RF and Optical Bolometer

When RF energy is delivered to a resistive load it dissipates heat. If the load has a relatively poor thermal coupling to its surrounding environment its temperature will rise. By measuring the temperature rise it is possible to determine the average power delivered to the load. There are various problems with the approach though, calibrating it is troublesome as ambient temperatures change and many of the coefficients involved are unknown and would need to be determined experimentally. It was during my design of the calibration system I discovered it is fairly easy to just put the temperature sensor and load into a servo loop and maintain a constant temperature above the ambient. In this way the change in DC power required to maintain the sensor at a constant temperature is directly the amount of power delivered to it by other means.

Basic Bolometer Scheme Diagram

To achieve the thermal control system I started with thermistors but rapidly changed to using common silicon diodes who have a temperature coefficient of about 2 mV/K. An Op-Amp with offset compensation (a CA3130 was in the junkbox) amplifies the drop voltage difference between two diodes and drives a resistor heating element to keep one junction a constant temperature above the other. The amplifier gain is about 30 dB and is not completely open-loop except for the thermal feedback to give it some stability. The circuit in all is very simple and functions adequately for the task.

Thermal Balance Bolometer Circuit

In the first version I used normal leaded components held together with a drop of liquid electrical tape. This arrangement allowed me to test the concept on a solderless breadboard. A centre-nulling meter facilitated "zeroing" the difference by biasing its other terminal to equal the quiescent voltage across the heater. This way I could watch the sensor drift and bias either sensor with my fingers resulting in a swing in the appropriate direction. Such an arrangement does not yield direct-reading of power, but can be calibrated at a particular ambient temperature and is handy for trending. For direct reading the absolute power levels in the heater load must be measured and subtracted to yield an accurate figure of power delivered by the external source.

Bolometer Circuit Initial Lash-Up

I considered using the same load for RF as the heater. In theory this is quite practical; an RF choke implementing a bias-Tee to deliver the DC heating power while the RF is delivered through a DC blocking capacitor. I wanted to be able to calibrate the device with DC (which is easily measured), so I went for a dual heater design. A separate 50 Ohm load is thermally coupled to the same heater/diode pair used in the control loop. This mandates calibration as the heating effect of the current in each load may not be precisely communicated at the same level to the diode but in practice it was shown to work very well. For RF-only measurement the single heater/sensor system is simple and direct reading (but may require a high heater supply compliance to drive the 50 Ohm load).

Single Load Resistor Head Diagram

The power delivered to the load/sensor system need not be via the resistor. I discovered external heat sources (like my soldering iron) would produce measurable deflections from quite some distance. This enables direct measurement of electromagnetic radiation that the sensor can absorb. I tried aiming a toy laser pointer at the initial leaded detector arrangement and measured roughly 2 mW of dissipation difference, this seemed consistent with its < 5 mW compliance labelling.

Lashed-Up Two Resistor Detector Head

The time constant of the lashed-up detector was fairly long, and to minimise this I decided to build a more physically compact detector. I used SMD components soldered to a small square of brass shim stock. I reasoned the surface area of the plate could be measured and used to calculate optical fluxes. I soldered the SMDs directly to each other after bonding them together with superglue and then soldered their common ground to the plate. The entire assembly was then suspended over a larger brass plate which holds the reference diode. The larger plate can be attached to a physically large heatsink to form a stable ambient reference, while radiation and conduction to the reference mass from the sensor implements the heat-leak required. To facilitate the very low reflectance required for radiation measurements I eventually painted the top side of the brass sensor plate flat-black using Lampblack mixed with a little Red Gum in alcohol to bind it.

Bolometer Sensor Using SMDs
Bolometer Head Including Sensor and Reference

It is important to shield the detector and reference junction from drafts and differences in ambient illumination. The sensor is especially sensitive to drafts and I reason you could calibrate it quite accurately as an Anemometer (kinda neat, no moving parts!), at least at constant barometric pressure. I used a small potting box to cover the detector assembly to exclude drafts. This improved the baseline stability enormously. A small hole in the box allows a radiation beam to enter and hit the sensor plate facilitating its measurement.

Quick Lash-Up Measuring Laser Power

With the improved detector the output of the laser pointer was once again measured. 2.4 mW or 3.8 dBm was measured. The DC input from its battery is 72.8 mW, giving it a rather unspectacular efficiency of 3.3%. This concerned me that the sensor reflectance might still be fairly high, but as the compliance data suggests the output is < 5 mW it seems at least consistent. Diode lasers are usually more efficient than that, but the drive electronics is likely wasting a lot of power. I did not dismantle the laser head to directly measure the power delivered to the diode. I'd need a calibrated optical source to check the sensor at optical frequencies. Lampblack should be fairly flat with respect to frequency, at least compared to other "black" pigments. A green laser pointer measured 3.6 mW (5.6 dBm) and is also labelled as < 5 mW. Its IR local oscillator must be filtered from the output fairly well, I was expecting an unusually large reading from its IR leakage.

At RF the detector performs very well and consistently. I measured my 50 MHz "16 dBm" signal source at 48 mW (16.8 dBm) The previous calibration was by comparison to a DC-calibrated diode detector, so I am amazed by the agreement actually. Similarly I measured attenuation steps of a ~100 mW (20 dBm) signal at 10 MHz the results being quite consistent with my previous attempts to calibrate the poorly constructed attenuator at DC.

RF Step-Attenuator Test Lash-Up (100 mW FSD being Delivered)

The system is configured to measure 1 mW to 100 mW, a 20 dB dynamic range. The poor dynamic range is typical of thermal sensors. Its lower limits could be improved by active cooling of the sensor head to achieve more sensitivity and more attention to noise filtering. Its upper limit is really only constrained by the temperature limit of the sensor assembly. I used 125 mW rated SMD resistors and set the bias power a little bit above that level (which is safe due to the heatsinking effect of the detector assembly). With larger resistors in the detector and a suitable power Darlington follower the circuit could be used up to kW. Efficiency is of course terrible, the sensor bias power must exceed the power to be measured. Larger RF powers are more easily measured with diode peak-voltage measurements, but as a thermal device is a natural integrator it can measure the true average power of complex waveforms containing multiple frequencies (including 0, DC biases which may or may not be a problem depending on the application). Attenuation from higher power is probably the most practical method. Amplification can be used to measure smaller signals, but the calibration of the amplifier then becomes an issue. MMICs with reasonably flat gain and compression points exceeding the detector range are available. The load offers a good return loss well into VHF and is therefore capable of absolute average power measurements from DC to several hundred MHz.

The general design can implement all kinds of radiant energy sensors. A pyrometer is simply a matter of optics and calibration. The device is already a fairly usable laser power meter.




title type size
Basic Bolometer Scheme Diagram Source application/postscript 10.429 kbytes
Thermal Balance Bolometer Circuit Source application/postscript 15.383 kbytes
Single Load Resistor Head Diagram Source application/postscript 11.452 kbytes


2009-03-22: Bolometer Head Completed
I finally assemble the bolometer sensor head into something resembling a completed unit.
2009-03-18: Bolometer Baseline Noise Improvement
Gradual improvement of the bolometer baseline noise and the RS-232 sampler data capture system over several days.
2009-03-01: Digital Display for the Bolometer
A multiplexed 7-segment digital display for the bolometer.