Double Balanced Mixer: How a Diode Ring Mixer Works

Almost every radio you own contains at least one mixer, and in many high-performance transceivers the circuit that decides how well the receiver behaves near a strong signal is a small black box with four diodes and two tiny transformers inside. This is the double balanced mixer, the workhorse of communications receivers, transmitters and test instruments since the 1960s. This guide explains what a mixer does, how the diode ring achieves its remarkable properties, what the numbers on a data sheet such as drive level, conversion loss and isolation mean in practice, and why the choice of mixer has so much influence on contest-grade receiver performance.

What a Mixer Actually Does

A mixer is a circuit that multiplies two signals. If you feed it a signal at frequency A and another at frequency B, the output contains the sum A + B and the difference A − B. In a superheterodyne receiver the incoming signal is combined with a local oscillator, usually written LO, so that the wanted station always appears at the same intermediate frequency, where the narrow filters can do their work. The principle is the same one that creates the unwanted response described in our article on the image frequency, because a mixer cannot tell the wanted sideband from the unwanted one without help from filters.

A perfect multiplier does not exist, so real mixers approximate it with a nonlinear device or with a switch. The simplest mixer is a single diode, which produces the sum and difference, but also passes the original signals and a long list of other products to the output. Balancing is the technique that removes the most troublesome of these products by cancelling them. A single balanced mixer cancels one of the input signals, while a double balanced mixer cancels both, which is why the output contains little else than the sum and the difference.

The Diode Ring

The classic double balanced mixer has four Schottky diodes connected in a ring, with one broadband transformer on the RF side and another on the LO side. The strong LO signal switches the diodes on and off. During one half of the LO cycle two of the diodes conduct and connect the RF signal to the output with one polarity, and during the other half the other two diodes conduct and connect it with the opposite polarity. The result is that the RF signal is multiplied by a square wave of the LO frequency, and a square wave contains the fundamental and odd harmonics but no direct current.

That last point is the secret. Because the switching function has no DC component, the LO frequency itself does not appear at the IF port, and because the circuit is symmetrical, the RF signal does not appear there either. Both are suppressed by the balance of the transformers and the matching of the diodes, typically by 30 to 50 decibels in a good commercial part. This is the property that makes the double balanced mixer so valuable in a transmitter, where an unwanted LO leak would otherwise be radiated as a carrier or a spur, and in a receiver, where the LO leak would otherwise go to the antenna or disturb the IF stages.

The three connections are named the RF port, the LO port and the IF port. The mixer is passive and bilateral in a sense, which means it can be used in either direction. A receiver feeds the antenna signal to the RF port and takes the intermediate frequency from the IF port, and a transmitter uses the same device the other way round, feeding the low-frequency signal into the IF port and taking the output at the RF port. This symmetry is one reason why the same part number turns up in so many designs.

Drive Level: What Does Level 7 Mean?

Because the diodes act as switches, they need a strong LO signal to turn on fully. Mixers are specified by this drive level, and the figure also hints at the signal-handling capability. A level 7 mixer is designed for an LO power of about +7 dBm, which is 5 milliwatts. A level 10 mixer needs +10 dBm, a level 17 mixer needs +17 dBm (50 milliwatts), and special high-level parts need +23 dBm or more. These figures are the nominal values, and a mixer works within a couple of decibels of them. If you drive the mixer too weakly, the diodes are switched incompletely, the conversion loss rises and the intermodulation performance falls sharply.

The reason for choosing a higher level is dynamic range. The strongest signal that a diode mixer can handle without distortion is roughly 10 dB below the LO level, so an LO of +17 dBm allows a much stronger incoming signal than +7 dBm. A rule of thumb says that the input third-order intercept point is about 10 to 15 decibels above the LO level, so a level 7 mixer has an intercept near +17 to +20 dBm, while a level 17 mixer reaches +27 to +30 dBm. These numbers are the reason why high-end HF receivers and many DX and contest transceivers use high-level diode mixers, or the switching mixers described below. The meaning of the intercept point and the dynamic range is explained in the article on receiver dynamic range and IMD.

Conversion Loss and Isolation

A passive diode mixer does not amplify, and it also loses signal in the conversion. The conversion loss of a good ring is around 6 to 8 dB, of which 3.9 dB is the theoretical minimum because half of the signal power goes to the unwanted sideband. The loss matters because it adds directly to the noise figure of the stage. On the lower HF bands the noise from the sky and from man-made sources is far above the receiver noise, so a mixer loss is not a problem and a preamplifier would only worsen overload, as explained in the article on the HF preamplifier and attenuator. On 10 metres and above, or on VHF, the loss may need to be compensated by gain before or after the mixer.

The second set of numbers describes isolation between ports. LO-to-RF isolation tells how much of the oscillator leaks to the antenna side. LO-to-IF isolation tells how much appears at the output, where it may be amplified by the IF stages or detected as a carrier. RF-to-IF isolation shows the leakage of the input signal straight to the output. A typical data sheet gives 30 to 45 dB for each of them. In practice the isolation can be much worse if the mixer is not terminated properly, because the balance depends on all three ports seeing the impedance for which the mixer was designed.

