Graupner R600  (BACK)


Type: R600 Single Conversion.

Pulse decoder= 74AC164 and LM339, Receiver system= Single converting 4116
IF=455kHz, IF filter=455kHz ceramic, FM decoder= ceramic resonator,
RX frequency was accurate within 40Hz using original crystal.

Antenna wire de-soldered (Length=99cm) and 50 Ohm RF test generator attached.
1kHz sine and 2.5kHz deviation.

On the PCB I found a via/pad 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 -98dBm (2.5uV) not super good.

with -70dBm input level, the frequency width is measured to = 17.0kHz
with -50dBm input level, the frequency width is measured to = 20.6kHz

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 : 74mVpp not good, if used in electric planes, or with noisy digital servos
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 single conversion so it recieves two frequencies,
the wanted and the mirror, both with same sensitivity, normal for all single conversion types.

29th of April 2006, Thomas Scherrer