IviDownconverterExternalMixer Extension Group
The IviDownconverterExternalMixer extension group supports using an external mixer. Use the IviDownconverterExternalMixer properties to enable and disable the mixer as well as to control the harmonic and the bias level and limit. This extension group also defines VIs that configure these properties.
When you use an external mixer, you must convert many of the downconverter settings carefully so that you can read the values. Specifically, ensure that you carefully configure the frequency, harmonic number, mixer configuration, and conversion loss to use 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, as shown in the following formula:
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. Although both even and odd harmonics can be used, the harmonic is generally odd in balanced mixers. Even order terms are naturally attenuated. The selected harmonic, together with the setting range of the 1st LO, determines the limits of the settable frequency range. The following limits apply:
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 |
Mixer Configuration
The external mixers are typically configured either as two-port or three-port devices. Single-diode mixers generally require a DC voltage that is applied through the LO line. This DC voltage is 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 used to feed the LO signal to the mixer. The measured signal 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 measured signal is fed to the RF input of the external mixer.
Conversion Loss
The maximum input level depends on the external mixer’s conversion loss, the LO drive level to the mixer, and the mixer configuration. Conversion loss is the difference between a test signal directly connected to the RF input of the downconverter or the IF input to the signal strength at the output of the external mixer’s IF output port for the same drive level. For example, let the desired IF signal be -20dBm at 250MHz. If an external mixer downconverted a -20dBm 30GHz signal to 250MHz, at -44dBm, it would have a conversion loss of 24dB.
In general the maximum input signal to an external mixer’s input, the RF port, should be 15dB lower than the mixer’s LO drive level to ensure proper linear operation. An external mixer with a +13dBm LO drive level should not have an input above -2dBm, for example. External amplifiers and attenuators can affect this level. Operating well below the LO drive level eliminates RF input compression on the main signal. Although the system usually provides filtering to minimize unwanted mixing products and still downconverts larger signals, overdriven input signals can be reproduced inaccurately at the IF output.
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 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.
IviDownconverterExternalMixer VIs
IviDownconverter Configure External Mixer Bias
IviDownconverterExternalMixer Properties
IviDownconverter External Mixer Bias Enabled
IviDownconverter External Mixer Bias Level
IviDownconverter External Mixer Bias Limit
IviDownconverter External Mixer Enabled