Index

The KuPol Receiver

The Kupol receiver is a pseudo-correlation dual-polarization receiver. It has two horns that are symmetrically placed either side of the bore-sight of the telescope. During observations the ANT (antenna) horn is pointed at an astronomical source and the REF (reference) horn also points at the same elevation as the source, but at the azimuth of the source minus 12.95 arcminutes. Left and right circular polarizations are continuously received from both horns.

The kupol receiver has two backends, the universal backend (UBE), which samples the full bandwidth of the receiver (13.5 to 17.2GHz), and provides LCP, RCP and total power outputs, and an FPGA based digital backend that splits the bandpass into 6 bands.

Using KuPol via the Universal Backend

The essential components of the RF signal path between the outputs of the horns and the inputs of the universal backend, are shown in the following diagram. The diagram only shows the signal paths for left-circularly polarized signals. The equivalent diagram for the right-circularly polarized signals would look identical, except that UBE channels 0 and 1 would become channels 2 and 3. Within this diagram, there are four sub-diagrams, each of which show the signals for one of the 4 possible phase-switch states.

Following the signals from left to right, the ANT and REF signals go to the inputs of a 180-degree hybrid-coupler. The two outputs of this coupler are half the sum and half the difference of the ANT and REF input signals. Igoring the phase-switches for the moment, these sum and difference outputs are passed through a pair of low-noise amplifiers of similar gains. The amplified ANT and REF signals are then recovered from the sum and difference signals by a second 180 degree hybrid-coupler. By this point the recovered signals have both been amplified by the sum of the gains of the two low-noise amplifiers, so any drift in the gain of either amplifier affects both signals equally.

The outputs of the second hybrid-coupler are not purely ANT and REF. The ANT output is contaminated by the REF signal scaled by the difference between the gains of the two amplifiers. Similarly the REF output is contaminated to the same degree by the ANT signal. To make this contamination insignificant, the difference between the amplifier gains must be kept much less than the sum of the gains.

The ANT and REF outputs of the second hybrid each go to different square-law detectors. The outputs of these detectors are proportional to the power of the amplified ANT and REF signals. In the universal backend, each of the these outputs goes to an integrator, followed by a sample-and-hold circuit, and finally an analog to digital converter. Since the two outputs traverse different analog paths between the final hybrid and the analog to digital converter, they are affected by slightly different gains and DC-offsets. This makes the raw difference between the digitized ANT and REF samples a poor estimate of the difference between the signals received by the ANT and REF horns.

The effects of the mismatched analog signal paths can be greatly reduced by periodically alternating the ANT and REF signals between the two signal paths, summing samples of ANT from each of the two signal paths, to get an ANT signal that is affected by the sum of the gains and the sum of the offsets of the two signal paths, and doing the same for the REF signal. Differencing these ANT and REF sums then largely cancels out the unbalanced gains and offsets, leaving a much better estimate of the true difference between the signals that entered the ANT and REF horns. This works well provided that the rate at which the signals are alternated between the two signal paths is significantly greater than the rate at which the gains and offsets of the two signal paths vary.

The job of alternating the ANT and REF signals between the two analog signal paths is accomplished by a pair of 180-degree phase switches situated between the two 180-degree hybrid couplers. When these are both in the same state, the sum and difference outputs of the final hybrid coupler are ANT and REF respectively, whereas when the switches are in opposing states, the outputs become REF and ANT respectively. The following table lists the 4 possible combinations of phase-switch states in the order that the universal backend steps through them during each phase-switch cycle. For each state the corresponding signals that emerge from the 4 receiver output channels are shown.

Phase Switch UBE Channel Signals
2 1 0 1 2 3
off off ref L ant L ref R ant R
off on ant L ref L ant R ref R
on on ref L ant L ref R ant R
on off ant L ref L ant R ref R

The universal backend can only transfer one digitized sample to the 40m control computer per millisecond. The kupol receiver has 4 channels that are simultaneously integrated and later digitized, and it takes 4ms to transfer the resulting samples to the computer. While the result of one integration is being transfered to the computer, the next integration is accumulated. Thus the 4ms transfer time also dictates an integration time of 4ms. The states of the phase switches are set by control bits sent by the computer to the universal backend every millisecond. The states of these bits are changed at the start of each integration, then held in the same states for the next 4ms. There are 4 possible pairs of phase-switch states, so it takes four 4ms integration periods to cycle through all of the states. During this time 16 samples are transfered to the computer. Starting from the beginning of a phase-switch cycle, the following table shows the samples that are received from the universal backend during one phase-switch cycle and the corresponding states of the phase switches.

