Small Isolation Chamber

Open Source Instruments Inc. has moved from 397 Moody St, Waltham, MA, to 5 Pratt Ave, Waltham, MA. Our new office is a 180-m² basement with a concrete floor and plenty of half-windows.


Figure: New OSI Location: 5 Pratt Ave, Waltham, MA.

On Moody St, interference power picked up by a Damped Loop Antenna (A3015C) in the 925-928 MHz band was up to −58 dBm (1.6 nW). Here on Pratt Ave, the same 925-928 MHz power peaks at −45 dBm (32 nW). On a work bench, reception from a subcutaneous transmitter only at ranges less than 10 cm. Interference power in our new office is four times greater than in any other place we have measured (London, Oxford, Edinburgh, Guildford, Waltham, Boston). Our new location is a fine place to test the efficacy of radio-frequency isolation chambers.

We set up a small isolation chamber. It consists of six AN-77 absorbers, gray sides inwards, standing on the concrete floor. We wrap an outer layer of resistive sheet around the absorbers. We put a cover of the same resistive sheet on top. We assume the floor is a perfect absorber, because we are in a basement.

Figure: Small Isolation Chamber on Concrete Floor.

We set up our A3008C spectrometer and measure 925-928 MHz interference power for one minute with the pick-up antenna on the corner of our work bench, on the floor out in the open, in the center of our isolation chamber, and in the corner of our isolation chamber.


Figure: Interference Power in Separate Minutes and in Various Locations.

The peak interference power outside the enclosure is −45 dBm. Inside the chamber, the power never rises above −68 dBm (160 pW). The chamber appears to provide 23 dB of isolation. We place a Subcutaneous Transmitter (A3019D) inside the chamber 50 cm from each of two antennas connected to an Octal Data Receiver (A3027C). Over the course of one minute we obtain 100.0% reception. We remove the enclosure lid and reception drops to 29%.

Command Transmission Noise

We turn on two Subcutaneous Transmitters (A3019). One has 150-mm leads and the other has 45-mm leads. We place the tips of the leads in water, so that they are connected by an impedance of order 100 kΩ. This connection reduces the amount of mains hum the transmitters pick up, allowing us to see other sources of noise. We turn on our Command Transmitter (A3023CT), which emits pulses of 146-MHz radio-frequency power to stimulate Implantable Lamps (A3024). We place the command antenna 70 cm from our two subcutaneous transmitters, and instruct the command transmitter to emit 10-ms pulses at 10 Hz. We see the following response from the subcutaneous transmitters.


Figure: Command Transmission Noise. Two A3019 subcutaneous transmitters responding to pulses of 146-MHz command pulses. Vertical range is 27 mV, horizontal range is 500 ms. Pulses are 10 ms at 10 Hz. Pink trace: 150-mm leads. Orange trace: 45-mm leads.
If we remove the leads from an A3019, we see no response to the command transmission. We record with a two-channel Subcutaneous Transmitter (A3028D) and observe the same pulses on both channels. We suspect that the leads pick up 146-MHz power, which penetrates the transmitter circuit and is somewhere demodulated by a non-linear element in the amplifier. The demodulated signal is added to the input, resulting in pulses. We record the following trace from an A3019 without encapsulation and equipped with 150-mm wire leads.


Figure: More Command Transmission Noise. One un-encapsulated A3019 responding to pulses of 146-MHz command pulses. Vertical range is 27 mV, horizontal range is 500 ms. Pulses are 10 ms at 10 Hz.
We add a 100-kΩ resistor in series with each lead, and a 10-pF capacitor at the far side of the resistors. Thus the resistors form a high-pass filter with cut-off frequency 80 kHz. At 146 MHz, the filter attenuates by a factor of two thousand in amplitude. A 20-mV pulse of command transmission should be reduced to 1μV. With the filter installed, we recording the following trace from the subcutaneous transmitter with the same command pulses.


Figure: Eliminated Command Transmission Noise. One un-encapsulated A3019 equipped with 80-kHz low-pass filter showing no response to pulses of 146-MHz command pulses. Vertical range is 27 mV, horizontal range is 500 ms. Pulses are 10 ms at 10 Hz.
The 10-pF capacitor of the filter appears in parallel with the 10-MΩ input impedance of the subcutaneous transmitter's EEG amplifier, making a low-pass filter with cut-off frequency 1.6 kHz, which is above the 1.3-kHz bandwidth we would obtain from an A3019 running at 4096 SPS. The two 100-kΩ resistors form a divider with the 10-MΩ input impedance, which reduces the EEG signal amplitude by 2%, which is insignificant. We claim this filter will have no effect upon the detection of EEG, and yet eliminates the command transmission noise.

Taper Power Measurement

We have three ways to measure the power emitted by a glass taper. For all methods, we mask the LED with black tape and we use our SD445 photodiode. The photodiode measures 10 mm × 10 mm. With no reverse bias, this photodiode has sensitivity 0.18 mA/mW at 460 nm (see LED Efficiency). In the first method, we measure the light intensity emitted in an array of directions and integrate over a hemisphere to obtain total emitted power. In the second method, we place our photodiode in five locations to make a virtual cube of photodiode 10 mm on each side, centered on the taper. We add the photocurrents together and so obtain an estimate of total power. In the third method, we hold the photodiode as shown below.


Figure: Single-Location Measurement of Taper Power Output.
The transparent plastic surface of the photodiode reflects light with angle of incidence greater than 80°. The above arrangement results in the photodiode receiving light emitted by the taper within a ±80° wedge in the horizontal plane. To obtain total power emitted, we multiply the photocurrent by 360°/160° = 9/4 and divide by the sensitivity 0.18 mA/mW.

