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GX1600 STANDARD HORIZON - Circuit Description | Radioaficion Ham Radio

GX1600 STANDARD HORIZON - Circuit Description

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GX1600 Circuit Description

Reception and transmission are switched the MPU IC Q2002 (R5F3650MNFB) on the CNTL Unit. The receiver uses double-conversion superheterodyne circuitry, with a 21.7 MHz 1st IF and 450 kHz 2nd IF. The 1st local is produced by a PLL synthesizer, yielding the 21.7 MHz 1st IF. The 2nd local uses a 21.250 MHz crystal oscillator, yielding the 450 kHz 2nd IF. The 2nd mixer and other circuits use a custom IC to convert and amplify the 2nd IF and detect FM to obtain demodulated signals. During transmit, the PLL synthesizer oscillates at the desired frequency directly, for amplification to obtain RF power output. During transmit, voice modulation is applied to this synthesizer. Transceiver functions, such as TX/RX control, PLL synthesizer settings, and channel programming, are controlled using the MPU.


Incoming RF signals from the antenna connector are delivered to the RF Unit, and pass through a low-pass filter (LPF) consisting of coils L1001 & L1002 and capacitors C1001,C1004,C1016&C1017, and antenna switching diode D1004 • D1005 for delivery to the receiver front end.

Signals within the frequency range of the transceiver are then passed through a SAW filter XF1001(HDF160F), before RF amplification by Q1016 (3SK294).

The amplified RF is then passed through a SAW filter XF1002(HDF160F), the pure in-band input signal is delivered to 1st mixer Q1027(3SK293).

Buffered output from the VCO is amplified by Q1021 (2SC5006) and low-pass filtered by coils L1027 & L1029 and capacitors C1150, C1151, C1155, C1158 & C1164 to provide a pure 1st local signal between 134.35MHz and 141.575 MHz for delivery to the 1st FET mixer Q1027(3SK293).

The 21.7 MHz 1st mixer product then passes through monolithic crystal filters XF1004 & XF1005 , and is amplified by Q1031 (2SC4400) and delivered to the input of the FM IF subsystem IC Q1035 (NJM2591V). This IC contains the 2nd mixer, 2nd local oscillator, limiter amplifier, FM detector, noise amplifier, and squelch gates.

The 2nd local in the FM IF subsystem IC is produced from TCXO X1002 (21.250 MHz), and the 1st IF is converted to 450 kHz by the 2nd mixer and stripped of unwanted components by ceramic filter CF1001.

After passing through a limiter amplifier, the signal is demodulated by the FM detector. Demodulated receive audio from the FM IF subsystem IC is amplified by Q1004(LM2902PW).

The audio signal is passed through the amplifier Q1002(LM2902PW) & the Electric Volume Q1028(AK2330), to the AF power amplifier Q1044 (LA4425A), the audio signal is delivered to the 8 Ohm internal loudspeaker and external Speaker terminal in the accessory cable. PLL SYNTHESIZER

The 1st Local signal maintains stability from the PLL synthesizer by using a 21.250 MHz reference signal from TCXO X1002. PLL synthesizer IC Q1037 (MB15A02) consists of a prescaler, reference counter, swallow counter, programmable counter, a serial data input port to set these counters based on the external data, a phase comparator, and a charge pump.

The PLL synthesizer IC divides the 21.250 MHz reference signal by 1700 using the reference counter (12.5 kHz comparison frequency).

The VCO output is divided by the prescaler, swallow counter and programmable counter. These two signals are compared by the phase comparator, and applied to the charge pump. A voltage proportional to their phase difference is delivered to the low-pass filter circuit, then fed back to the VCO as a voltage with phase error, controlling and stabilizing the oscillating frequency. This synthesizer also operates as a modulator during transmit.

The VCO is consisted of Q1024 (2SK210GR) and varactor diodes D1012, D1014 (all HVC359), which oscillates at 21.7 MHz below from the receiving frequency. The VCO output passes through buffer amplifier Q1021,Q1023(2SC5006) to obtain stable output, then applied to the 1st mixer Q1027(3SK293). The DC supply for the VCO is regulated by Q1018 (2SC4617).


Voice audio from the microphone is delivered via the MIC connector to the MAIN Unit. After passing through microphone amplifier Q1003 (NJM12902V), a pre-emphasis network, limiter (IDC: instantaneous deviation control) and low-pass filter network Q1003 (NJM12902V), the Electric Volume Q1028(AK2330), the audio is adjusted for optimum deviation level and delivered to the next stage.

Voice or DSC (Digital Selective Calling) encode signal inputs from the low-pass filter network Q1003 (NJM12902V) is by the VCO Q1024 (2SK210GR) and varactor diode D1012, D1014 (all HVC359), which oscillates the fundamental transmit frequency with direct frequency-modulation using varactor diode D1013(1SV214). The modulated signal passes through buffer amplifier Q1023 and Q1021 (both 2SC5006), diode switch D1008 (DAN235U) to drive amplifiers Q1014 (2SC5006) and RF power amplifier module Q1011 (RA33H1516M1).

The RF energy is passes through antenna switch D1004 (AKP350KV) and low-pass filter (LPF) consisting of coils L1001 & L1002 and capacitors C1001, C1004, C1016 & C1017 and finally to the antenna connector.

RF output power from the RF power amplifier module Q1011 (RA33H1516M1) is sampled by C1009 and C1015 and is rectified by D1003 (RB715F). The resulting DC is fed through

Automatic Power Controllers Q1001(LM2904PW) to RF power amplifier module Q1011 (RA33H1516M1), thus providing positive control of the power output.

Generation of spurious products by the transmitter is minimized by the fundamental carrier frequency being equal to the final transmitting frequency, modulated directly in the transmit VCO. Additional harmonic suppression is provided by a low-pass filter consisting of coils and capacitors, resulting in more than 80 dB of harmonic suppression prior to delivery of the RF energy to the antenna.

DSC Encoder/ Decoder


The DSC (Digital Selective Calling) encode signal which D/A converted in the MPU IC Q2002 (R5F3650MNFB) on the CNTL Unit is fed through the low-pass filter Q1003 (NJM12902V) & the Electric Volume Q1028(AK2330) on the MAIN Unit, then supply to the VCO Q1024 (2SK210GR).


The receiving DSC code is demodulated by the FM IC Q1039 (NJM2591V), then fed through the low-pass filter Q1043 (2SC4617) to the DSC Decoder IC Q1038 (NJM2211M) which the receiving DSC code is decoded. The decoded DSC signal delivered to the MPU IC Q2002(R5F3650MNFB)

1050 Hz Weather Alert Decoder

1050 Hz Weather Alert signal from the FM IF subsystem IC Q1035 (NJM2591V) is demodulated by Q1004 (LM2902PW) then is applied to the MPU IC Q2002 (R5F3650MNFB ) on the CNTL Unit.


Operation is controlled by 8-bit MPU IC Q2004 (R5F3650MNFB). The system clock uses a 9.8304 MHz crystal for a time base.


The EE-PROM Q2003 (R1EX24256ASAS0I) retains TX and RX data for all memory channels, prescaler dividing, IF frequency, local oscillator injection side, and reference oscillator data.


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