FM Modulation

HP 8360

User’s Handbook

FM Modulation

Frequency modulation can be accepted from an external source at the FM connector or can be internally generated by synthesizers with Option 002. The damage level of the FM input is 4~15 V DC. The input impedance is set to 50R. A jumper on the All FM Driver board allows you to change the input impedance to 600R.

(See “Adjustments” in the Service Guide.) The FM sensitivity can be scaled to either 100 kHz/V, 1 MHz/V, or 10 MHz/V.

When internal FM is chosen (Option OOZ), the rate and deviation are set by softkeys in the FM menu. The waveform menu provides a choice of sine, square, triangle, ramp, or noise waveforms. The monitor menu lets you output the internally generated modulation waveforms to the rear panel AM/FM OUTPUT connector. The scale of the FM output depends on the FM deviation chosen. The following table shows the scale versus deviation. The monitor menu also lets you display the value of the FM deviation.

FM Coupling

Whether provided from an external source or generated internally (Option 002), the FM system can be either AC or DC coupled. If you choose AC coupled FM, you will be modulating a phase-locked carrier. This is the specified synthesized operation. The modulation rate must be 100 kHz or greater. If not, the frequency changes caused by the modulation are inside the phase-lock loop bandwidth and the output will not be linear FM. For modulation frequencies below 100 kHz, choose DC coupled FM. In this mode, the phase locked loop is de-activated. This means that the synthesizer is operating as an open loop sweeper. The synthesizer will not be phase locked, and therefore, be aware that the phase noise and CW frequency accuracy specifications do not apply.

OVERMOD Message (Maximum Deviation)

The maximum FM deviation is limited by the following two conditions:

•Maximum FM deviation must be less than 8 MHz and

•Maximum FM deviation must be less than n x 5 x FM rate (refer to the “Frequency Bands” specification for the value of n).

The following chart shows the limits of each band given these two conditions. For example, in band 1 at a 1 MHz FM rate, the FM deviation must be less than 5 MHz.

n (1) x 5 x FM Rate (1 MHz) = 5 MHz.

Operating and Programming Reference M-17