Of our three methods, the first is the most time-consuming and prone to error. The second is prone to error due to incorrect placement of the photodiode. The third is the easiest. We measured the power output of A3024HFB No2.9 with Method Two and obtained 10.8 mW at 30 mA forward current. We applied Method Three and again obtained 10.8 mW. From now on we will use Method Three and assume it is accurate to ±10%. We will refer to Method Three as the angled photodiode method. We use the angled photodiode method to measure total power output versus forward LED current for A3024HFB No2.9, and obtain the following plot.


Figure: Taper 2.9 Output Power versus LED Forward Current.
We see that the taper emits roughly 11 mW at 30 mA, 18 mW at 50 mA, and 22 mW at 70 mA. When the forward current is above 50 mA, however, the output power drops during the seconds after we apply the current. Thirty seconds after we apply 80 mA, the power has dropped by 10%. This drop is due to self-heating of the LED.

New Head Fixture

We have a new head fixture printed circuit board. It provides a notch for the guide cannula, rather than a hole. There is a two-way plug for delivering power during assembly, and two miniature sockets for pins during implantation. We mount ten C460EZ500 LEDs in QFN-8 packages into ten such circuit boards and measure their power output at various forward currents.


Figure: We give diameter of fiber we later glued to the LED and the coupling efficiency. The coupling efficiency is the fraction of light emitted at 30 mA that emerges from the fiber tip.

In series with the LED is a surface-mount resistor that allows us to set the LED forward current once we know the length of the power supply leads. We expect the leads to be 50 mm long in mice and 150 mm long in rats. The resistance of our stretched helical 100-μm wire leads is 10 Ω per 50 mm. The power supply is 5 V and the LED forward voltage is 3.2±0.1 V for currents of order 50 mA. With a 0-Ω resistor and 150 mm leads we will get 32 mA. With a 7-Ω resistor and 50-mm leads we will get 70 mA, assuming the battery can supply the required current to its boost regulator.

We make an 8-mm long fiber of higher-index glass. The fiber has a 2-mm taper at one end a flat on the other. Its diameter is 440 μm. We its base onto the center of the LED. We squash down the bond wire, but there remains a gap of roughly 100 μm between the fiber base and the LED surface. Our calculated coupling efficiency for a fiber with outer diameter 440-μm and numerical aperture 0.86 is 47%. We expect to lose 2% of this light in the yellow glass of the fiber, which will reduce our coupling efficiency to 46%. Our calculation assumes, however, that the LED and fiber surfaces are touching.


Figure: Higher-Index Taper Glued to Blue LED. Assembly number A3024HFB, serial number 2.10.

We mask the base of the fiber with tape and pass 30 mA through the LED. We measure the photocurrent in our 10 mm × 10 mm photodiode at range 20 mm for the fiber tip. We vary the angle, θ, between the axis of the fiber and the line joining the tip to the center of the photodiode. We orient the photodiode so its surface is always perpendicular to this line. For θ = 90° we go around the fiber in a horizontal plane in 45° steps. The light intensity varies from 50-110 μW/cm², with an average of 80 μW/cm². We move the photodiode in a vertical plane and vary θ from 90° to −90°. We obtain the following plot of intensity versus angle.


Figure: Intensity versus Off-Axis Angle for Fiber Tip. The fiber base is masked with black tape. We move our 1 cm² photodiode around in a vertical plane at range 2 cm. Assembly number A3024HFB, serial number 2.10.

By integrating this curve for the hemisphere above the taper, we obtain a crude estimate of 9 mW for the total emitted power. To obtain a better estimate, we place our 1 cm² photodiode in five positions on four sides and above the tip of the fiber, at range 5 mm, so as to make a cube that receives all emitted power. We add the photocurrents thus obtained, after subtracting for background light, and arrive at a total of 1.76 mA, which suggests a total output power of 9.8 mW at 30 mA forward current. Given that this LED emitted 26.7 mW at 30 mA, our coupling efficiency to the tip is 37%. We increase the LED current to 50 mW and optical power output increases by 45% to approximately 14.3 mW. For the first time, we have a taper that emits roughly 10 mW with forward current 30 mA. If we increase the LED current to 70 mA, which will be possible with a rat-sized lithium-ion battery, this fiber tip will emit 19 mW. This fiber's coupling efficiency is 37%, compared to a theoretical maximum of 46%.

To complete the head fixture, we need a 9-mm silica guide cannula. Our existing guides are 7-mm long. Our plan is to glue the tip of the guid to the base of the taper, and measuring with the existing taper, the guide cannula length below the plastic thread must be 9 mm.

UPDATE: [24-DEC-13] In No2.2, we use the offset placement of the fiber shown here, which allows us to press the fiber base onto the surface of the LED. We get 5.7 mW for 30 mA, or 22% coupling efficiency.

UPDATE: [23-JAN-14] By squeezing the fiber clamp with our fingers, we force the fiber base closer to the LED surface. This appears to increase our coupling efficiency from 37% to 38%.

UPDATE: [24-JAN-14] We take out head fixture A3024HFB No2.6 and measure its power output with 30 mA. We mask the LED with aluminum foil. With angled photodiode we get 10.4 mW total power output. With the fiber perpendicular to the photodiode and the tip touching the photodiode, we get 1.3 mA of photocurrent, or 7.3 mW incident light. Given that the photodiode extends for 5 mm from the tip, and the light emission takes place along a 1-mm length, everything emitted within ±84° of the fiber axis is incident upon the photodiode. The missing 3 mW must be emitted at an angle greater than ±84°.