Noise blanker NB circuit
The input of the NB circuit is extracted from the outputs of second-stage RX mix FETs Q13 and Q14 (2SK520(K44)) and is applied to the NB hybrid IC1 (KCX01). The input signal is amplified and detected by IC1 then output as a noise blanking signal. The operation of IF amplifier Q15 (3SK13KM)) is stopped by NB switching transistor Q16. Noise components are then eliminated.
NB1 is used for short-duration pulses such as ignition noise. NB7 is used for longer-duration, long duration pulse noise such as the woodpecker.
Transmitting Circuit Configuration
The transmitter utilizes a double-conversion system. An audio signal from the microphone is amplified by microphone amplifiers Q37 and Q38 (2SC3324(G)) and modulated by double-balanced mixer (BM) IC6 ((O.PC1037A). The modulated output is converted to a DSB signal, passed through 10.695 MHz single-sideband (SSB) filter XF2, then converted to an SSB signal. The SSB signal is then amplified by TX IF amplifier Q45 (3SK131(M)) in the first stage. An ALC voltage is applied to the second gate of Q45 by a dual MOS FET IF amplifier to control the transmitter output.
Audio signals in the H3E mode are also modulated, like an SSB signal, passed through a filter, then amplified by Q45. A carrier signal is added to the amplified signal by a circuit consisting of Q48 and Q49 producing an H3E signal.
The 10.695 MHz signal amplified by Q45 is mixed with a 60.6 MHz signal by second-stage mixer IC7 (SN16913P), then converted to a 71.295 MHz signal. Spurious components in the 71.295 MHz signal are eliminated by a monolithic crystal filter (MCF) (XF1). The resultant signal is amplified by dual gate MOS FET amplifier Q50 (3SK129(L)). When a high SWR is felt at the antenna, the ALC2 voltage at the second gate of Q50 is reduced and the transmitting output level is lowered to protect the transistor in the final stage. The amplified transmitting IF signal is input to a double-balanced mixer consisting of FETs Q51 and Q52 (3SK179(L)), then converted to the desired transmitting frequency. The converted signal is passes through a low-pass filter to eliminate higher harmonic components and is then amplified to the signal level required for a final-stage drive circuit by transistor Q53 (2SC2053).
The drive output is sent to the final-stage unit and amplified to a sufficient output level by wideband pre-drive amplifier Q1 (2SC1971), wideband push-pull drive amplifiers Q2 and Q3 (2SC3133), and wideband push-pull amplifiers Q4 and Q5 (2SC2879(0,Y)) in the final stage.
Predrive amplifier Q1 has a fixed bias and is thermally coupled with diode D1. The bias currents of drive amplifiers Q2 and Q3 and final-stage amplifiers Q4 and Q5 are adjusted by VR1 and VR2. Diode D2 is thermally coupled with Q2, D3 with Q4, and D4 with Q6. Diodes D1 through D4 compensate for the temperature in each stage and prevent thermal runaway.
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