SRT Circuit Descriptions

(This is a re-formatted version of the file found at
http://www.haystack.mit.edu/edu/undergrad/srt/receiver/receiver_circuit.html.)

B) Image Rejection Mixer
A pair of mixers with 90° quadrature phased L.O. drive followed by a 90° I.F. phasing networks make a "phasing type" single sideband receiver.  The lower sideband is only rejected by 20 dB but since the unwanted "image" sideband is adjacent to the wanted sideband we don't expect strong signals in the radio astronomy band to be present in the image.  Both the preamp and the MAR-6SM amplifier provide a large amount of reverse isolation so that the level of L.O. radiation, which might interfere with others, is minimized.  The unconventional use of the single conversion to a low "baseband" frequency I.F. has the advantage of avoiding a difficult image problem, allows simple active I.F. filtering, as well as allowing for easy future digital signal processing of the baseband.  The disadvantage is that we need to rely on amplifier reverse isolation to prevent significant L.O. radiation.  The AD8011 operational amplifiers are unit gain all_pass filters with 90 degree phase difference determined by c1 and c2.  Since most of the radiometer's gain is in operational amplifiers it is very stable and requires only infrequent calibration.

C) Gain Control
A 10 dB attenuation can be switched in to prevent overloading the power detector when observing strong signals like those from the sun when is in an active state.

D) Active I.F. Filter/Amplifiers
A pair of AD8011operational amplifiers provide 53 dB of gain and high/low pass filtering.  The high pass cuts-off frequencies below 10 kHz while the low pass cuts-off frequencies above 70 kHz.  The high pass is a 4-pole Butterworth filter with 56 kHz 3 dB passband.

E) Square Law Detector
The conversion of the I.F. frequency voltage waveform to a voltage which is proportional to the I.F. signal power uses a back-diode or Schottky diode in the "squaring" region.  Because the square law region is at a very low level the diode output is amplified by a factor of 50 using a OP-27 amplifier.

F) Analog to Digital Conversion
The I.F. power is an analog signal which is converted to a pulse whose duration is inversely proportional to the average power present during the pulse period.  The AD654 and the stamp pulsin routine performs this "integrating" analog to digital conversion (ADC).  The AD 654 output is a 12 ms positive pulse for a nominal 0.5 volt output from the OP-27 followed by a fixed 3 ms negative pulse.  This negative pulse is shortened by the current through a diode when the output goes low.  See the AD654 data sheet for the details of its operation.  The stamp uses the pulsin routine to measure the duration of the positive pulses.

G) Local Oscillator Synthesizer
A voltage turned oscillator (JTOS-2000) is "phase locked" to the 8 MHz crystal oscillator using a LMX2325 synthesizer chip.  The details of the LMX2325 and the principles of the phase lock are described in the data sheet.  A MAR-4SM amplifier is used to isolate the oscillator from the mixers and an OP-27 is used to boost the LMX2325 phase detector voltage so that the oscillator can reach the OH frequencies at 1665 MHz which require a tuning voltage above +5v.

H) Serial Communication
The D.C. power along with bi-directional serial communication are all multiplexed onto a single conductor coax cable which connects the "ground" electronics from the receiver electronics on the antenna.  The voltage from the LM317T conveys the D.C. power and the "uplink" to the receiver all follows:

RS-232 input	LM317 out	Voltage on cable
+12v	22v	19v
-12v	17v	14v

I) Stamp Controllers
The radiometer and antenna drives are controlled via stamp 1 controllers.  The RS-232 serial port is used to talk to both stamps using a different "keyword" for each stamp and a wired logical "OR" for the response from each stamp.  This method allows communication with one stamp at a time.  Under most circumstances this is acceptable since we don't need the radiometry data while the antenna is in motion.

J) Biasing the Operational Amplifiers
Only a positive voltage is supplied to the radiometer.  Since the operational amplifiers require both polarities the inputs are biased to a voltage above ground potential with resistor network.  In addition there are many decoupling capacitors to filter the power and bias voltages.

K) Motor Control
Uses power H-bridges (LMD-18200) to control and reverse the +24v D.C. power to the motors.

L) Antenna Feed
The radiometer is connected to a feed which illuminates a 10 foot diameter dish with f/D ratio of 0.4.  The feed is made by adding a λ/4 probe to the C-band (4.2 GHz) feed.  The outer diameter of the C-band feed is about 8" which supports a TE₁₁ circular waveguide mode with a/λ ≅ 0.5 which is close to optimum for a f/D ≅ 0.4 [see Methods of Experiment Physics, Astrophysics edited by Marton, vol. 12B page 32]. The aperture efficiency should be close to 60% (or 50% when the feed and feed support blockage is taken into account) so that the antenna temperature is about

T_ant = (FAη / 2k) x 10^-26

where
A = area of the reflector in m²
F = radio source flux density in w m^-2 Hz^-1
η = aperture efficiency ≅ 0.5
k = Boltzmann's constant 1.38 x 10^-23 w Hz^-1 K^-1.

