A Temperature Controlled Head Amplifier for 1420MHz Radio Astronomy
This document describes the design and build of a temperature controlled head amplifier assembly for use in Radio Astronomy at 1420MHz. The design is compact so that the amplifier can be mounted close coupled to the dish antenna feed, in this case to a 6 inch diameter cylindrical waveguide with a coaxial output. The system uses a Peltier semiconductor cooling / heating unit driven remotely from a temperature control electronics unit which takes real time temperature readings from the cold plate on which the Peltier element and the head amplifiers are mounted. The system schematic is shown below:
The low noise amplifier is a 1420MHz unit with 28dB gain designed by W5AGO and available from Radio Astronomy Supplies http://www.radioastronomysupplies.com The filter is a 1200MHz high pass SMA type VHF-1200+ available from Mini Circuits http://www.mincircuits.com. . The line amplifier is a BGA616 MMIC gain block with a gain of 15dB @ 14220MHz mounted on a silvered PCB inside a die cast aluminium enclosure fitted with SMA connectors.
The gain of the combined amplifier / filter chain @ 1420MHz is 43dB. The purpose of the temperature stabilisation is to maintain this figure to a 0.1dB over long periods of time during observations. This requires that the closed loop temperature control is better than 10C for an outside temperature variation of 200C In order to achieve this the amplifiers are well thermally bonded to a thick aluminium cold plate with significant thermal capacity. The Peltier cooling element is bonded to the other side of this plate and to the heat sink from which the fan draws waste heat. The unit on the cold side of the plate is surrounded with expanded polystyrene insulation and the whole enclosed in bright aluminised Mylar foil to reflect heat from outside. Finally the entire assembly is fitted inside a white painted plastic rain cover.
The temperature sensor is an LM35DZ IC connected to a CA3140EZ Mosfet Operational Amplifier providing a gain of 10x and able to drive the 30m of twisted pair cable to the control electronics.
1420MHz Antenna and Temperature controlled Head Amplifiers
The thermo electric temperature unit is fitted on the back of the 6 inch waveguide feed as shown below. Both units are enclosed in white painted light weight weather proof plastic containers.
Below is the actual head amplifier assembly with the components indicated.
The Radio Astronomy Supplies’ LNA and the BGA616 line amplifier were mounted to the cold plate using heat sink compound to ensure good thermal conductivity and fast heat transfer to the active components. The Peltier cooling element cannot be seen in these pictures as it lies at the heart of the assembly between the cold plate and the multi finned heat sink. The temperature sensing IC is spring loaded onto the cold plate and faced with heat sink compound. The whole assembly is ~ 100mm diameter and fits inside the plastic weatherproof container shown below.
The graph above shows the operation of the temperature controlled head amplifier assembly. The light blue trace is the temperature of the cold plate as a function of time over a period of ~ 1 hour. The yellow trace is the temperature of the RAS LNA (which shows some thermal lag behind the cold plate). The cooling is switched on at the start of the trace and reduces the temperature from 180C to 70C in a few minutes. Once the required control temperature is reached the Peltier element drive current [dark blue trace] reduces from maximum drive and, following a short settling period, keeps the temperature stable. Twice during the hour long test a 1kW fan heater is used to warm up the outer skin of the head amplifier assembly (the second instance is marked on the graph).The Peltier drive current responds by increasing and thus keeps the Amp and Plate temperatures constant to within 0.20DegreesC
The evidence of the use of the heater can be seen also in the brown trace which is the air temperature in the control room where the test is being performed. The Temperature scale is 1V = 100DegreesC
The temperature control unit is a straightforward OP Amp based analogue system where the actual sensed temperature of the head amplifier is compared with the required temperature (determined by a front panel setting). The difference signal is amplified with sufficient current (up to + - 3A to quickly drive the Peltier semiconductor element in the correct direction to reduce the temperature difference to zero. Temperature control is better than 10C for air temperatures of –100DegreesC to +300DegreesC. Temperature control to this accuracy ensures that the noise level and gain of the head amplifier does not affect the stability of the measured astronomical signal over long periods of up to 24 hours.