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Cold ElectronicsThe most likely scanning strategies to be used with Bolocam will result in a very slow modulation of the astronomical signal. To achieve the best stability of these slow signals, it is best to AC bias the bolometers. Bolocam uses a pseudo-current bias that is generated by applying an AC bias voltage to a bolometer circuit loaded with resistors significantly larger (20MΩ) than the bolometer operating resistance (~5MΩ at 300mK). The figure below depicts a simplified model of the bolometer readout circuit used in Bolocam II. Using a current bias (as opposed to a voltage bias) is preferred due to the germanium thermistors having a negative dR/dT (i.e. resistance decreases with increasing temperature), thus resulting in negative electro-thermal feedback. 
Figure 1: Simplified bolometer readout electronics employed in Bolocam II. The bias signal is also sent to the warm readout electronics for demodulation purposes. The JFETs operate with optimally low noise at 130K. The range of appropriate bias frequencies is constricted by three major concerns. It is important that the bolometers effectively see a constant bias power, therefore, the bias frequency must be significantly faster than the bolometer time constant (τ ~ 10 - 20msec). The bias frequency must also be slow enough to avoid attenuation from the effective RC filter arising from the bolometers and capacitance between the wires (with Rbolo ~ 5MΩ and C < 100pF, the cutoff frequency is fc = 1/(2πRC) > 320 Hz). Finally, the bias frequency may not be near any resonant frequency within system. Taking these restrictions into consideration, a bias frequency of 200Hz was chosen for Bolocam II. Following each bolometer is a low-noise, unity-gain JFET amplifier that transforms the differential circuit to much lower impedance, thus reducing the circuit’s susceptibility to microphonics, EMI noise, and RC attenuation. These cold JFETs demonstrate optimal low-noise performance around 130K. To successfully operate at these temperatures, the JFETs must be kept thermally distant from the bolometers while still remaining physically close to avoid noise and attenuation problems. To accomplish this, we have suspended the JFET modules (each module contains the JFETs for one hextant of the bolometer array) in a G-10 cage within the center cavity of the cryostat. 
Figure 2: G-10 JFET cage. Attached to 4K working surface (top). Lower level sunk to 77K bath, while the JFETs self-heat to operating temperature at the center stage. For further EMI protection, all wiring is pi-filtered as it enters the 4K workspace. For cold filtering, we chose to use custom pi-filtered connector saver modules manufactured by Cristek Interconnects Inc. These are compact modules containing shielded back-to-back female and male connectors with pi filters between them. EMI filtering is necessary due to RF noise that enters the cryostat through the dewar window and couples to the wires between the 4K and 300K stages. Therefore, it is important to also filter the signals as they leave the cryostat and enter the shielded warm readout electronics. This warm filtering will be achieved using commercially available back-to-back d-sub-50 connector savers from Spectrum Control that exhibit excellent filtering performance at 300K for a reasonable cost. Back to BOLOCAM II main page. For further information on BOLOCAM II, contact Grant Wilson or Jay Austermann. |