M) Digital Receiver
The digital receiver replaces the single 40 kHz filter and analog square law detector with a 8-bit analog to digital down converter (AD9283) digital downconverter GC1011A (see www.ti.com - search on Graychip) and Motorola 56F803 digital signal processor (DSP).

a] R.F. Section
The output of the LNA is further amplified by a MAR6SM and down converted using an image reject mixer. Current firmware selects an I.F. frequency of 800 kHz and a high side L.O. has been chosen. i.e. the frequency synthesizer is set to a frequency 800 kHz higher than the desired R.F. center frequency. A total I.F. gain of 50 dB before A/D conversion.

b] A/D Conversion
The 8-bit A/D converter provides an offset binary code which is connected to bits 3 thru 10 of the digital down converter. The MSB (bit 11) is set to zero as are bits 0 thru 2. This scheme avoids the need to invert the MSB of the A/D to generate the twos complement data expected by the Graychip.

c] Digital Down Conversion
The GC1011A uses quadrature digital mixers with numerical local oscillator. The "sine" and "cosine" outputs are low pass filtered in stages to avoid aliasing. The low pass filtered 16 bit outputs are accessed by the 56F803 DSP.

d] DSP
The 56F803 DSP is a general purpose processor the performs the following functions (with current firmware code):

1) Uses the built-in UART to listen for the keyword "freq" which will be followed by the mode.
2) Sets up the LMX2325 L.O. frequency synthesizer.
3) Sets up the GC1011A Graychip digital L.O. frequency, decimation ratio, gain, and selects the default 80% filter.
4) Reads output from the Graychip 64 complex samples in a block.
5) Each 64 sample block is Fourier transformed using 64 point complex FFT.
6) The FFT outputs are squared to obtain the spectral power.
7) The spectral power from each block is accumulated for 4096 blocks.
8) The accumulated outputs are sent back at 2400 baud.

The code is placed in flash memory of the 56F803 using the Metrowerks embedded development system via the JTAG port. The source code (which is mostly in C is available) requires the development system and JTAG converter to be altered and re-downloaded.

e] Power, Communication and Digital Clocks
As in the analog receiver the ground controller provides the DC power and 2-way serial communication. The communication from the controller to the 56F803 is performed by the supply voltage which changes from a nominal 14 volts to 18.8 volts for the "uplink". (this is based on a current of 210 ma - which is typical of the receiver and a 13.3 ohm series resistor in the controller.) The down link is obtained by increasing the current from about 210 ma to about 400 ma. The 3.3 volts for the A/D, Graychip and DSP is provided by an efficient DC to DC converter (LT1616). The fundamental clock for the receiver is the 8 MHz crystal of the R.F. synthesizer. The 8 MHz is sent to the 56F803 whose internal clock circuitry multiples the 8 MHz to 40 MHz for the A/D sample clock and the Graychip clock and to 80 MHz for the internal core DSP logic.

f] Ground Controller
A new ground controller circuit provides control for the analog receiver (series resistor of 20 ohms - jumper removed) or the digital receiver (series resistor of 13.3 ohms - jumper in place). The new circuit also includes a true RS-232 level converter to ensure reliable communication for longer cables between the PC and the controller.

N) Radiometer Stamp Control Firmware
The radiometer stamp communicates with the PC serial port, controls the L.O. frequency synthesizer, the 10 dB attenuator and measures the radiometer power.

a] L.O. frequency synthesizer
The reference divides (R register) is set to 200 to provide a 8 MHz/200 = 40 kHz reference.  [This could be changed by small amounts for added frequency flexibility but large changes may require changing the loop filter capacitor values.]  The prescalor is set to 64/65 by making S15 = 0.  The A register is set by first calculating its value in the host PC j = (freq/0.04) + 0.5;

Where	Freq = frequency in MHz
	j = 32 bit unsigned integer

Then separating j into bytes

b11 = (j >> 14)& 0xFF
b10 = (j >> 6)& 0xFF
b 9 = j & 0x3F

Because the synthesizer is quantized the actual frequency will be

freq = ((256 * b11 + b10) * 64 + b9) * 0.04

in the case that the frequency you selected is not a multiple of the 40 kHz reference frequency.  The bytes b11, b10, b9 along with a byte b8 to control the attenuator are sent to the stamp following the keyword "freq".

b] Power measurement
The radiometer power is measured by using the pulsin routine (see stamp 1 manual) to count the stamp clock cycles in a pulse from the AD654 voltage to frequency converter.  To enhance the accuracy we count up to 10⁴ 10 microsecond cycles.  During this time we may have several AD654 positive pulses so we return the total count w1 and the number of pulses counted w2.  The host PC then calculates the power

Power = scalef  x (w2 / w1)

so that, for example, a 0.5 volt out of the OP-27 which produces a pulse length of 12 ms so that

w2 = 8 = b4
w1 = 9600 = b3 x 256 + b2
and power = 833.33 when scalef = 106

The scale factor of 106 is approximately the value which makes the output in units of K but each individual radiometer needs to be calibrated if you want the power to be in units of Kelvins.

O) Antenna Drive Control Stamp
The antenna azimuth and elevation drives are controlled one at a time with a stamp 1 in the power supply unit which is located near the control PC.  The motors are activated when the stamp detects the keyword "move".  The keyword is followed by a byte which gives the axis and direction to the move and the number of "counts" of the reed microswitch on the drive gear to move.  The motors drive a magnetic disk with 12 poles giving 12 contact closures per rotation which in turn drive a sprocket gear with 8 teeth which chain drives a large 54 tooth sprocket gear which in turn drive a sector gear of 52 teeth via a worm gear.  Thus the number of counts per degree is

12 x 54 x 52 / (8 x 360) = 11.70

Note added by JDL: The "move" command is used internally by the java program and is not accessible from the console or a commands file. The synopsis is: move mm counts where the values of mm mean