter. CF1001 (20 kHz/ -6 dB) and noise blanker gate diodes D1020/D1021 (1SS53) to the main IF filters: CF1002 (SSB/AM-N) CF1003 (AM-M), and CF1004 (AM-W), with filter selection made via the mode switch. The filtered IF signal is then delivered to the main IF amplifier chain, consisting of Q1024 , Q1025 (3SK73GR), and Q1026 (2SC945A-Q).
In the SSB and CW modes, the IF signal is coupled to the product detector, a diode ring demodulator consisting of D1030 - D1033 (1N60), which converts the IF signal into audio using the carrier signal delivered from Q1033 (2SC945A-Q). The audio signal is fed to the audio amplifier, Q1034 (2SC575C2), which delivers 1.5 watts of audio power to the speaker.
In the AM mode, the IF signal is coupled from Q1026 via C125 to buffer amplifier Q1027 (2SC945A-Q). The signal is then detected at D1028 / D1029 (1N60) and the resulting audio signal is fed to the audio amplifier via buffer amplifier Q1030 (2SC945A-Q).
A portion of the output from the second mixer is fed through buffer Q1021 (2SC945A-Q) to amplifiers Q1038 and Q1039 (2SC945A Q). When a carrier or noise free modulated signal is received, the IF signal is rectified by D1037 and D1038 (1N60), producing a DC voltage. The DC voltage is amplified by Q1040 (2SC945A-Q) and fed to gate 2 of Q1036 and Q1037, controlling the gain of those stages.
When pulse noise is received, D1035 and D1036 (1SS53) rectify the IF signal, producing a DC voltage which controls the noise blanker switching diodes (D1020 / D1021). Noise pulses have a very short duration, but extremely high amplitude. Because of the very slow time constant of the AGC circuit feeding back to Q1036 and Q1037 , these short duration pulses will not induce AGC action, and those stages will operate at full gain. When a pulse is received, however, Q1023 biases D1020 and D1021 to block the signal path momentarily.
When a noise pulse and a desired signal are received simultaneously, the blanking action is not impaired, because the relative amplitude difference between the desired signal and the noise pulse is high.