Last updated 1998 June 26

This document is based upon a Radar Interface discussion, 1995 November 1, with Don Campbell, Eddie Castro, Alice Hine, Mike Nolan, Phil Perrilat, Bill Sisk, Mike Sulzer, and Bob Zimmerman present. This is a dynamic document intended to reflect current plans, not a meeting record.

This document was updated based upon a conversation with Bill Sisk 1996 March 25. I kept the previous version in case anybody reads these.

Radar Data Acquisition

The radar data acquisition system has three primary modes of operation for planetary radar: hardware decoding of a coded signal, direct sampling of a coded signal, and CW spectrometry. Construction of the hardware to receive the first is underway, and can probably be finished by first light. CW and direct sampling will still be done with much of the pre-upgrade hardware. At the meeting, a new purely digital scheme for CW data acquisition was blocked out. This new system will not work by first light, but, since much of the pre-upgrade hardware can probably be made to work, albeit inelegantly and not as well, it was concluded that it was better for the new system to be done well rather than soon. Our decision as to how to improve will probably depend on observing experience, so the scheme described here is very preliminary.

Synopsis

The initial input for this system is a 260 MHz IF (left and right circular down-converted from 2380 MHz upstairs). Because of our narrow-band signal, conversion to circular in the turnstile upstairs probably gives us cleaner polarizations than using the downstairs IF/LO. This signal has not been Doppler corrected. This signal comes down on an optical fiber, then out of the downstairs IF/LO system on the .2-.4 GHz channel. The analog gain is set in the IF/LO, with a fine gain control later in the signal path. There are two independent (polarization) channels in this hardware, but there is only one set of frequency synthesizers. The 260 MHz signal is passed through a many-pole 20 MHz analog bandpass filter. This filtered 260 MHz signal is then down-converted to baseband by a Doppler-correcting mixer, then analog low-pass filtered for sideband rejection. The signal power is measured at this point for setting the gains to get good dynamic range into the digitizers.

Decoder

At this point, the signal is fed to an 8-bit digitizer running at 80 MHz. The digitizer feeds the software-selectable digital sinc filters, which provide separate 9-bit I and Q outputs. The filters provides 4-bit input to the hardware decoder. Which 4 bits is software selectable.

Sampler / CW

Along the decoder signal path, before the digitizers, there is a BNC front-panel output that can be fed to other hardware. The ``Radar Interface'' will be used in this mode, with patch-corded analog filters feeding the RI inputs. There are no digital filters for either direct sampling or CW in the current design, though those will clearly be in future plans.

Decoder / Sampler

The ``sampler'' portion of this program has been deleted to save engineering time. Sampling will be done through the pre-upgrade analog filters. The decoder itself is separately discussed, here I describe the input signal path.

The data acquisition system for the hardware decoder takes a 260 MHz IF (two circular polarizations down-converted from 2380 MHz upstairs). This signal has not been Doppler corrected, though that may be possible and useful in the future. This signal comes down on an optical fiber, then out of the downstairs IF/LO system on the .2-.4 GHz channel. The analog gain is set in the IF/LO. There are two independent (polarization) channels in this hardware, but there is only one set of frequency synthesizers. The system will be described as if there were only one channel, for simplicity.

The 260 MHz signal is passed through a 20 MHz (1-dB down) many-pole analog filter for rejection of out-of-band signals. The 20 MHz filter was chosen because it is a substantial fraction of the transmitter bandwidth (26 MHz at -1 dB), and because there are cellular allocations in the 2390-2417 MHz band (but possibly not in Puerto Rico).

This filtered 260 MHz signal is then down-converted to 20 (or 30) MHz by a Doppler-correcting high-side mixer with an LO at 280 (or 290) MHz, then analog low-pass filtered for sideband rejection. The high side was chosen to avoid intermodulation products with the sampling frequency, since the LO for a lower sideband mixer would be at 240 MHz, which is 3 times the sampling rate. Note that the sign of the Doppler correction depends on how many high-side LOs are used before the Doppler-correcting mixer, but not on whether it is high- or low-side itself. Currently (June 1998) we are using a single high-side LO upstairs at 2640 MHz, and so the Doppler correction is inverted.

