Company: Multiplex (BACK)


Type: RX9 DS IPD Dual Conversion, on this picture the MPX connector version is shown,
there exist also a UNIVERSAL connector version, for same price.

Microcontroller decoder IPD (Atmel AT90LS4433 with 4MHz resonator)
Receiver system: double converting (6131 and TA776)
1. OSC=39.7MHz, 2. OSC=4.965MHz (RX ch80, 35.200MHz)
1. IF =4.5MHz, 2.IF=455kHz
FM decoder: ceramic resonator
IF filter, 4.5MHz ceramic filter and 455kHz 5pole ceramic.
RX frequency was accurate within100Hz using original DS multiplex crystal.

Antenna wire desoldered, and 50 Ohm RF test generator attached.
1kHz sine and 2.5kHz deviation.

On the bottom side a tantal capacitor labeled C225 with demodulated audio signal,
so it is easy to see with a scope how the receiver perform, when wanted signal is equal to noise,
the RF input signal level is recorded.

Center frequency with best sensitivity -115dBm (0.4uV)

with -70dBm input level, the frequency width is measured to = 17.9kHz
with -50dBm input level, the frequency width is measured to = 26.9kHz

Super good sensitivity (long range), channel selectivity impressive at low RF level,
but lack of gain reduction range when input goes up to -50dBm.

Sensitivity to injected noise on the power line
I inject sine wave signals via a 400uH coil and transfer capacitor 100nF,
directly into the power supply connector of the receiver board.
This simulates a bad motor regulator BEC or Servo current pulse, or noise picked up on long power and servo wires
35MHz RF input signal level at -90dBm
Demodulated signal observes as with the sensitivity test.
Noise frequency with best interference = 455KHz,
Input Voltage level when the wanted signal is jammed to 50% signal to noise.
Measured to : 32mVpp VERY BAD !!
Risk of problem in electric planes = HIGH ! This was expected due to lack of series coils in power lines
before internal regulators, by using capacitors for decoupling like here, the input noise generates current,
this flow all over the boards and inject unwanted voltages into highly sensitive receiver section.

Mirror frequency and other issues:
This Receiver uses 4.5MHz first IF, so you MUST use original Multiplex DS crystals,
The mirror will be 4.5 + 4.5 over, be coarse 1. Osc is swinging at 39.7MHz (the crystal in socket)
Sensitivity at 44.220MHz is -47dBm, so mirror attenuation is 68dB.
Sensitivity at 4.5Mhz on antenna connector is -35dBm, and on power connector 170mVpp.

13. April 2006, Thomas Scherrer

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