QM1004-2-18 Combined RF Upconverter/Downconverter
User Manual
2.7 FREQuency:LO1:PLLMode [INT
|
FRAC
|
1
|
0]
Note:
This v6 command replaces the FREQ:PLLM command available in previous hardware versions.
The GUI currently supporting v4, v5, and v6 hardware uses both commands to maintain support, while
v6 hardware only uses this latest command.
This command sets the PLL mode of the tunable Local Oscillator (LO1) located inside the QM1004-2-18
for both channels. A PLL mode of
INT
or
1
indicates that the PLL is operating in Integer mode, while a
PLL mode of
FRAC
or
0
indicates that the PLL is operating in Fractional Mode.
When the PLL is placed in
Integer
mode (also referred to as
Integer-N
mode), the output frequency is an
integer multiple of the reference oscillator frequency, divided by the reference divider, multiplied by the
hardware prescaler. The QM1004-2-18 has a fixed hardware prescaler of 4 and a reference frequency of
100 MHz. With the reference divider set to 1, the frequency resolution of FREQuency:TUNE is 400 MHz.
This means that 10.15 GHz and 10.55 GHz would both be tunable frequencies, as the corresponding LO
frequencies (14.8 GHz and 15.2 GHz) are divisible by 400 MHz, but 10.35 GHz would not be tunable
with the default LO1 reference divider as the corresponding LO frequency (15 GHz) is not divisible by
400 MHz. The user can manipulate the frequency step size in integer mode by changing the reference
divider. By increasing the reference divider value to 2, the tuneable frequency step size becomes 200
MHz, making 10.35 GHz a tunable frequency. While increasing the reference divider value decreases the
tunable frequency step size, phase noise performance is degraded.
When the PLL is placed in
Fractional
mode, the tuneable frequency can tuned with a resolution of 100
kHz. In the above example, the synthesizer would be capable of outputting a frequency of 10.0001 GHz
when operated in
Fractional
mode. When operating in fractional mode, the reference divider should al-
ways be 1 for best phase noise performance.
Table 4.6 summarizes the advantages and disadvantages of the two synthesizer operational modes.
Table 4.6:
Comparison between Integer and Fractional PLL modes
Mode
Advantages
Disadvantages
Integer
Lower Phase Noise
Frequency must be an inte-
ger multiple of
𝑓 𝑟𝑒𝑓
𝑁
*
𝑝𝑟𝑒𝑠𝑐𝑎𝑙𝑒𝑟
,
where
𝑓
𝑟𝑒𝑓
= 100 and
𝑝𝑟𝑒𝑠𝑐𝑎𝑙𝑒𝑟
= 4 for the QM1004-2-18.
Fractional
Can tune in 100 kHz frequency
steps
Increased Phase Noise
Increased spurious products
Quonset Microwave
Revision 6.1.0
37