Practical experiences, desensibilisation

In practice, unexpected problems uccurred. The main problem was a limit in suppression of sidebandnoise, even when inserting extra notches on the receiver frequency.

The couse was intermodulation between the second trnsmitter harmonic and the noise energy in the non-suppressed sideband. This can occur in te circulator or in the receiver input stages.

Calculation for a 70cm repeater on RB0 (Rx freq. 434.6 MHz, Tx freq. 433.0 MHz).

Ft 2x Ft Fs   (Ft-Fr) Tx lower sideband
433.0 866.0 1.6 Ft-1.6=431.4
2Ft-(Ft-Fs) =  

866-(433-1.6) =

866-431.4 =  
434.6 =  



This calculation shows, that a non-lineair component can generate a noisesignal on the receiver input frequency  Fr . The signal components are:

a.    The noise energy on the non-suppressed lower transmittersideband frequency: Ft-Fs
b.    The second harmonic of the transmitter carrier frequency: 2 x Ft

By reducing either one a better result can be achieved.

    Typical levels.

1. Noiselevel in lower sideband (see fig.1):  -70 to - 80 dB
2. 2nd harmonic of transmitter carrier: fi. -80 tot -90 dB
3. Mixing losses in the intermodulating part: fi. 30 dB
4. Resulting intermod signal level: -120 to -130 dB.

Intermod level is 20 to 30 dB above the receiver noise level. In this case, the receiver sensitivity can degrade 30dB (5 S-points!).

Most effective showed an extra notch on the non-suppressed under sideband (ft-fs). A sharp bandpass filter helps, but can shown to be not effective enough.

   Adding an extra notch on the mirror frequency (70 cm).

Fig. 13

Fig. 14