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Spurious signals that appear when there is no input signal are known as non-input-related spurs. These spurs are combinations of internal signals that mix and result in a signal in the NI 5665 IF. A common type of input-related spur is generated by the internal LO signals. If one or more of the following equations is satisfied, a spur of this type may be created:

mf_LO1 + nf_LO2 + pf_LO3 = f_IF1

mf_LO1 + nf_LO2 + pf_LO3 = f_IF2

mf_LO1 + nf_LO2 + pf_LO3 = f_IF3 (NI 5603 only)

where   m is an integer

n is an integer

p is an integer

f_LO2 is equal to 4 GHz (NI 5603 or NI 5605 for RF frequencies ≤ 3.6 GHz) or 800 MHz (NI 5605 for RF frequencies > 3.6 GHz)

f_LO3 is equal to 800 MHz (NI 5603 or NI 5605 for RF frequencies < 3.6 GHz)

Note  For the NI 5665 (14 GHz), the 4 GHz LO2 IN front panel connector is connected to the LO2 OUT NI 5653, but the connection is not used for RF input frequencies greater than 3.6 GHz. The LO2 connection can still cause spurs in the high band.

Depending on the IF Filter Selection option you choose, f_LO1 and f_IFx can be calculated as shown in the following tables.

NI 5603 and NI 5605 (for RF Frequencies ≤ 3.6 GHz)
IF Filter Selection	fIF1	fIF2	fIF3	fLO1 Range
300 kHz	4601.0 MHz	601.0 MHz	199.0 MHz	4601.0 MHz to 8201.0 MHz
5 MHz (NI 5605 only)	4610.0 MHz	610.0 MHz	190.0 MHz	4610.0 MHz to 8210.0 MHz
Through	4612.5 MHz	612.5 MHz	187.5 MHz	4612.5 MHz to 8212.5 MHz

NI 5605 (for RF Frequencies > 3.6 GHz and ≤ 7.5 GHz)
IF Filter Selection	fIF1	fIF2	fLO1 Range
300 kHz	601.0 MHz	199.0 MHz	4201.0 MHz to 8101.0 MHz
5 MHz	610.0 MHz	190.0 MHz	4210.0 MHz to 8110.0 MHz
Through	612.5 MHz	187.5 MHz	4212.5 MHz to 8112.5 MHz

NI 5605 (for RF Frequencies > 7.5 GHz)
IF Filter Selection	fIF1	fIF2	fLO1 Range
300 kHz	601.0 MHz	199.0 MHz	4050.50 MHz to 7300.50 MHz
5 MHz	610.0 MHz	190.0 MHz	4055.00 MHz to 7305.00 MHz
Through	612.5 MHz	187.5 MHz	4056.25 MHz to 7306.25 MHz

LO Leakage to RF and IF

LO leakage is a portion of the LO signal that leaks from the NI 5603 or the NI 5605 LO IN to the RF IN or IF OUT front panel connectors. LO leakage to the RF IN front panel connector is most often seen at low tuned frequencies. The NI 5665 has three LO frequencies: LO1 tunes along with the RF input frequency, LO2 is fixed at 4 GHz, and LO3 is fixed at 800 MHz.

The following LO leakage signal frequencies are related to the frequency of the LO signal:

• LO3 leakage frequency: 800 MHz
• LO2 leakage frequency: 4000 MHz
• The LO1 frequency can be calculated, as shown in the following table, where F_tune is the tuned frequency of the NI 5665 system.

IF Filter Selection	fLO1 Frequency
	NI 5603 and NI 5605 (for Frequencies ≤ 3.6 GHz)	NI 5605 (for Frequencies > 3.6 GHz and ≤ 7.5 GHz)	NI 5605 (for Frequencies > 7.5 GHz)
300 kHz	Ftune + 4601.0 MHz	Ftune + 601.0 MHz	(Ftune + 601.0 MHz)/2
5 MHz (NI 5605 only)	Ftune + 4610.0 MHz	Ftune + 610.0 MHz	(Ftune + 610.0 MHz)/2
Through	Ftune + 4612.5 MHz	Ftune + 612.5 MHz	(Ftune + 612.5 MHz)/2