The 280 MHz LO is doppler shifted by the following procedure:

1. The programmable DDS provides a doppler-shifted 20 MHz tone.

2. This signal is mixed with a fixed 100 MHz LO to generate an 80 MHz signal

3. This signal is mixed with a (programmable) 360 (or 370) MHz LO in the IF/LO to generate the doppler-shifted 280 (or 290) MHz signal.

The signal power is measured at this point for setting the gains to get good dynamic range into the digitizers. is this true? The signal is then split, and goes to a front-panel output and to the decoder. This IF is nominally 40 MHz wide, and will be sampled at 80 MHz, but because of the filtering, it is really only 20 MHz wide. The extra will be filtered off in half-band filters in the sinc-filter hardware.

There may be a narrow-band filter (1.25 MHz centered at 20.625 MHz) at this point, to be used to clean the signal and avoid overflow for the longest bauds. The accumulators can in principle overflow for bauds longer than 6.4 μs with 80 MHz sampling, but a properly placed bandpass filter cleans the signal and also allows the system to simply ignore some of the samples, rather than having to average them, eliminating the possibility of overflow. This particular filter allows the system to still have a 20 MHz IF and sample at 80 MHz, but only keep every 32nd sample and still be complex sampled at 1.6 μs and longer. The overlap allows two complex samples/baud at all baud lengths except 50 ns.

At this point, the signal is fed to an 8-bit digitizer running at 80 MHz. This 80 MHz signal is generated from the second DDS by subtracting from a fixed 100 MHz, and is doppler-shifted at the code rate. The digitizer feeds the software-selectable digital sinc filters, which provide separate 9-bit I and Q outputs. The filters provides 4-bit input to the hardware decoder. Which 4 bits is software selectable. The original scheme was to select the bits based upon a formula from the measured power either from the power counter or the RI. This system amounts to an AGC, and, while convenient for some things, makes the system uncalibratable. The general feeling at the meeting was that, while the option of this AGC would be nice, a purely manual system would be better than a purely automatic one. DC pointed out that, since the antenna gain should be more constant after the upgrade, the need for an AGC should be reduced. "Manual system" in this context means a user-entered value, not a big black knob. Phil suggested that the software would compare the power counter reading to the I output, and flag a mismatch to the user. If this problem were due to a signal out of the (final) bandpass that was not so large that the digitizer dynamic range was exceeded, the output bits could be selected manually and observing could proceed. Note that this procedure may be dangerous with sinc filters because of their sidelobes.

The sinc filter unit begins with a half-band filter to convert the 80 MHz samples into separate I+Q channels, each 0-20 MHz. These are half-band filtered again to provide I+Q at 0-10 MHz (each). This signal is fed to the sinc filters and then on to the decoder. Note that two samples per baud are only available at 10 MHz and below.

There is a single monitor D/A converter located after the sinc filters that can provide either I or Q from either polarization channel, selected by front panel toggle switches.

The set B 9-bit I+Q filter outputs have been deleted, so all discussion of them is now moot.

There will be a set of high-speed computer controlled variable attenuators somewhere in the signal path for gain setting. They can be anywhere between the 260 MHz input and the point where the signal splits to the front panel, and the site has not been chosen. The attenuators don't work below 10 MHz, which is probably OK, since anything below 10 MHz is out-of-band, but is a reason to keep them in the higher-frequency part of the path.

Clocking

There was a discussion of how the various digitizers should be clocked. It is necessary for the Decoder, and convenient for the Radar Interface, for the digitizer clock to be synchronized with the transmitter, including Doppler-induced time dilation. Another issue is that we sample the transmitter output for some experiments, and don't want the samples to happen on code transitions. In practice, it's probably easier if all of the clocks are the same, both for implementation and for monitoring, and nobody came up with a good reason not to. In the current scheme, both of the DDS are used at 80 MHz, and the decoder wil have to be driven by dividing down the 80 MHz signal. Bill thinks dividing down the 80 MHz signal will provide a much cleaner signal than would multiplying up the 20 MHz one (which was the other option).

