Graupner SMC19 (BACK)
Type: SMC19 Dual Conversion.
Pulse decoder=LM339 and Motorola micronontroler, Receiver system= Double converting IC=??
1IF=10.7 and 2IF=455kHz, 1IF Filter ceramic, 2IF filter=455kHz ceramic, FM decoder=Coil resonator,
RX frequency was accurate within 40Hz using original crystal.
Antenna wire de-soldered (99cm length), and 50 Ohm RF test generator attached.
1kHz sine and 2.5kHz deviation.
On the 3 pin board interconnect I found a pin 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 -104dBm (1.4uV)
with -70dBm input level, the frequency width is measured to = 23.1kHz
with -50dBm input level, the frequency width is measured to = 25.3kHz
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 : 51mVpp Verry good, can cause problems if used in electric planes, or with noisy digital servos
30. April 2006, Thomas Scherrer