Conversion Loss Using the Frequency Offset Mode

Conversion loss is the measure of efficiency of a mixer. It is the ratio of side-band IF power to RF signal power, and is usually expressed in dB. (‘lb express ratios in dR, the dBm power in the denominator must be subtracted from the dBm power in the numerator.) The mixer translates the incoming signal, (RF), to a replica, (IF), displaced in frequency by the local oscillator, (Lo>.

Frequency translation is characterized by a loss in signal amplitude and the generation of additional sidebands. For a given translation, two equal output signals are expected, a lower sideband and an upper sideband.

FREQUENCY

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Figure 3-4.

An Example Spectrum of RF, LO, and IF Signals Present in a

Conversion Loss Measurement

The analyzer allows you to make a swept RF/IF conversion loss measurement holding the Lo frequency fixed. You can make this measurement by using the analyzer’s frequency offset measurement mode. This mode of operation allows you to offset the analyzer’s source by a Rxed value, above or below the analyzer’s receiver. That is, this allows you to use a device input frequency range that is different from the receiver input frequency range.

The following procedure describes the swept IF frequency conversion loss measurement of a broadband component mixer:

1. Set the LO source to the desired CW frequency and power level.

CW frequency = 1000 MHz Rower = 13 dBm

2.Set the desired source power to the value which will provide -10 dRm or less to the R channel input. Press:

3.Calibrate and zero the power meter.

4.Connect the measurement equipment as shown in Figure 3-5.

q The low pass filter is required to limit the range of frequencies passed into the R channel input port. The flter is selected to pass the IF frequencies for the measurement but prevent the L0 feedthrough and unwanted mixer products from confusing the phase lock loop operation.

q A pad is used to isolate the titer and improve the IF port match for the mixer.

q The attenuation of the power splitter is used to improve the RF port match for the mixer.

Making Mixer Measurements 3-7

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HP 8753E manual Conversion Loss Using the Frequency Offset Mode