MIXER SPECIFICATION GUIDE
All specified parameters are measured in a 50 ohm system with the high level CARRIER applied to the LO port unless otherwise stated. ETTI mixers are available with nominal LO drive levels from +3 dBm, for low power and battery operated systems, to +27 dBm, where wide dynamic range or low spurious outputs are required. All specified performance is for an LO drive level of +7 dBm unless otherwise stated.
ISOLATION: The parameter of greatest concern in most mixer applications is the port-to-port isolation. Mixer isolation is defined as the ratio, in dB, of the signal power available into one port of the mixer to the measured power level of that signal at one of the other mixer ports in a 50 ohm system. Available power is defined as the power that would be delivered to a perfect 50 ohm load, generally expressed in decibels above one milliwatt (dBm). Most mixer designs result in maximum isolation from the LO to the RF ports. The LO to RF isolation is generally slightly less at the higher frequencies and the RF to IF isolation is the poorest. Single balanced mixers require all RF to IF isolation to be provided externally. Mixer isolation is a function of the equality of the diode dynamic characteristics and the accuracy of the transformer’s balance. High second harmonic content in the CARRIER signal will also be detrimental to mixer isolation. Isolation deteriorates at the higher frequencies due to the parasitic reactances in the circuit.
CONVERSION LOSS: Conversion loss is a measure of mixer efficiency. Mixer conversion loss is defined as the ratio, in dB, of the INPUT signal level to the level of one of the OUTPUT sidebands in a 50 ohm system with the CARRIER drive set at the specified nominal available power level. In the ideal mixer, one half of the available INPUT signal power resides in each of the OUTPUT sidebands. The mixer loss in excess of this inherent 3 dB reduction is due to spurious product generation, diode insertion loss, and transformer core losses at mid-frequency. Conversion loss will deteriorate at the high and low frequency extremes due to transformer roll-off. The DC coupled IF port does not display a low frequency roll-off as long as the INPUT signal remains within the mid-frequency range. Single balanced mixers require external decoupling of the IF and RF ports to achieve their specified conversion loss. Conversion loss can be improved somewhat by providing a short circuit impedance at the OUTPUT port for the undesired sideband. Conversion loss is a function of the CARRIER drive level as shown in Fig. 4.
Fig. 4 - IF-RF Conversion Loss vs. LO Drive
NOISE FIGURE: The I/F diode noise for the Schottky-Barrier diodes employed is negligible for frequencies above 10 kHz. For frequencies above 400 kHz the SSB noise figure is, for all practical purposes, the conversion loss. Noise figure is deteriorated by CARRIER drive level above the specified nominal.
SPURIOUS OUTPUT: The ideal mixer would be a perfect multiplier. By a simple trigonometric identity it can be shown that a perfect multiplier would produce only two frequency components at its OUTPUT, the CARRIER frequency plus and minus the INPUT signal frequency. The diodes transfer characteristic can only be represented by a power series expansion that results in a continuum of frequency components all harmonically related to the CARRIER and INPUT signal frequencies. In the perfectly balanced double balanced mixer, the fundamental and all harmonics of the CARRIER and INPUT signals, as well as their even ordered products, will cancel in the mixer and will not appear at the OUTPUT. Since perfection has not quite been achieved, these components will appear at the OUTPUT attenuated by the degree of mixer balance. The lower odd ordered mixer products will be the highest level at the OUTPUT. The actual magnitude of this multitude of spurious mixer products is dependent on CARRIER and INPUT signal levels, frequency, load impedance, and temperature. Most of these spurious products will fall far outside the desired OUTPUT bandwidth. A variety of simple graphical techniques are available for determining what product or products will cross over or fall within the OUTPUT passband.
Fig. 5 - Typical Mixer Spurious Level -- mfL ± nfR
INTERMODULATION DISTORTION (IMD): When the INPUT signal consists of two or more simultaneous frequency components, a whole new family of spurious products result. These have come to be known as intermodulation distortion (IMD) products and are the result of harmonically related combinations of the INPUT signal frequencies. The most undesirable of these are the odd order difference IMD products which generally fall well within the OUTPUT signal passband. The IMD performance of the mixer is determined by the magnitude of the third order products of two closely spaced equal INPUT tones. Theoretically, the third order IMD products will increase 3dB for each 1 dB increase in the level of the two equal tones. By specifying the typical input coordinates of the intercept of the extrapolated transfer characteristics of one of the desired first order tones and one of the third order products, the actual level of the third order product output at any two tone input level can be estimated. A first order approximation of the ratio in dB of the power level of one of the equal tones to the level of one of the third order IMD products at the OUTPUT can be obtained from the relation:
IMD ratio (dB) = [Input (dBm) – intercept (dBm)] x 2
The third order intercept point is a theoretical point and cannot be realized. The third order response deviates from linear as saturation is approached. The true intercept point is essentially independent of the saturation level and is primarily a function only of the third order curvature of the diode transfer characteristic. However, as the INPUT approaches saturation, the transfer characteristic is modified when the INPUT signal begins to effect the diode conduction cycle. This change in conduction period will often temporarily reduce the third order curvature before it abruptly deteriorates. For best IMD performance, high level models should be considered.
CROSS MODULATION: Cross modulation is also a third order effect. A direct mathematical relation exists between two tone third order IMD and cross modulation performance. Since they are identical phenomena, the third order intercept point is also a good relative indicator of mixer cross modulation performance.
Fig. 6 - Two Tone IMD Performance
CONVERSION COMPRESSION: As the INPUT signal is increased it will reach a point where its peak envelope voltage becomes appreciable with respect to the carrier. As this occurs, some INPUT signal power is converted to switching power and the OUTPUT sideband transfer curve begins to depart from its linear response. The conversion compression point is defined as the INPUT signal level at which one of the OUTPUT sidebands deviates from linear by 1dB. A good first approximation of the compression point is the INPUT signal level, which is 6 dB less than the CARRIER level.
DESENSITIZATION: Desensitization is the same phenomena as conversion compression. The desensitization point is defined as the INPUT level of a strong adjacent channel signal which will cause one of the OUTPUT sidebands of a low level signal to compress by 1 dB. A good first approximation of the desensitization point is 2 dB less than the compression point.
DYNAMIC RANGE: The mixer’s dynamic range is defined as the ratio of the INPUT signal which produces the minimum acceptable OUTPUT signal-to-noise ratio and the INPUT signal level at OUTPUT saturation. The actual dynamic range value is determined by the specifications of the application. In some applications the saturation level may be specified as the compression point or a number of dB below the compression point. In linear systems, it may be defined as the INPUT level at which certain spurious or IMD products reach a specified level. Once the system specifications have been arrived at, the dynamic range of the mixer in any application may be determined from the relation:
Dynamic Range (dB) = 144 – mixer noise figure – minimum acceptable S/N ratio