A6A8 YIG-Tuned Mixer (YTX),A6A5 Amplifier/Coupler/Load Unit (ACLU), Circuit DescriptionThe YTX consists of a three-pole tunable bandpass filter and a single-diode microwave harmonic mixer. The 2 to 6 GHz local oscillator (LO) signal enters the ACLU, and is amplified and coupled into the YTX to turn its diode on and off at the LO rate. The diode conduction angle is adjusted with DC diode bias to be optimum for the harmonic of the LO being used. The incoming microwave signal passes through the bandpass filter, through the mixer diode, as the diode switches on and off, and into the precision 500 load of the ACLU. The IF signal at 321.4 MHz travels down the same line as the higher frequency LO, but a matching network couples the 321.4 MHz IF signal to the ACLU output while blocking the LO.
The three-pole filter has two purposes:
1.To reject the image response of the mixer 642.8 MHz away from the desired signal.
2.To reject the LO signal that would otherwise come out of the mixer’s microwave input port, and out of the instrument front panel.
Handling the YIG-Tuned Mixer
Caution The YIG-Tuned Mixer (YTX) contains an extremely small and sensitive diode. A reverse voltage in excess of 3 V may damage it. It is connected directly to the center conductor of the LO/IF connector, and is especially susceptible to blowout from electrostatic discharge. Before connecting any coaxial cable to the YTX, connect the YTX BIAS pin to ground, discharge the coaxial cable by connecting the center conductor to ground (YTX body), then simultaneously touch both to chassis ground or the YTX package. It is recommended that the other end of the cable be connected first before going through this procedure.
Once the cable from the YTX to the ACLU is properly installed, the BIAS pin of the ACLU is directly connected to the YTX diode. Therefore, treat the BIAS pin with extreme caution, especially avoiding electrostatic discharge.
As a general rule, connect all devices to the chassis before connecting to or touching the BIAS pin on the ACLU.