Log Detector/Controller Scales 0.05 To 3.5 GHz

May 17, 2006
Accurate power measurements are essential in many types of commercial and military systems. Logarithmic detectors have traditionally provided a reliable means of measuring power, but over limited frequency ranges. The new HMC600LP4 logarithmic ...

Accurate power measurements are essential in many types of commercial and military systems. Logarithmic detectors have traditionally provided a reliable means of measuring power, but over limited frequency ranges. The new HMC600LP4 logarithmic detector/controller from Hittite Microwave Corp. (Chelmsford, MA) combines a wide frequency range with a wide dynamic range to support a variety of power-measurement applications at frequencies through 4 GHz. It is the first in a family of power detectors.

The HMC600LP4 converts RF input signals to proportional DC output voltages across an RF input range of 0.05 to 3.5 GHz (and usable to 4 GHz). It employs successive compression by means of a cascade of eight amplifier cells, each with 8.5-dB gain stabilized over temperature and voltage. Square-law detectors located before and after each gain stage convert the RF signals into DC current which is summed and then amplified and conditioned with a current amplifier. For low input power levels, all of the amplifiers operate in their linear amplification region and the output response is linear. When the input power increases, it drives the amplifier stages into saturation one by one, creating an accurate approximation of a logarithmic function. The current signal is finally converted into voltage-domain with a current-to-voltage converter, and buffered to drive the HMC600LP4's LOGOUT output pin.

In the logarithmic detection mode, LOGOUT pin is externallyconnected to the VSET input pin (closing the feedback of the output buffer) resulting in a nominal logarithmic slope of 19 mV/dB and an intercept point of -95 dBm, both stable with temperature, supply, and frequency. In this mode, the typical output voltage range is +0.6 to +1.9 VDC.

In the controller mode, the HMC600LP4's VSET input accepts an external voltage control input, as shown in the automatic-gaincontrol (AGC) example of the block diagram. In the example, a model HMC552LP4 1.6-to-3.0-GHz double-balanced mixer is used to upconvert an IF signal for transmission. The RF output of the mixer is routed through a model HMC473MS8 voltage-variable attenuator and then coupled into the RF input of the HMC600LP4. The DC output of the HMC600LP4 is compared with an external setpoint voltage, and provides a feedback control to the HMC473MS8. The attenuator accepts a single positive control voltage and provides an attenuation range of 48 dB. This loop ensures that the appropriate signal level is applied to the HMC482ST89 gain block, and to the subsequent RF amplification stages.

The HMC600LP4 provides superb detection accuracy and stability with temperature for such a wide-dynamic-range, broadband device (see table). For input frequencies of 900 MHz and 2500 MHz at +25C, the HMC600LP4 exhibits 1 dB dynamic range of 67 and 59 dB, respectively, both with 1 dB accuracy. At 3500 MHz, the HMC600LP4 still delivers 39 dB dynamic range with 1 dB accuracy. The RSSI slope is very consistent with frequency, providing a typical output voltage range of +0.6 to +1.9 V, and an intercept level (dBm) between -99.6 and -96.5 dBm (see figure) at 900 MHz. The RSSI error versus temperature performance is stable from -40 to +85C, exhibiting dynamic ranges of 72, 69, and 54 dB, respectively, at 900, 1900, and 3000 MHz, each with 3 dB accuracy. If a user has the opportunity to normalize the error at +25C (as done by some competing products), the HMC600LP4 is then capable of 90 dB dynamic range from 900 to 3000 MHz with 3 dB accuracy.

The HMC600LP4's RF input can be driven-with differential or single-ended signals. The RF signal path is internally matched to 50 ohms and the device can operate with supply voltages of +2.7 to +5.5 VDC with no significant changes in performance. The device consumes 30 mA at room temperature and a CMOS-logic-compatible power-down pin allows the user to decrease DC power consumption when the power detection function is not required. As with the company's new T/H amplifier, samples and an evaluation board for the log detector/controller are available now.

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