IviSpecAnExternalMixer Extension Group
Many spectrum analyzers have outputs and inputs that allow external equipment to use the IF or mixer signal that the spectrum analyzer uses. In this case, external equipment can be used to mix signals to convert them to measurable frequencies. This allows the use of an analyzer to measure values that are outside of the normal frequency range of the equipment. When using an external mixer, many of the settings of the analyzer have to be carefully converted to allow you to know what is meant by the values read. Specifically, the frequency, the harmonic number, mixer configuration, and conversion loss must be configured carefully to be able to use the external mixing successfully. The frequency of the input signal can be expressed as a function of the local oscillator (LO) frequency and the selected harmonic of the 1st LO is as follows:
fin = n * fLO +/- fIF Where: fin frequency of input signal n order of harmonic used for conversion fLO frequency of 1st LO fIF intermediate frequency
The Harmonic number defines the order n of the harmonic used for conversion. Both even and odd harmonics can be used. The selected harmonic, together with the setting range of the 1st LO, determines the limits of the settable frequency range. The following applies:
Lower frequency limit: fmin = n * fLO,min - fIF Upper frequency limit: fmax = n * fLO,max + fIF Where: fLO,min lower frequency limit of LO fLO,max upper frequency limit of LO
The following sections describe the mixer configuration and the conversion loss table configuration.
Mixer Configuration
The external mixers are typically configured either as two-port or three port mixers. Single-diode mixers generally require a DC voltage which is applied via the LO line. This DC voltage is to be tuned to the minimum conversion loss versus frequency. Some instruments can define a limit for the BIAS current. The two-port mixer connects the LO OUT / IF IN output of the analyzer to the LO/IF port of the external mixer. The diplexer is contained in the analyzer and the IF signal can be tapped from the line which is used to feed the LO signal to the mixer. The signal to be measured is fed to the RF input of the external mixer. On the other hand, the three-port mixer connects the LO OUT / IF IN output of the analyzer to the LO port of the external mixer. The IF IN input of the analyzer is connected to the IF port of the external mixer. The signal to be measured is fed to the RF input of the external mixer.
Conversion Loss
The maximum settable reference level depends on the external mixers conversion loss which is defined by average conversion loss or by using the conversion loss table. For example, if an IF signal with a level of -20 dBm is applied to the LO OUT / IF IN or IF IN input of the spectrum analyzer, full screen level is attained. Consequently, the maximum settable reference level is -20 dBm at a set conversion loss of 0 dB. If a conversion loss > 0 dB is entered, the maximum settable reference level increases in the same proportion. If the maximum possible reference level is set on the analyzer, this level is reduced if a smaller conversion loss is entered. In addition to the dynamic range of the spectrum analyzer the 1 dB compression point of the mixer has to be taken into account. The levels of the input signals lie well below this value to avoid generation of harmonics of these signals in the mixer. These are converted by the LO signals harmonics of higher order and appear in the displayed spectrum. Some instruments allow the definition of conversion loss tables. The Conversion loss table allows the conversion loss of the mixer in the selected band to be taken into account as a function of frequency. Correction values for frequencies between the individual reference values are obtained by interpolation (Linear interpolation). Outside the frequency range covered by the table the conversion loss is assumed to be the same as that for the reference value marking the table limit.
IviSpecAnExternalMixer VIs
IviSpecAn_ConfigureConversionLossTable
IviSpecAn_ConfigureConversionLossTableEnabled
IviSpecAn_ConfigureExternalMixer
IviSpecAn_ConfigureExternalMixerBias
IviSpecAn_ConfigureExternalMixerBiasEnabled
IviSpecAn_ConfigureExternalMixerEnabled
IviSpecAn_ConfigureExternalMixerNumberOfPorts
IviSpecAnExternalMixer Properties
IVISPECAN_ATTR_EXTERNAL_MIXER_AVERAGE_CONVERSION_LOSS
IVISPECAN_ATTR_EXTERNAL_MIXER_BIAS
IVISPECAN_ATTR_EXTERNAL_MIXER_BIAS_ENABLED
IVISPECAN_ATTR_EXTERNAL_MIXER_BIAS_LIMIT
IVISPECAN_ATTR_EXTERNAL_MIXER_CONVERSION_LOSS_TABLE_ENABLED
IVISPECAN_ATTR_EXTERNAL_MIXER_ENABLED