The Spurious Avoidance submenu defines whether the analyzer measures with a local oscillator frequency LO below or above the RF input frequency.
This feature can be used to eliminate known spurious components in the input signal that can distort the measurement, especially in the low frequency range.
In Auto mode, the analyzer auto-selects the local oscillator frequency, depending on the receiver (RF) frequency and the test port. This mode systematically avoids known spurious signals provided that no frequency conversion occurs in the test setup.
LO > RF means that the LO frequency is always above the measured RF frequency. This mode is appropriate for avoiding single, known spurious signals.
LO < RF means that the LO frequency is always below the measured RF frequency. This mode is appropriate for avoiding single, known spurious signals.
In the presence of several spurious signals, setting the Spurious Avoidance parameter globally may not be sufficient. To improve the result, perform a Segmented Frequency sweep and assign independent LO frequencies to the individual sweep segments.
Application example
Consider the following test setup with strongly reflecting DUT (e.g. a bandpass in its stop band) that is measured in transmission. a1 is generated at a frequency RF.The reflected wave b1 falls into the receiver mixer of the analyzer port 1, where a small fraction of the mixer product RF + 2*IF can be reflected back towards the DUT. If this spurious wave a'1 passes the DUT, then it is received as b'2 at port 2, together with the wanted signal b2.
LO > RF implies that LO = RF + IF. The mixer at port 2 converts both the wanted signal b2 and the spurious signal b'2 which is at the frequency RF' = IF + LO, to the same IF frequency. The response of an ideal, infinitely steep bandpass filter with a pass band between B- and B+ looks as follows:
For a wide bandpass, the spurious response flattens the filter edges.
The spurious signal can be eliminated by dividing the sweep range into two segments with different LO settings:
In the low-frequency segment, ranging up to the center frequency of the bandpass filter, the frequency of the local oscillator is set to LO < RF. This ensures that the spurious signal b'2 is not measured at port 2.
In the high-frequency segment, starting at the center frequency of the bandpass filter, the frequency of the local oscillator is set to LO > RF. If the center frequency is larger than B+ – 2*IF, then there is no distortion from b'2.
Remote control:
[SENSe<Ch>:]FREQuency:SBANd POSitive | NEGative | AUTO
Enables and configures the Automatic Level Control (ALC) for all channels. ALC keeps the level of the a waves (source level) at a constant value, irrespective of the DUT's input impedance. The measurement speed is slightly reduced.
In the Source section of the Port Configuration dialog, a subset of the ALC settings can be defined independently for each analyzer port. Individual port settings override the general ALC configuration.
If the analyzer cannot reach the designated source level at a particular sweep point, it displays the message Ports n: Source level failure. Check the data sheet for an overview of the available source levels.
The settings in the dialog are available as long as ALC is on.
ALC Bandwidth selects the bandwidth in the ALC control loop. With the automatic selection, the analyzer selects an appropriate bandwidth, depending on the source frequency and level. The bandwidth has an impact on the ALC Parameters; see below.
ALC Settling controls the automatic level control during the measurement. Clamp ALC during Measurement suspends the ALC mechanism while the analyzer acquires measurement data. The Control Range is the maximum change of the source signal level due to the ALC; a value of 5 dB means that the source signal level may vary in a 10 dB wide symmetric range around the start value. With a wider control range, the ALC can compensate for larger attenuation/gain factors. A possible limitation of the port powers (System Configuration – Power – Port Power Limits) is ignored while the ALC loop is active. Therefore a narrower control range (small value) may be required to protect sensitive DUTs from excess input levels. A narrow control range is often sufficient if the source power is relatively well known, e.g. from a previous power calibration.
Settling Tolerance Window defines the maximum variation of the source signal level after the ALC has settled. The analyzer will generate an error message Port <i> output power unleveled if the actual source power exceeds the window. The settling tolerance window has no impact on the ALC algorithm.
ALC Parameters defines the tuning parameters of the Proportional-Integral (PI) controller which the analyzer uses as a feedback controller for the ALC. With automatic settings, the parameters are determined by the ALC bandwidth. It is also possible to vary the parameters manually; see background information below.
ALC Start Offset increases/decreases the source signal level before the ALC loop is started. An appropriate value can speed up the ALC. If Use ALC Offset of Prev. Measurement is active, the ALC correction value (level offset) of the previous measurement (i.e. the last sweep point measured) is used. This setting is effective only for sweeps comprising a single partial measurement; it is ignored at the first sweep point or after a channel change.
ALC parameters
Proportional-Integral (PI) controllers provide two control parameters:
The Proportional Gain controls the change of the controller output in response to the current error value. If the value is too high, the ALC may become unstable. If it is too low, the ALC may not respond sufficiently to errors and become too slow.
The Integration Time controls the change of the controller output based on the integral of the error over time. If the value is high, the ALC becomes slower. If it is too low, the ALC may overshoot and thus become unstable.
The proportional and integration terms are summed to calculate the controller output, so there is a tradeoff between the two terms. With automatic ALC parameter setting, the control parameters are determined by the selected ALC bandwidth as shown in the following table.
ALC bandwidth
Proportional Gain
Integration Time
7.12 MHz
1.1200
4.3750E-007 s
1 MHz
0.7600
7.5000E-007 s
100 kHz
0.8800
1.3000E-005 s
10 kHz
1.7600
3.2000E-005 s
1 kHz
1.9200
3.2000E-004 s
500 Hz
0.0150
1.1000E-005 s
200 Hz
0.0500
7.6000E-005 s
100 Hz
0.0475
1.1200E-004 s
50 Hz
0.0138
7.2000E-005 s
30 Hz
0.0044
3.8000E-005 s
20 Hz
4.4800
For some test scenarios, an adjustment of the ALC parameters (loop tuning) can improve the stability and speed of the ALC.
DIAGnostic:ALC:SETTings[:STATe] ON | OFF DIAGnostic:ALC:AUBW DIAGnostic:ALC:BW DIAGnostic:ALC:CLAMp DIAGnostic:ALC:RANGe DIAGnostic:ALC:STOLerance DIAGnostic:ALC:PIParameter DIAGnostic:ALC:PIParameter:GAIN DIAGnostic:ALC:PIParameter:ITIMe DIAGnostic:ALC:SOFFset DIAGnostic:ALC:POFFset
Reduces the phase noise of the source signals. The measurement speed is slightly reduced.
[SENSe<Ch>:]FREQuency<1>:LPNoise ON | OFF