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Table 8-1. Numbered Error Messages (4 of 17)

2 ff the modulation bandwidth of the VCO source you are using is not at least 10 times greater

(cont'd)

tnan tn

e PLL bandwidth for the measurement, there are generally three possible actions you

can take.

• Increase the modulation bandwidth of the VCO source. If the VCO source is a bread-

board or prototype oscillator, it may be possible to make the component changes

necessary to provide sufficient modulation bandwidth.

• Use a different VCO source with greater modulation bandwidth.

• Decrease the PLL bandwidth to 1/10 the VCO's modulation bandwidth by decreas-

ing the Peak Tuning Range (PTR) for the measurement. Use Figure 8-2 to determine the

PTR required. The PTR can be reduced by decreasing the Tuning Voltage Range value

entered.

Divide the required PTR by the VCO's Tuning Constant to determine the ap-

propriate Voltage Tuning Range to enter. (Keep in mind that reducing the PTR limits the

HP 3048A's Frequency Dnft Tracking Range for the measurement. For further information

about changing the PTR, refer to Changing the PTR in Chapter 3

of the Operating Manual.)

If you do not know the modulation bandwidth for your VCO source, you can determine

it by locating the 3 dB roll-off frequency of the VCO tune port. The following steps

describe a procedure for determining the modulation bandwidth.

a. Connect a tracking generator output (such as the HP 3585A) to the tuning input port

on your VCO.

b. Set the tracking generator output to sweep the audio spectrum.

c. Connect the RF output from your VCO to a modulation analyzer (such as an

HP 8901

A).

Set the modulation analyzer to measure FM.

e. Connect an audio spectrum analyzer to the MODULATION OUTPUT on the

HP 8901 A.

f. Increase the frequency spectrum of the tracking generator, if necessary, until

the 3 dB roll-off frequency can be seen on the audio analyzer's display. The

3 dB frequency should be > ten times the PLL bandwidth for the measurement.

The modulation bandwidth of a VCO can also be determined using the Bessel Null technique.

Apply a series of audio frequencies to the tune port of the VCO. Observe the VCO's output

signal on an RF Spectrum Analyzer and adjust the audio modulation level for the first carrier

null.

Calculate the VCO's modulation sensitivity at each modulation frequency using the level

and frequency of the audio signal at the null setting. Plot the modulation sensitivity versus the

audio frequency to determine the modulation bandwidth.

rev.05SEP89 Numbered Error Messages 8-5

1 2 ... 395 396 397 398 399 400 401 402 403 404 405 ... 452 453

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Agilent Technologies HP 8642A/B Specifiche Pagina 400