Termination: The Detail That Decides Performance

A diode ring mixer expects 50 ohms at each port over a wide frequency range. At the IF port that includes frequencies far from the wanted IF, since the mixer also produces the sum, the harmonics and many other products. If the IF port sees a crystal filter, which is a reactive load outside its passband, the unwanted products are reflected back into the diodes and mix again, which creates intermodulation and a higher noise figure. Good receiver designs therefore place a diplexer or a resistive pad at the IF port. A diplexer passes the wanted IF to the filter and sends everything else to a 50 ohm resistor, so that the mixer sees a constant load. The same idea of a stable termination is used in other circuits, and you can read about pads in the article on RF attenuator pads.

The RF side needs a band-pass filter ahead of the mixer, because the mixer will happily convert any signal that reaches it, whether it is the wanted one, the image, or a harmonic of the LO that mixes with something far away from the band. A diode ring responds to odd harmonics of the LO, so it is vulnerable to signals at three times and five times the LO frequency, again scaled by the IF. This is why a receiver with a high-level mixer still has a switched set of band filters in front of it.

Other Mixer Families

The Gilbert cell is a transistor circuit that performs a four-quadrant multiplication. The SA612 (the successor of the NE602) is the best known example among builders, because it is cheap, needs only a small LO signal and gives conversion gain. The price is a low intercept point, a typical +5 dBm or less on the input, and a susceptibility to overload from strong broadcast stations in the evening. Such a mixer is excellent for a QRP transceiver or a simple receiver, and a poor choice for a contest station near a big signal.

The fast bus switch circuits, such as the 74CBT3253 and similar parts, work as a pair of fast analogue switches. Driven by a square wave, they behave like a ring mixer with very low loss and an intercept point that can match a good diode ring, which is why they are common in modern software defined radios and in the quadrature sampling detector often called the Tayloe detector. The article on direct sampling and superheterodyne receivers shows how these ideas relate to the receivers you can buy. A frequency mixer is also the heart of the product detector discussed in our guide to the direct conversion receiver.

Spurious Products and the LO Quality

A mixer is only as clean as its oscillator. Any noise on the LO is transferred to the output, and a strong adjacent signal can mix with the sidebands of the oscillator noise and fall inside the wanted channel. This is called reciprocal mixing, and it appears as a rising noise floor whenever a strong station is nearby. The topic is covered in the article on phase noise. A high-level mixer with a poor LO is a waste, which is why synthesizers such as those described in the guide to PLL and DDS circuits are designed with such care.

Spurious products, also called spurs, follow a simple formula: they appear at m times the RF frequency plus or minus n times the LO frequency, for whole numbers m and n. The order of the product is m + n, and the lower the order, the stronger the spur. Designers plot these products in a spur chart and choose the IF so that the strongest ones fall outside the band of interest. A double balanced mixer has the advantage that products with even m or even n are strongly suppressed, which removes a large part of the chart. Operators see such spurs as birdies, fixed whistles that appear on the same frequency no matter what antenna is connected.

Practical Advice for Builders and Buyers

If you build a receiver, the following habits give the best results. Provide the specified LO level with a buffer amplifier, since a mixer that is under-driven will lose its intercept point. Use a diplexer or a pad at the IF port. Place a band-pass filter before the RF port. Keep the three ports well separated on the circuit board and use short connections to the transformers. Measure the conversion loss with a signal generator and a spectrum analyser or a power meter, and compare it with the data sheet. If you buy a transceiver, the receiver data on independent test sites show the third-order intercept at 20 kHz spacing, which is a direct sign of how good the mixer and the front end are. The topic is explained further in the article on receiver sensitivity and noise floor.

For repair work, remember that the diodes of a ring are matched and that the part is often sold as a sealed module. Replace it as a whole, because replacing one diode destroys the balance. Static discharge from an antenna or an RF burst from a nearby transmitter can damage Schottky diodes, which shows up as a loss of sensitivity or a rise in the LO leak. A diode protection circuit in front of the mixer and a good antenna ground reduce the risk.

The Bottom Line

A double balanced mixer multiplies two signals with a ring of four Schottky diodes, and its balance suppresses the LO and the RF signal at the output. The LO drive level, from +7 to +17 dBm and above, sets the signal-handling ability, while conversion loss of 6 to 8 dB and port isolation of 30 to 45 dB describe the rest of the performance. A correct 50 ohm termination, especially at the IF port, and a clean oscillator are as important as the mixer itself. Active Gilbert cell mixers are easy and economical, while switching mixers offer low loss and a high intercept point, so the right choice depends on whether you build a simple receiver or a contest station.

Frequently Asked Questions

What is a double balanced mixer?

It is a mixer, usually a ring of four diodes with two transformers, that multiplies the RF and LO signals and cancels both input signals at the output, leaving mainly the sum and difference frequencies.

What does a level 7 mixer mean?

The number is the nominal local oscillator power in dBm. A level 7 mixer needs about +7 dBm, or 5 milliwatts, and a level 17 mixer needs about +17 dBm, or 50 milliwatts.

Why do mixers need a 50 ohm termination at the IF port?

The mixer produces many unwanted products. A proper termination absorbs them, while a reactive load such as a crystal filter reflects them back into the diodes and creates intermodulation.

Is conversion loss a problem on HF?

Usually not on the lower bands, where atmospheric and man-made noise is higher than the receiver noise. It can matter on 10 metres, VHF and above, where gain before or after the mixer helps.

Can the same mixer be used in a transmitter?

Yes. A passive diode mixer works in both directions, so a low-frequency signal at the IF port can be converted to the transmit frequency at the RF port.

Which is better for a contest receiver, a Gilbert cell or a diode ring?

A high-level diode ring or a switching mixer is better, because its intercept point is far higher and strong neighbouring stations cause less intermodulation.