Downlink Sample Integration Phase Switch Signal
2 1
0 0 off off ref L
1 0 off off ant L
2 0 off off ref R
3 0 off off ant R
4 1 off on ant L
5 1 off on ref L
6 1 off on ant R
7 1 off on ref R
8 2 on on ref L
9 2 on on ant L
10 2 on on ref R
11 2 on on ant R
12 3 on off ant L
13 3 on off ref L
14 3 on off ant R
15 3 on off ref R

The raw samples that are received from the backend are made available via the ukupol.rx.sample_values[] archive/monitoring register. This contains up to 1001 samples that are received from the backend each second. The actual number in each 1 second archive frame is indicated by the value of the integer ukupol.rx.nsample register. The times at which each of these samples were received are recorded in the ukupol.rx.sample_times[] register, and the origin of each sample is encoded in the corresponding element of the ukupol.rx.sample_origins[] register.

After each completed 16ms phase-switch cycle, the control computer takes the mean of the 4 samples of each of the 4 desired signals, to compute phase-switch demodulated signals. The phase-switch demodulated samples of the ANT LCP signal are recorded in the ukupol.rx.demod_ant_ll[] register. Similarly those of the ANT RCP, REF LCP and REF RCP signals are recorded in the ukupol.rx.demod_ant_rr[], ukupol.rx.demod_ref_ll[], and ukupol.rx.demod_ref_rr[] registers, respectively. These are array registers that hold up to 63 demodulated 16ms samples each (1 second/16ms = 62.5 demodulated samples per second). The corresponding timestamps, which are recorded in the ukupol.rx.demod_times[] register, indicate the central times of each 16ms integration interval. The actual number of demodulated integrations that are recorded in the above registers, is indicated by the integer value of the ukupol.rx.ndemod register.

A useful value to be able to plot and use for data analysis, is the mean of all of the demodulated samples of a given signal during the 1 second duration of an archive frame. Similarly, the sum of the mean LCP and RCP demodulated samples gives the mean value of the total intensity (I polarization). The mean values of the ANT RCP samples are recorded in the ukupol.mean_ant_rr register, those of ANT LCP in ukupol.mean_ant_ll, and the sum of these, which is the mean total intensity from the antenna horn, is recorded in the ukupol.mean_ant_i. Similar registers exist for the signals from the reference horn. The mean differences between the demodulated ANT and REF samples are also recorded for LCP, RCP and total intensity, in the ukupol.mean_diff_rr, ukupol.mean_diff_ll, ukupol.mean_diff_i registers, respectively.

The kupol receiver has two noise diodes in the ANT signal path and two in the REF signal path. Of the two diodes in each of these signal paths, the one that is refered to as the "cal" diode is a weak source of noise (~ 3K), whereas the one that is refered to as the "noise" diode is a strong source of noise (~ 30K). Commands are described below that turn these diodes on and off individually. All commands to change the states of these noise diodes are delayed until the start of a new phase switch cycle.

Commands accepted by the kupol universal backend

Commands that are specific to the Kupol receiver, are sent as text command strings using the tell command. Beware that the contents of these strings are not checked by the script parser when scheduling scripts are parsed, but rather by the receiver controller when they are actually sent to it.

Setting configuration values

Receiver configuration values are sent to the receiver using the set command. For example, the following command turns off the calibration signal for the antenna horn, and sets an attenuation of 4dB. 
  tell ukupol, "ant_cal=off, atten=4"
Note that this command set 3 different parameters. Other parameters that are not changed by a given set command, are left unchanged. The following is a list of the parameters that can be assigned, along with the values that can be assigned to them, via the set command:
ant_cal
Set the on/off state of the antenna-beam calibration signal, as follows:

ant_cal=on
Turn on the antenna-beam calibration signal.
ant_cal=off
Turn off the antenna-beam calibration signal.

ref_cal
Set the on/off state of the reference-beam calibration signal, as follows:

ref_cal=on
Turn on the reference-beam calibration signal.
ref_cal=off
Turn off the reference-beam calibration signal.

ant_noise
Set the on/off state of the antenna-beam noise signal, as follows:

ant_noise=on
Turn on the antenna-beam noise signal.
ant_noise=off
Turn off the antenna-beam noise signal.

