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QYT KT-8900 VHF/UHF mobile transceiver

introduction permalink:
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1 x QYT KT8900 radio
1 x DTMF microphone
1 x Car Power Cable
1 x Fuse
1 x Mounting bracket
1 x Screw set
1 x User"s Manual

TX/RX frequency range: 136...174 MHz (VJF) and 400...480 MHz (UHF)
Memery channels: 200
Channel Spacing: 5 / 6,25 / 10 / 12,5 / 15 / 25 KHz
Supply voltage: 13,8 VDC +/-15%
Squelch way: CTCSS / DCS / 5 Tone / 2 Tone / DTMF
Frequency Stability: 2.5ppm
Operating Temperature: -20...+60 'C
Dimension: 98 mm wide, 35 mm high and 118 mm deep
Weight: 408 grams
Bandwidth selection: wide and narrow
Sensitivity: ≤;; 0,25 μ;;V (wide) / ≤;; 0,35 μ;;V (narrow)
Channel choice: ≥;;70 dB (wide) / ≥;;60 dB (narrow)
Intermodulation: ≥;;65 dB (wide) / ≥;;60 dB (narrow)
Spurious Rejection: ≥;;70 dB (wide) / ≥;;70 dB (narrow)
Audio response: +1...-3 dB @ 0,3...3 kHz (wide) / +1...-3 dB @ 0,3...2.55 kHz (narrow)
Signal to noise ratio: ≥;;45 dB (wide) / ≥;;40 dB (narrow)
Audio distortion: ≤;;5 %
Audio output power: ≥;;2 W @ 10%
Output power: 25 W / 20 W (VHF/UHF)
Modulation Mode: 16KΦ;;F3E / 11KΦ;;F3E
Channel power: ≥;;70 dB / ≥;;60 dB
Signal to noise ratio: ≥;;40 dB / ≥;;36 dB
Parasitic harmonic: ≥;;60 dB / ≥;;60 dB

spurious radiation on VHF permalink:
According to the Dutch "Gebruikersbepalingen amateurfrequentiegebruik" number 212-3.3 of the 20th of augustus 2012 part 3.1 it's not allowed to radiate spurious signals of -36 dBm or -60 dBc between 50...1.000 MHz wherever the highest number is applicable. The highest harmonic (at 435 MHz) is -29,55 dBc (decibell relative to the carrier). This is approximately 30 dB (1.000 times) worse than alowed. The transmit power during the test is 20 Watts. At 435 MHz this is approximately 30 dB less of 20 Watts. This is approximately 20 mW / 13 dBm. The maximum limit is -26 dBm. Therefore the spurious radiaton doesn't meet the limits. The third harmonic (the worst one) is represented at the 70 cm band. (144 MHz * 3 = 432 MHz and 146 MHz * 3 = 428 MHz) When transmitted at the 70 cm band, the -60 dBc limits are met at a first glance and therefore shouldn't be a problem.
To meet the regulations on VHF an additional low pass filter should be used! I'll look into it if it's possible to add a low pass filter to the VHF circuit. I someone can help me finding the schematic and/or service manual I would like to hear this!

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measurement setup
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The used equipment is:
- Rigol DSA-815 TG spectrum analyser.
- Radio System AB RS-9694 measuring coupler.
- Spinner BN 52-77-66 400 W dummyload.

update 20161012 - It seems there are two (8-pole?!) low pass filters on the circuit board. The antenna signal is split in two ways and switched by PIN diodes. Therefore the VHF and UHF path should be able to suppress harmonics. I used the computer program ELSIE to determine what supression should be able to get on VHF. I designed a 7-pole Chebycheff low pass filter for 148 MHz with a passband ripple value of 0,1. The component values are rounded to the nearest real wordl values. The suppression for 290 MHz is 55 dB, at 435 MHz 82 dB and at 580 MHz is the suppresion 100 dB. If there is no loss at the carrier, the dB values can be considered as a negative dBc value. Since the third (theoretical strongest) harmonic is the strongest for real, it's possible the applied low pass filter doesn't work what it's designed for.
My plan is to bridge the VHF low pass filter en measure the results and do the same measurement without the filter bridged (original configuration). Than can be determined if the filter is working at all. Hopefully it's possible to recalculate the component values and therefore it should be possible to replace the components by correct values. It's likely the replacement of inductor (L) and capacitor (C) components will fix the problem. Well, let's measure first and navigate from there. To be continue...

Replacement filter sketch:
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CTCSS test permalink:
I tested the CTCSS sensitivity. I set the receiver to the desired 77 Hz tone. I set the R&S SMT-02 signale generator to a 1 kHz audible tone with 2,5 kHz deviation and a second 77 Hz (sub)audio tone with a deviation of also 2,5 kHz. The -100 dBm signal from the generator is fed into the antenna port and the received 1 kHz tone is audible. The 77 Hz tone of the generator is changed to determine how critical the CTCSS circuit is. The squelch is opened between 76,3...78,1 Hz. This means that there is a tone span of 1,8 Hz for the 77 Hz tone. This seems good.

digital noise on audio permalink:
There's a flaw in the design of the radio, of should i say microphone? When the volume is turned off (knob fully CCW), and the [MONI] butting is pressed to open the squelch a digital noise is comming from the speaker. When the microphone is unplugged, the noise is gone. This flaw was reported earlier by IK1ZYW. It's likely this is noise is generated by the digital signals between the microphone and the radio. The buttons are connector to logic that "translates" the butten pressing into a digital signal that can be handled by the CPU. This sames a lot of wiring. I'm looking to find a solution. maybe some ferrite or a strategic placed capacitor... To be continued...

reverse engineering permalink:
output stage amplifier
The final stage of the power amplifier is based on a wideband AFT09MS015NT1 N-channel RF power LDMOS transistor. This component is designed for mobile two way radio equipment with frequencies from 136...941 MHz to generate 16 Watts of RF power at 12,5 VDC. The LDMOS is fed with a RF signal of 230...380 mW (depending on VHF of UHF use) and has a gain of 16...18,5 dB. The output power is approximately 15...16 Watts @ 12,5 VDC. It has integrated ESD protection and has high lineairty for TETRA and SSB use. Since the KT-8900 is designed for 13,8 VDC, the output stage amplifier is running on higher voltages and therefore has more output gain and more heat generation...

radio microcontroller
The 'beating heart" of the KT-8900 is a RDA1846 chip. This is a highly integrated single CMOS chip microcontroller for two way radio applications like portable transceivers. This chip handles the translation from RF carrier to voice in the RX path and from voice to RF carrier in the TX path completely. The RDA1846 has a digital signal processor, which makes it have optimum voicequality, flexible function options and robust performance under varying reception conditions according to the RDA datasheet. The chip can handle signals of 134...174 MHz and 400...500 MHz. This chip is also used in the Baofeng UV-5R portable radio. Altough it is a powerfull chip with a lot of processing power, it's only 5 by 5 mm (QFN package). There's an onboard VCO, supports AFC, AGC, pre/de-emphasis, VOX, squelch, CTCSS, DCS, DTMF, 8 GPIO's for external controls, I2C bus interface, 8 dBm RF "power" amplifier and analog /digital volume control possibility. That's a lot of smart things stuffed in this small package...

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