RF Single balanced mixer vs double balanced mixer vs triple balanced mixer | difference between Single balanced mixer, double balanced mixer and triple balanced mixer
This page compares RF Single balanced mixer(SBM) vs double balanced mixer(DBM) vs triple balanced mixer and mention difference between Single balanced mixer and double balanced mixer type. It also compares these two types with doubly double balanced mixer i.e. triple balanced mixer.
There are two main types of RF mixers viz. passive mixer and active mixer. Passive mixers are made of using diode devices. Active mixers are made of using BJTs or FETs. Refer RF Mixer basics which describes circuit symbol of RF mixer, operation of RF mixer using mathematical equation and rules of thumb to be considered while using mixer in RF design.
RF Mixers are widely used in RF and Microwave circuit design mainly RF up conversion and down conversion. The RF device which does this is referred as RF Transceiver. Refer Design of RF frequency converter using discrete RF mixers. RF Mixers are available as connectorized modules as well as in the form of drop-in modules.
Single device mixer or unbalanced mixer
Figure-1 depicts single device mixer type. It uses one nonlinear diode device. To achieve best performance i.e. to obtain maximum conversion gain, LO node should be SC(short circuit) at RF and IF frequencies. RF node should be SC at the LO frequency in order to prevent LO leakage into RF port.
It is extremely useful for very high frequency in the millimeter wave frequency band.
It is very simple in design and can help achieve broad bandwidth.
Disadvantage: It does not attenuate LO AM noise and needs injection filter.
As mentioned unbalanced mixers allow unwanted considerable levels of LO and RF signals. Expensive filters are needed to remove these. Moreover it is inconvenient to employ these filters in the design. In order to avoid these disadvantages of unbalanced type of mixers, balanced type of RF mixers have been designed and developed.
Following are the benefits of balanced RF mixer type over unbalanced single device mixer type:
• Rejection of spurious responses and intermodulation products.
• Better LO-to-RF, RF-to-IF, and LO-to-IF isolation.
• Rejection of AM noise in the LO as mentioned above.
The demerits of balanced mixers are their requirement of greater LO power. Moreover they are complex in design compare to single device type.
There are three types of balanced RF mixers i.e. single balanced, double balanced and triple balanced (Doubly double balanced). Let us compare and differentiate these RF mixer types with respect to their merits (advantages) and demerits(disadvantages).
Single Balanced Mixer
Figure-2 depicts Single Balanced Mixer type. It provides isolation between LO and RF inputs to solve the issues faced by single device mixer type. It uses two diode devices. It can be realized using two single device mixer types connected via 90 degree hybrid or 180 degree hybrid circuit.
Double Balanced Mixer
Figure-3 depicts Double Balanced Mixer type. As shown it will have 4 diodes arranged in ring. It will demonstrate same effect as 2-diode mixer circuit.
There are two double balanced RF mixers e.g. star mixer and ring mixer.
Doubly Double Balanced Mixer-Triple Balanced Mixer
Figure-4 depicts Triple Balanced Mixer type.
This mixer type is created using two double balanced mixers driven in
push pull configuration.
This "doubly double balanced" mixer is also known as "triple balanced mixer".
The key features of triple balanced mixers are as follows:
• Overlap of RF , LO and IF frequency bands
• High spurious suppression
Following table mentions difference between Single balanced mixer, double balanced mixer and triple balanced mixer types.
|RF Mixer type||Merit
|Single Balanced Mixer||• Provide either LO or RF Rejection (20-30 dB) at the IF output
• Rejection of certain mixer spurious products depending on the exact configuration
• Suppression of Amplitude Modulated (AM) LO noise
|• Require a higher LO drive level
|Double Balanced Mixer||• Both LO and RF are balanced, providing both LO and RF Rejection at the IF output
• All ports of the mixer are inherently isolated from each other
• Increased linearity compared to singly balanced
• Improved suppression of spurious products (all even order products of the LO and/or the RF are suppressed)
• Reasonable conversion loss on signal RF (about 7dB)
• Consumes no power except for the losses incurred in conversion
• Broadband in nature and therefore suited to multi-band designs
• High intercept points
|• Require a higher LO drive level
• Require two baluns
• Relative high noise figure, about the same as the conversion loss
• Ports highly sensitive to reactive terminations.
• High quality, high speed diodes which will take the necessary saturating current and large reverse voltages across the non-conducting diodes are an absolute must where performance counts
• Diodes need to be well "matched"
• The transmission line transformers require great care in design and construction. The actual construction will determine the bandwidth.
|Triple Balanced Mixer||• Increased linearity
||• Increased complexity (3 baluns and 8 diodes are required)
• Higher level of LO drive must be provided
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