Agilent Technologies 856290216 Fractional N PLL, Postscaler Divide Numbers, Synthesizer Section

Table 1 l-10. Postscaler Divide Numbers

Divide	I
	Dll DlO
Number
7	0
6	1
5	1

-7

D9 Input output Range (MHz) Range (MHz)

(A14J304) (A14TP4)

840 to 973 60.0 to 69.5

834 to 987.96 69.5 to 82.33

823.2 to 960 82.33 to 96.0

If the output frequency is wrong by less than 1 MHz, the phase locked loop is not unlocked but still requires repair. Continue with the “Fractional N Oscillator PLL” section.

Fractional N PLL

The fractional N PLL provides a synthesized frequency in the range of 60 MHz to 96 MHz. The 800 MHz to 1020 MHz voltage controlled oscillator (VCO) in the loop is divided down to lock with the 2.5 MHz reference. Simultaneously, the VCO is divided by two and then by either 5, 6, or 7 to generate the 60 MHz to 96 MHz output.

The prescaler (function block AR) supplies the clock signal for the fractional divider and is required for the fractional divider to operate. At the start of a fractional N sweep, the fractional divider is set to a value for the start frequency and a sweep rate. It then sweeps for as long as HSCAN is high. Use the following procedure to troubleshoot unlocked loop problems or problems of locking to the wrong frequency (by less than 1 MHz):

1.Check the two LEDs on Al4 frequency control assembly. If either LED is lit, the fractional N phase locked loop is not locked.

2.The 10 MHz reference is required for fractional N operation. It is divided by four to

2.5 MHz in the reference divider circuitry, block AN. It is used to lock the divided voltage controlled oscillator (VCO) frequency. Check that the 10 MHz reference is present at A14J301. The 10 MHz reference is derived from the 600 MHz reference on the Al5 RF assembly.

3.Change the spectrum analyzer from the fractional N span to 0 Hz.

4.Check the frequency at A14TPl. It should equal the value found by pressing a,

MORE 1 OF 2, FREXj DIA$ZNOSE , and R$W OSC FREq .

5.Check the tune voltage at R240 in function block AQ.

6.Look up the expected problem area in Table 11-11 with the information from steps 4 and 5. Go to the appropriate troubleshooting steps.

1 l-32 Synthesizer Section