Linear Technology just released its model LTC5583, a 40MHz to 6GHz dual-channel, matched RMS power detector that seems exceptionally well suited to monitor VSWR in a basestation transmitter RF power amplifier (PA).
Why is this important? Imagine a scenario where a base station antenna is hit by lighting, causing reflected power. In a base station where the RF PA is transmitting at 20 to 40W, the reflected power could seriously damage the PA, which is a major and expensive part of the base station system. Damage to the PA also will likely result in a disruption of service.
How can this RMS detector help?
In RF PA applications, the LTC5583 provides a simple solution for accurately measuring forward power, reverse power, and voltage standing wave ratio (VSWR). The device is made up of a pair of 60dB dynamic range RMS detectors that are matched to 1.25dB, and it targets LTE, WiMAX, W-CDMA, TD-SCDMA and CDMA2000 3G or 4G basestations and other high-performance radios employing complex modulation waveforms.
According to Linear Technology, the LTC5583 has best-in-class channel-to-channel isolation of over 55dB at 2.14GHz, when driven differentially. It can operate single-ended for RF input frequencies up to 2.14GHz, requiring no external balun transformers.
I recently spoke with James Wong, Linear's Product Marketing Manager of High Frequency Products for some more information on this product. Here's what he had to say:
RF & Microwave Designline (RFMWD): What are the major RF/microwave applications for this product? James Wong (JW): A major application for a dual RMS detector like the LTC5583 is for VSWR monitor of a basestation transmitter RFPA (power amplifier). The purpose is to monitor the health of the PA and its antenna connection. This is accomplished by having one channel of the RMS detector sensing the forward power using a directional coupler strategically connected to the PA output, while the other channel senses its reflected power. The LTC5583’s difference amplifier output provides the VSWR information.
This VSWR monitor is applicable to all types of high power transmitters, such as broadcast radios, cable networks, airborne or shipboard radios, microwave radios, broadband wireless access, etc.
Another application is RFPA power control for MIMO (Multiple Inputs / Multiple Outputs) transmitters. That is, the LTC5583’s two independent RF detectors can be used to measure two channel’s RFPAs. The matched performance of the LTC5583 allows tight RF output power matching of two transmitters.
Still another application is for one channel to monitor one node of a transmitter or receiver chain, while using the second channel to measure the signal level in another node of that signal chain to provide gain feedback so that the gain can be set and corrected continuous. (It is a form of automatic gain control.)
RFMWD: What are the key 'care abouts' for engineers about this new product?
JW: Engineers care about such a product’s linearity, temperature variations (error), dynamic range (particularly having low end sensitivity) and power consumption.
Linearity: having good linearity reduces measurement error. Thus providing accurate and reliable power control and minimizing the need for calibration or correction.
Temperature variation: An RFPA can generate a large amount of heat. With cellular basestation equipment being designed to be mounted on cellular towers that are exposed to the environment, large variation due to temperature changes can cause substantial cellular signal change and hence coverage degradation. This is because a cellular network’s coverage depends heavily on each sector’s prescribed transmission power – by design. Any deviation can seriously degrade its coverage.
Dynamic range: this is an important spec which ensures the system has adequate margin to maintain constant level of performance as the tolerances accumulates through the entire signal chain.
Power consumption: more and more systems are built into a smaller form factor. For example, a cellular basestation is evolving from a large, rack mounted system that sits inside an air-conditioned room to that of a remote radio head (RRU) that houses multi-channels of transceiver electronics inside a sealed, weatherproof case that is mounted on top of a broadcast tower. So thermal management is a huge challenge. Low power consumption is always a point of trade-off with performance and size.
RFMWD: Besides above, any other competitive advantages? JW: There is one other key competitive advantage that the part also offers. The LTC5583’s RF inputs can operate single-ended, eliminating the need for external balun transformers. This reduces the solution costs significantly. In fact, the product’s specifications are based on single-ended inputs within a frequency range up to 2.2GHz. Other competitors’ specifications are based on having to use external balun transformers. For these devices, single-ended operation results in substantially degraded performance.
RFMWD: How does it differ from any previous generations of similar products you may offer? JW: This dual RMS detector offers more than 20dB higher dynamic range and better sensitivity than previous generations of similar products. The LTC5583 represents the state-of-the-art RMS detector design that overcomes many challenges.
RFMWD: What was your greatest technical challenge in developing this product, how did you overcome it?
JW:There were many challenges to develop this product.
Channel-to-channel isolation - this is always a tough challenge, made significantly more difficult being a 6GHz part. So great care was attended to in the chip layout to minimize inter-layer and channel-to-channel coupling.
Temperature stability - We exercised care in matching circuits, temperature stabilized reference to build the bias circuitry, and incorporated a temperature compensation circuit to allow the users to compensate the device for extra accuracy over temperature.
RFMWD: What inspired you to develop this product? JW:This product features and specifications are primarily based on customer inputs who shared with us their design challenges they face. We believe we largely addressed most of their concerns and difficulties.
Operating Frequency Range: 40MHz to 6GHz
Difference Output for VSWR or Power-Gain Measurements
Channel-to-Channel Matching: <1.25dB
Channel-to-Channel Isolation: >55dB
Log-Linear Dynamic Range (Modulated Signals)
at 880MHz 61dB
at 2.14GHz 60dB
at 5.8GHz 49dB
Accurate RF Measurement Range -58dBm to 2dBm
Accuracy Over Temperature (-40°C to +85°C) + 1dB
Flat Detector Response from 700MHz to 2.7GHz
Single-Ended RF Input (40MHz to 2.2GHz) No External Transformer
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