Carrier spectrum at full power from a IC-290E. The serial number is 01135.

Carrier spectrum at full power from another IC-290E. The serial number is 02881. This unit has accidentally been connected to a 28V DC power supply. The power amplifier failed and has been replaced.

The IC-290 has a 10kHz synthetizer and when the VCO voltage needs a change 10kHz pulses are emitted from the phase detector. As can be seen on the upper spectrum, the associated spurs surrounding the carrier every 10 kHz are not very strong under stationary conditions.

The spectrum of the unit that suffered from a 28V DC supply shows strong and unstable spurs every 10 kHz. Note that the peak level (green) of the spurs is much higher than the value at the time when the image was captured. This reflects the fact that the level of the spurs goes up and down by about 30 dB. The spectrum of the properly working unit can be made to look identical to the defective unit by tuning the voltage control on the power supply up and down slightly. The variation of supply voltage causes the PLL to emit 10 kHz pulses to keep the VCO locked.

The problem with the defective unit is most probably that the PLL system is not fed with a stable DC voltage because something was damaged by the 28V supply. Consequently the PLL loop has to compensate vor supply voltage variations.