I think we’re all well aware that wireless communication is serious business these days. Smartphones, wearables, home automation, satellites, military systems—the market for RF products is large and steadily expanding.
But if you’ve spent much time with RF design, you know that it’s not the simplest of engineering disciplines. And the situation only gets worse when you’re faced with the high (sometimes very high) frequencies used in the latest wireless systems. Fortunately, semiconductor companies employ some seriously skilled engineers who have the expertise and the resources needed to create high-performance, user-friendly RF products.
In this article, we’re focusing on the receive path. A standard component in RF receiver circuitry is a mixer—more specifically, a downconverting mixer.
Don’t let the name lead you astray: an RF mixer doesn’t add signals together; rather, it multiplies them in order to achieve frequency translation. In a heterodyne receiver, the output of the mixer is referred to as the “IF,” because it has been shifted down to the “intermediate frequency.” The inputs are the received RF signal and the LO (“local oscillator”) signal; the intermediate frequency is governed by the difference between the RF frequency and the LO frequency.
Based on the above description, a basic mixer would be an analog multiplier with two inputs and one output, and that is exactly what we have in the HMC219B.
Image courtesy of Analog Devices
However, the HMC219B is a rather high-performance type of “basic.” It’s a “double-balanced” mixer; this topology provides higher isolation and lower even-order distortion in the output signal. Its intended frequency range starts at 2.5 GHz and extends to 7 GHz. It requires no external matching circuitry. And despite all this, it’s tiny, as new ICs generally are these days.
The HMC219B can actually be used as a downconverting mixer or an upconverting mixer, but the LTC5566 is intended only for downconversion. This part is good up to 5 GHz (or up to 6 GHz with “degraded performance,” according to the datasheet); however, the lower end of the frequency range—300 MHz—is much more friendly for those who don’t need billions of cycles per second to move their data around.
The LTC5566 is in the same general category as the HMC219B, but we can’t really compare the functionality, because there is a lot more going on in the LTC5566:
Image courtesy of Linear Tech
As you can see, there are actually two mixers in this part, along with a variable-gain amplifier for each channel and a convenient SPI interface for gain control and RF-input impedance tuning. Suggested applications include 4G and 5G receivers, distributed antenna systems, and software-defined radios.
With the ADC32RF80 from TI, we have moved into the realm of intense integration. This 72-pin behemoth is a dual-channel device that supports RF frequencies up to 4 GHz. It incorporates 3 GSPS analog-to-digital conversion, digital downconverters, a JESD204B output interface, and front-end measurement circuitry for use with an external AGC circuit.
Image courtesy of Texas Instruments
For those of you who don’t feel like hand soldering 72 surface-mount pins (and good luck with the exposed thermal pad), an evaluation board is available.
Is there a new RF product that you’re thinking about using in your next design? If so, feel free to mention it in the comments.