HP 3048A Operating Manual page 175

Table 3-25. Numbered Error Messages (8 of 17)
4 If you do not know the modulation bandwidth for your VCO source, you can determine it
(cont'd) 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).
d. 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 on
an RF Spectrum Analyzer and adjust the audio modulation level for the first carrier null.
Calculate the VCO's modulation sensitivity at each audio frequency using the level and
frequency of the audio signal at the null setting.
7. The noise on one or both sources exceeds the small angle limit. For information
about verifying the noise level and recommended action, refer to Evaluating Noise
Above the Small Angle Line in Chapter 3 of this manual. For information about
measuring sources with high close-in noise, refer to Ignore Out of Lock in the Test
Mode Section of Chapter 5 of the HP 3048A Reference Manual.
5 Measured VCO peak tuning range exceeds 500 kHz (or 200 MHz.) The Peak
Tuning Range limit for the system is 500 kHz (or 200 MHz if an RF analyzer is configured
in the system). The Peak Tuning Range is the product of the VCO Tuning Constant
multiplied by the Voltage Tuning Range of the VCO. (If the system measures the VCO's
Tuning Constant, it will use the measured value to compute the Peak Tuning Range for
the measurement.)
Action: Reduce the Voltage Tuning Range entered for the VCO source you are using, or
select a VCO source with a lower Tuning Constant.
6 Unable to establish quadrature. Occurs when source is under system control and
system is unable to acquire quadrature. The system attempts to acquire quadrature at
the beginning of the measurement and after it has completed a measurement segment if
quadrature was lost at any time while the segment was being measured.
Action: Manually adjust for quadrature by adjusting the source frequency (or the Phase
Shifter if one is being used). Adjust for center scale on the HP 11848A Quadrature Meter.
3-128 Signal Source Applications
rev.l2JUL88