Phase Noise Measurement: Phase Detectors

A double balanced mixer employing Schottky diodes with an IF response extending to dc is the usual choice for low noise systems

Selecting the mixer

When using a mixer as a phase detector it is operated with 90 degree phase difference between the RF and LO signals as this maximises the sensitivity to PM noise and minimises the sensitivity to AM noise. When using a phase lock loop to maintain this it is important to note that the PLL actually adjusts the phase between the mixer inputs so that the dc mixer output is zero. Real mixers have a significant dc offset due to diode mismatch and transformer asymmetry. Thus to achieve minimum sensitivity to AM noise a dc offset has to be added to the PLL amplifier to achieve phase quadrature. The dc offset of most mixers isn't specified (it varies with input frequency, signal level, and temperature as well as IF port termination ), however it can be estimated from the mixers port to port isolation specs. Mixers specified for phase detector operation have low dc offset with a low dc offset tempco they also have high port to port isolation. Operating the mixer with both RF and IF ports saturated minimises its sensitivity to AM noise. The electrical length from the RF to IF port of a mixer isn't necessarily equal to the electrical length from the LO to the IF port so even if the mixer has zero dc offset due to diode mismatch and transformer asymmetry the RF and LO input signals may not be in phase quadrature at balance so a dc offset at the PLL amplifier input may be required to achieve exact phase quadrature between the RF and LO signals. This effect can be more significant at higher phase detector input frequencies.




2) mixers as phase detectors

3) Environmental Effects in Mixers an Frequency Distribution Systems

Mixer Port Termination

IF Port

The mixer noise is lowest when the IF port is terminated in a capacitor, however this reduces the IF port high frequency cutoff to less than 100kHz. The sum frequency should see a relatively low impedance (50 ohms or a short) whilst the mixer dc load impedance should be high.

RF and LO Ports

To minimise VSWR at the RF and LO ports a low value (~33 ohms) resistor can be connected in series with the RF and LO ports of the mixer The optimum value depends on the mixer drive level and the particular mixer. A 50 ohm (3-6dB) attenuator pad connected between RF and IF port sources and the corresponding series resistor further reduces the VSWR of the RF and LO ports to relatively low values.




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