page 2RF passive mixers

RF mixers useful informations

The passive mixers are versatile and very useful components, they are suitable for other applications in addition to theirs classic, some of these applications are well known but others a little less. Here follows some little tips on how to best use them.

Balanced Modulator Demodulator DSB	Balanced mixers and modulators are substantially equal, mixers are also used as audio modulators or demodulators provided that the IF port is DC coupled, in fact some years ago they were called also as balaced modulators (see VHF Commication magazine 2-96).	in the reverse mode as demodulator
Phase detector	Applying two identical frequencies are identical to the RF and OL ports, in IF there will be d.c. voltage proportional to the phase difference of the two signals. This is obviously valid only for those mixers who have the IF port coupled in d.c.. The early synthesizers in the 60s had the phase comparator made by a balaced mixer. Some mixers have a positive polarity output other negative, eg. SBL-1 and SRA-1 have negative polarity with offset of 1mV.	relative phase between two signals
PSK QPSK Modulator Demodulator	In simple words we can say that two mixers, a 0° hybrid and a 90° hybrid can form a modulator or demodulator for digital signals, this happened for example at the beginning with the first GSM module. In this case, however, the bandwidth is limited to 10-15% of the mixer bandwdth.
Attenuator and Limiter	By applying a signal to the LO port with RF output port it is possible, through the IF input port, make an attenuation with typical range of 3 - 40 dB. The same function is as limiter, pulse modulator, etc.. For example, the mixer SRA-1 and SBL-1 have the minimum attenuation at 3dB with 20 mA of bias.

For more informations see the manufacturers' catalogues about balanced mixers like: Merrimac, Ma-Com, Anzac, RHG, Mini-Circuits, etc… there are often available many application notes of considerable interest.

Suggestions on how to use passive mixers

• The RF and IF ports can be freely swapped. For example in the case of conversion up (as in a spectrum analyzer or TX UP converter) RF port is exchanged with the IF as the RF port often has a larger range than IF.
• The passive mixers offer very large IMD dynamics, the IF port should be well adapted to broadband for example by a 2 - 4 dB attenuator on IF output to improve the matching (see not connectorized attenuators).
• A mixer can be used also beyond its frequency specification, being a broadband aperiodic circuit the slope of degradation is slow. In practical experience degradation is the worsening of insulation between the ports (or of the balance if it is used as a balanced modulator) going up versus frequency the LO port would seem to be more tolerant to overcome its limits either at higher or at lower frequencies.