ref_noise
Set the on/off state of the reference-beam noise signal, as follows:

ref_noise=on
Turn on the reference-beam noise signal.
ref_noise=off
Turn off the reference-beam noise signal.

atten
Specify the attenuation to add in the signal path, as follows:

atten=n
Set the attenuation to n dB, where n must be an integer from 0 to 31.

time_nfit
Specify the number of recent seconds whose raw 1ms timestamps are to be used to fit for the timestamps of the current second, in place of using the raw measured timestamps.

time_nfit=n
Set the number of seconds to n. At the end of each second, a line will be fitted through the raw measured timestamps of this second and the raw timestamps of the previous n-1 seconds, fitted versus the ordinal sample number. The recorded timestamps of the second will be taken from this line at the ordinal sample number of each sample in the second.
time_nfit=0
Don't fit for corrected timestamps. Just record the raw timestamps.

Archive registers of the kupol universal backend

Archive registers of the kupol universal backend are collected into groups of related registers, refered to as boards. The registers of each board are listed below using the following columns: 
 board.name[dimension]    Description
Note that to select one of these registers in telviewer, read_registers etc, it is necessary to prefix the names shown in this web page with: "ukupol."

Also note that the two right-most columns of the tables of registers, show the raw units used to record the values as integers in the archive files, followed by the floating point units that result after scaling and offsetting these values by the numbers in the ukupol.cal instrumental calibration file.

Index of boards

Frame board registers:

Register name Description Raw units Cal units
frame.record The sequential record number. count count
frame.utc[0-1] The date and time of the frame, expressed as two elements containing MJD days and milli-seconds, respectively. This register is sought by telcontrol in the register maps of all client systems, and the time that it contains, rounded down to an integer number of seconds, is used to match simultaneous frames from the different systems. days, ms days, ms
frame.utc_status The usability of the timestamp contained in the frame.utc register, expressed as a FrameUtcStatus enumerator. code code

Receiver board registers:

Register name Description Raw units Cal units
rx.utc[0-1] The time of the 1-second tick at which the following information was archived, expressed in MJD days and milliseconds (rounded down to the start of the second). days, ms days, ms
rx.utc_status The usability of the timestamp contained in the above utc register, expressed as a FrameUtcStatus enumerator. code code
rx.noise_temp[0-1] The temperatures of noise diodes 1 and 2. milli Kelvin Kelvin
rx.temp_10k The temperature of the 10K plate. milli Kelvin Kelvin
rx.hybrid_temp The temperature of the first 180-degree hybrid-coupler. milli Kelvin Kelvin
rx.plate_temp The temperature of the cold plate (nominally 20C). milli Celsius Celsius
rx.rx_temp The temperature of the receiver support structure. milli Celsius Celsius
rx.pos_5v_v The voltage of the +5V supply. milli volts Volts
rx.pos_5v_i The current from the +5V supply. milli Amps Amps
rx.neg_5v_v The voltage of the -5V supply. milli volts Volts
rx.neg_5v_i The current from the -5V supply. milli Amps Amps
rx.pos_15v_v[0-1] The voltages of the stage 1 and 2 +15V power-supplies. milli volts Volts
rx.pos_15v_i[0-1] The currents being drawn from the stage 1 and 2 +15V power-supplies. milli Amps Amps
rx.pos_28v_v The voltage of the +28V supply. milli volts Volts
rx.pos_28v_i The current from the +28V supply. milli Amps Amps
rx.rf_power[0-3] The RF powers of the 4 receiver channels. mV/8.5 (legacy) Volts
rx.motor_speed The speed of the refrigerator motor (units unknown). uV Volts
rx.hotload_temp The hot-load temperature. milli Kelvin Kelvin
rx.backend_temp The backend temperature. milli Celsius Celsius
rx.heat_cool This indicates whether the backend is being cooled (-5v), heated (+5v) or left at the current temperature (0v). milli volts Volts
rx.flags A bitwise OR of the following configuration bits. Note that bit 0 is the least-significant bit.

0: (unused)
1: (unused)
2: (unused)
3: ANT_NOISE_ON
This flag is present when the ANT large-signal noise source is turned on.
4: ANT_CAL_ON
This flag is present when the ANT small-signal noise source is turned on.
5: REF_NOISE_ON
This flag is present when the REF large-signal noise source is turned on.
6: REF_CAL_ON
This flag is present when the REF small-signal noise source is turned on.

