Technical Close-Up: Mixer IC includes on-chip coupling transformers

MILPITAS, CA—The LT5557, a new high linearity, 3.3V, downconverting active RF mixer from Linear Technology elevates receiver dynamic range performance while extending bandwidth capability to cover 3G and WiMAX basestation frequencies to 3.8 GHz. The LT5557 offers 24.7 dBm IIP3, noise figure of 11.7 dB, and 2.9 dB of gain at 1.95 GHz. Its performance remains robust at WiMAX frequencies with 23.5 dBm IIP3, and 1.7 dB of gain at 3.6GHz. This performance is achieved with a low -3dBm LO input drive level, enabling best-in-class LO isolation performance. Typical LO-to-RF isolation is better than 42 dBc at 1.95 GHz. The LT5557's power consumption is low for this class of high linearity mixers, typically 270mW from a 3.3V supply. The device incorporates on-chip RF transformers to enable convenient single-ended, 50Ohm matching at the RF and LO inputs. The combination yields an overall cost-effective, compact, easy-to-use and high performance receiver solution for wireless basestations of all types.

The LT5557 incorporates a double-balanced active mixer core topology with on-chip LO buffer. Differential drive is employed internally throughout the mixer to maximize linearity and RF isolation. Conversion from single ended to differential is accomplished through on-chip RF balun transformers at the RF and LO inputs. The device's wide operating bandwidth covers the 850-965MHz GSM and US cellular bands, as well as 1.7GHz to 2.1GHz 3G wireless services. The LT5557 also supports WiMAX operating in the US at 2.6GHz and worldwide at up to 3.8 GHz.

The LT5557 operates from a single 3.3V supply, drawing a quiescent supply current of 81.6 mA. A shutdown feature is provided. When the chip is disabled, it draws a maximum sleep current of 100uA. The device comes in a 16-lead 4mm x 4mm QFN surface mount package.

It is pin compatible with other high performance Linear Technology downconverting mixers. Pricing starts at $5.95 each in 1,000-piece quantities. The product is available immediately from stock.

LTC won't disclose what type of on-substrate transformers are used on the LT5557. From a user's standpoint, it really doesn't matter, but from a technical point of view it would be nice to know whether the transformers are spiral structures with interleaved primaries and secondaries, or whether they're stacked structures, or perhaps some unique configuration.

In any case, kudos to LTC for including them on-chip; I personally don't know of any other IC maker who offers a monolithic device with such structures in place. With on-device transformers you may not have to resort to discrete LC matching networks off-chip.

LC Matching Networks

I say "you may not have to" because in some applications you'll still need lumped constant devices at the IC's RF input port and at its LO (local oscillator) port. Note that the headline in LTC's press release (on the left) states the device will work down to 400-MHz and as high as 3.8-GHz. As far as the datasheet goes, the LT5557 is characterized by LTC for IF (intermediate frequency) frequencies of 70-MHz, 140-MHz, 240-MHz, 360-MHz, and 450-MHz.

As it stands, the LT5557's RF input is matched to 50-ohms over the 1.6-GHz to 2.3-GHz span. Similarly, the LO input provides a 50-ohm match from 1-Hz to 5-GHz. However, if the driving stage's signal carries a DC component, you'll still need a series-connected blocking cap to isolate the DC from the delicate primary winding of the on-chip transformer at the RF-input port. Interestingly, the LO port already includes an on-chip series blocking cap.

If you decide to operate an LT5557 over other frequency ranges, you will need to provide external LC networks to complement the IC's internal transformers. As they stand, these on-chip transformers accept unbalanced single-ended 50-ohm inputs.

Two Configurations

LTC suggests two possible configurations (obviously you're free to devise countless others). One of LTC's suggested configurations, dubbed a standard evaluation circuit, uses transformer-based IF matching at the chip's balanced output. LTC claims this coupling to a subsequent IF strip will give you the best dynamic range and the widest IF bandwidth.

Click to view schematic

A second circuit replaces the IF output transformer with a discrete-component LC balun. The balun's output provides a 50-ohm match to downstream IF stages. This is likely a lower cost and possibly more compact approach. The balun circuit is claimed to deliver higher conversion gain, but as with most things in engineering, there's a tradeoff.

Click to view simplified schematic

In this case it's degraded IIP3 (third order input intercept point) and noise figure, and reduced IF bandwidth. (IIP3 is the point at which the power in the third-order product and the fundamental intersect; it's a point at which an amplifier stage is assumed to be linear. As such, IIP3 is a useful parameter to predict low-level intermodulation distortion). For lower IFs, the absolute bandwidth is small; at the higher IF frequencies you'll see wider bandwidth.

Further Optimization

Also, although the LTC5557's RF input is internally matched over the 1.6-GHz to 2.3-GHz range, input return loss is typically 12-dB at the band edges. As such, the input match at the lower band edge can be optimized with a small series capacitor at the chip's RF input pin. It will improve the 1.6-GHz return loss to greater than 25-dB. At 2.3-GHz, the match can be improved to greater than 25-dB with a small series inductor. And, a small LC network can simultaneously optimize both lower and upper band edges, expanding the RF input bandwidth to 1.2-GHz through 2.5-GHz.

Lastly, the input match can be shifted as low as 400-MHz, or up to the high end at 3.8-GHz, by adding a shunt capacitor across the LTC5557's RF input port, working in conjunction with a section of stripline on your circuit board or packaging substrate, or a small series inductor. Outboard matching permits the device to be used with designs at a number of frequencies, perhaps using the same circuit board layout.

Differential Output Use

The LTC5557's output can also be matched directly into a SAW (surface acoustic wave) filter, or an appropriate IF amplifier, eliminating the IF transformer. LTC shows a typical differential IF circuit matching the chip's 500-ohm differential output pins into a lower impedance differential SAW filter using a tapped-capacitor network.

Finally, LTC has an evaluation board for this IC. Equipped with miniature gold-plated threaded coaxial connectors, the board should help you get a handle on the device's capabilities, as well as ease your own layouts. LTC's datasheet also includes drawings for both transformer and balun output circuits.

Click here for a datasheet (in Adobe Acrobat .PDF format).

For more details, contact Linear Technology Corp., 1630 McCarthy Blvd., Milpitas, Calif. 95035-7417. Phone: 408-432-1900. Fax: 408-434-0507.

Linear Technology, 408-432-1900, www.linear.com