Matching is required at many frequencies

An ideal mixer will produce the sum and difference frequency from two ports to become the output of the third port. Real world mixers will also produce overtones and sums and differences between overtones.

Table 4 shows measured signals on a SBL-1 mixer with all ports terminated in 50 ohms. The input signals are 0dBm and the peak level when both are in phase is 6dBm. The output signal is compressed by about 2dB.

Table 4 gives the strongest signals at each port. The IF port has to be terminated in 50 ohms for all signals at or near the IF frequency, regardless of whether they are inside or outside a subsequent X-tal filter. The sum frequency at 277 as well as sum and difference frequencies with the third overtone of the LO have also to be terminated properly.

In case the IF is used for a narrow band only, the mixer feeding an X-tal filter via an amplifier, the circuitry connected to the IF port is not difficult because an amplifier with a noise figure of 2dB is good enough.

In case the IF is wideband for use with a shortwave radio or a wideband A/D converter the amplifier at the IF port must have a really low noise figure at the same time as the load impedance is 50 ohms. When the RF amplifiers are designed for a noise figure of 2dB at the mixer IF port, the noise temperature of 170K at the input of the IF chain should to a large extent be due to noise from the A/D converter or shortwave radio or whatever stage is limiting dynamic range. A noise figure of 0.6dB of the amplifier itself (43K) will allow 75% of the noise to come from subsequent stages.

Hiding the noise from later stages by use of more IF gain will degrade the dynamic range. The IF amplifier can be made 50 ohms for the IF frequency range, but for all the frequencies above 100MHz a resistor must be inserted. This resistor can be placed in series with a parallel LC circuit for the IF frequency band to prevent the resistor from degrading the noise figure at IF frequencies. The high frequency load at the IF port is layout sensitive! The impedance needs to be in the neighbourhood of 50 ohms from 100 to 500MHz.

The RF port has to be matched to 50 ohms for the main RF band and for the mirror image frequency. Matching the RF port at the sum and the difference frequencies is also a good idea although less important. Other frequencies are not important.

The LO port is not very sensitive. A parallel LC across it is ok. I have not investigated if a very high impedance at some frequency will create any problem.

INPUT SIGNALS
RF=144.0(-10dBm)  144.1 (-10dBm)
LO=133.3(+8dBm)

  Freq          Level
  (MHz)         (dBm)
------------------------------------------
SIGNALS OUT FROM IF PORT
 10.7,  10.8    -10      RF-LO  = IF
277.3, 277.4    -10      RF+LO  = 2LO+IF
255.8, 255.9    -22      3LO-RF = 2LO-IF
543.9, 544.0    -22      3LO+RF
144.0, 144.1    -29      RF     = LO+IF
 10.6, 10.9     -29      IM     = IF +/- dRF
266.6           -34      2LO
133.3           -37      LO
122.5, 122.6    -42      2LO-RF = LO-IF
143.9, 144.2    -46      RF +/- dRF

SIGNALS OUT FROM RF PORT
144.0, 144.1    -11      RF     = LO+IF
143.9, 144.3    -25      RF +/- dRF
277.3, 277.4    -35      RF+LO  = 2LO+IF
122.5, 122.6    -36      2LO-RF = LO-IF
122.4, 122.7    -38      LO-IF +/-dRF
 10.7, 10.8     -42      RF-LO = IF

SIGNAL OUT FROM LO PORT
133.3           -10       LO
399.9           -19      3LO
288.1           -48      2RF
288.0, 288.2    -51      2RF +/- dRF
266.6           -52      2LO


Table 4. Signal levels for a SBL-1, a level 7
schottky diode mixer. All ports terminated in 50 ohms.

Overtones

It is also a good idea to filter out overtones from the RF amplifier.

A wideband 50 ohm output amplifier would solve the matching problem very easily but second order harmonics will come already at low levels and they will mix with the sum frequencies and produce third order intermodulation.

The RF amplifier that preceeds the mixer should have a filter that removes the overtones. The matching to 50 ohms outside the filter passband can be done with a 3dB pad or some diplexer circuit.

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