Note that for C programs the values of the above bits are enumerated by the UkupolConfFlag datatype, which is defined in header file ukupol_regs.h. Note that the cal-diode bit assignments in this register are the same as those of the kuband receiver.

bits bits
rx.atten The setting of the attenuator (0 to 31) in dB. dB dB
rx.nsample The number of downlink samples, sample timestamps and sample origins in the rx.sample_values[], rx.sample_times[] and rx.sample_origins[] registers. Since the 1KHz sampling clock in the universal backend is not phase-locked to GPS, this may be fewer than 1000 samples, or up to 1001 samples. count count
rx.sample_times[0-1000] The times at which each of the 1KHz downlink samples were received by the control computer, expressed as a microsecond offset from the time-stamp in the rx.utc register. micro seconds seconds
rx.sample_values[0-1000] The raw 1KHz downlinked ADC counts that were received at the times specified in the rx.sample_times register. ADC counts ADC counts
rx.sample_origins[0-1000] The origin of each sample in the rx.sample_values[] register, expressed as an unsigned integer in the bits are partitioned as follows:

3-2: The 2-bit RF channel number of the sample.
1: The state of phase-switch 2
0: The state of phase-switch 1
 bits bits
rx.ndemod The number of phase-switch demodulated samples and associated values in the rx.demod_times[], rx.demod_rr_ant[], rx.demod_ll_ant[], rx.demod_rr_ref[], rx.demod_ll_ref[], registers. This may be between 61 and 63, because the average number of 16ms demodulated samples within the 1000ms duration of an archive frame is 62.5. count count
rx.demod_times[0-62] The central times of the 16ms intervals that were used to integrate each of the demodulated samples, expressed as a signed microsecond offset from the absolute time in the frame.utc register. micro-seconds Seconds
rx.demod_ant_ll[0-62] The phase-switch demodulated samples of the left-circular polarized signal received by the ANT horn. ADC counts ADC counts
rx.demod_ant_rr[0-62] The phase-switch demodulated samples of the right-circular polarized signal received by the ANT horn. ADC counts ADC counts
rx.demod_ref_ll[0-62] The phase-switch demodulated samples of the left-circular polarized signal received by the REF horn. ADC counts ADC counts
rx.demod_ref_rr[0-62] The phase-switch demodulated samples of the right-circular polarized signal received by the REF horn. ADC counts ADC counts
rx.mean_ant_ll The mean of the phase-switch demodulated samples of the ANT LCP signal. This is the mean of the values in the rx.demod_ant_ll[] register. ADC milli-counts ADC counts
rx.mean_ant_rr The mean of the phase-switch demodulated samples of the ANT RCP signal. This is the mean of the values in the rx.demod_ant_rr[] register. ADC milli-counts ADC counts
rx.mean_ant_i The mean phase-switch demodulated total intensity signal from the ANT horn. This is the sum of the mean of the values in the rx.demod_ant_ll[] register and the mean of the values in the rx.demod_ant_rr[] register. ADC milli-counts ADC counts
rx.mean_ref_ll The mean of the phase-switch demodulated samples of the REF LCP signal. This is the mean of the values in the rx.demod_ref_ll[] register. ADC milli-counts ADC counts
rx.mean_ref_rr The mean of the phase-switch demodulated samples of the REF RCP signal. This is the mean of the values in the rx.demod_ref_rr[] register. ADC milli-counts ADC counts
rx.mean_ref_i The mean phase-switch demodulated total intensity signal from the REF horn. This is the sum of the mean of the values in the rx.demod_ref_ll[] register and the mean of the values in the rx.demod_ref_rr[] register. ADC milli-counts ADC counts
rx.mean_diff_ll The mean of the difference between the phase-switch demodulated samples of the ANT and REF LCP signals. This is given by rx.mean_ant_ll - rx.mean_ref_ll. ADC milli-counts ADC counts
rx.mean_diff_rr The mean of the difference between the phase-switch demodulated samples of the ANT and REF LCP signals. This is given by rx.mean_ant_rr - rx.mean_ref_rr. ADC milli-counts ADC counts
rx.mean_diff_i The mean of the difference between the phase-switch meanulated samples of the ANT and REF total intensity signals. This is given by rx.mean_ant_i - rx.mean_ref_i. ADC milli-counts ADC counts
Martin Shepherd (2-Jun-2014)