10-9 Balanced mixer with low-pass filter output for mixing two RF sources. (A) RF low-pass filter connected to balanced mixer, (B) physical implementation.

A variation on the theme is shown in Fig. 10-8C. Although the output in Fig, 10 8A is taken from the emitter of the transistor, the variation in Fig. 10-8C uses a secondary winding over the tuning inductor (L¡) to output the signal.

Another variation useful for a wide variety of circuits is shown in Fig. 10-9A, and an actual example prototy] >ed by me is shown in Fig. 10-9B. This circuit uses a double-balanced mixer (DBM), such as the SRA-1 and related products by Mini-Circuits Laboratories, Inc. One signal is applied to the RF port of the DBM, and the other signal is applied to the LO port of the DBM (note the relative signal levels). The difference frequency is taken across the two shorted IF OUT ports. The low-pass filter is used to remove residual LO or RF signal, 1 used this scheme to produce a ham-band sweep generator. One oscillator was fixed tuned to 37 MHz then swept with a sawtooth tuning voltage ±1 MHz. The other oscillator was tuned to frequencies that would heterodyne the 37-MHz signal to the middle of various HF ham bands. A bank of potentiometers selects the dc bias required to achieve the correct output frequency

The RF signal generator circuit is generally well-behaved and thus is easy to build and align if proper technique is used. You can be quite successful making these circuits perform the tasks for which they were designed.


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