MIXERS AND FREQUENCY DISCRIMINATORS

Mixers are used to convert a signal from one frequency to another. This is done by combining the original RF signal with a local oscillator (LO) signal in a non-linear device such as a Schottky-barrier diode.

The output spectrum includes:

• The original inputs, LO and RF
• All higher order harmonics of LO and RF
• The two primary sidebands , LO +/- RF (m,n = 1)
• All higher order products of mLO +/- nRF (where m,n are integers)
• A DC output level

The desired output frequency, commonly called the intermediate frequency (IF), can be either the lower (LO-RF) or upper (LO+RF) sideband. When a mixer is used as a down converter, the lower sideband is the sideband of interest.

A microwave balanced mixer makes use of the 3 dB hybrid to divide and recombine the RF and LO inputs to two mixing diodes. The 3 dB hybrid can be either the 90° or 180° type. Each has certain advantages which will be covered later. The critical requirement is that the LO and RF signals be distributed uniformly (balanced) to each mixer diode.

Figure 1 is a typical balanced mixer block diagram. The mixer diodes are reversed relative to each other; the desired frequency (IF) components of each diode are then in-phase while the DC outputs are positive and negative respectively.

The two diode outputs are summed in a tee where the DC terms cancel and only the desired IF component exists at the IF port.

Other types of mixers exist, including the double-balanced mixer, and the Ortho-Quad® (quadrature fed dual) mixer. The relative advantages and disadvantages of each of the four types are summarized in Table 1.

Table 1. Mixer Comparison

NOTES:

1. Poor = 2.5:1 typical ; Good = 1.3:1 typical
2. Conversion loss: lowest: 5-7 dB typical; Low 7-9 dB typical
3. Poor: 10 dB typical ; Good: 20 dB typical ; Very Good: 25-30 dB typical ; Excellent: 35-40 dB typical4.
4. • Poor: partial rejection of LO/RF even harmonics
   • Fair: slightly better
   • Good: can reject all LO even harmonics
   • Very Good: can reject all LO and RF even harmonics

Used in various circuits, mixers can act as modulators, phase detectors, and frequency discriminators.

The phase discriminators can serve as a signal processing network for systems designed to monitor bearing, polarization, and frequency of AM or FM radiated signals.

A frequency discriminator uses a phase discriminator and adds a power divider and delay line at the RF input as shown in Figure 2.

The unknown RF signal "A" is divided between a reference and delay path. The differential delay ( T ) creates a phase difference () between the two signals which is a linear function of frequency ( f ) and is given by = 2f T.

When the two output signals are fed to the horizontal and vertical input of an oscilloscope, the resultant display angle will be a direct function of frequency.