CW systems

First-Light Hack

The "interim" (analog) CW system will consist of parts of the new Decoder combined with cabled analog hardware. While there is always the danger that "interim" + "budget cut" => "permanent", this system isn't really sufficient for the needs of the atmospheric group, and the proposed system seems like it will be.

Again, we have a 260 MHz IF into the Downstairs IF/LO. This will be run through a 20 MHz bandwidth filter (?), to the same Doppler-shifted mixer down to a second IF at 30 MHz. This IF is generated the same way as for the decoder, but with the LO at 290 MHz rather than 280 MHz, by resetting the programmable LO in the IF/LO to 370 MHz rather than 360 MHz. The 30 MHz signal will be sent to the front panel, where the user will patch-cord it to whatever detector is desired.

The feeling is that this setup can be working by first light, but that it's a hack, and that the available filters aren't optimal. Thus began the digital CW interface:

Digital CW Radar Interface

This description is very preliminary, and is more a document to discuss than a description of hardware. At this point, we will probably get some experiance with the observing with new hardware before designing the rest.

A digital CW filter set would eliminate the cabling nightmare and give us nice clean filters. It would be nice if this were a general-purpose system, as well. The following issues were raised:

• Dynamic Range. Mike Sulzer says he reallys needs 12 bits of dynamic range for ionospheric work.

• Filters. The CW system should have square (in frequency) filters, and sampling of a coded signal requires sinc filters. Mike Sulzer says he needs square filters for his work. Originally the decoder/sampler would have provided the sinc filters, but that is no longer in the design.

• Speed. Planetary Radar only needs a relatively narrow bandwidth, typically a few to a few hundred kHz. Aeronomy routinely needs several MHz, and Phil suggests pulsars would probably like 20 MHz (but now thinks they don't really want this system at all). The problem with sampling that fast is that it's difficult to record the data.

After some discussion, a scheme is blocked out. Again, the signal will come down at 260 MHz, and go through the 20 MHz / 5 Mhz bandwidth filter, be mixed down to baseband and low-pass filtered to remove the other sideband, all as in the Decoder / Sampler (probably the same hardware). The baseband signal will be sampled at 12 bits by a digitizer running at 40 MHz or 10 MHz, depending on which bandpass filter was selected. (Bill Sisk has seen ads for a 40 MHz 12 bit A-D by a reputable manufacturer). The digital signal then goes through up to 8 half-band filters, and comes out as 12-bit I+Q. There will then be hardware to select and pack the bits, and send them to some device that can in principle write the numbers down. Note that the minimum output sample rates are 80 kHz or 20 kHz (depending on the initial filter and sample rate) in each polarization and in each of I and Q, because there are only 8 filters, and 20 MHz / 2^8 = 78125 Hz. Adding more filters would be complicated. Programmable filters exist, but have lower dynamic range, which was the point in the first place. Most believed that this limit was acceptable because it is slow enough that, if desired, additional filtering could be done in software, and in any case is easily recorded on tape. Bill asked if real (rather than I+Q) outputs would be acceptable, as it would cut then number of output lines in half (and run twice as fast), and an 8-octave real filter board has already been designed. The scientists preferred I+Q if possible, because it keeps the math neater. Since the design must be changed anyway to allow packing, it will be I+Q.

D-A converters will be provided for monitoring. Because the half-band filters decimate the frequency (unlike the sinc filters), the output will have a messy frequency response, but an analog filter will be provided to smooth it (is some interpolation necessary anyway?). Because this output is only designed for monitoring, the relatively poor behavior of this analog filter is considered acceptable.

This system is not expected to be on line at first light. Bill was optimistic that it wouldn't take too long, as the boards are similar to some that are already designed. The current VME boards can only go at 15 MB/s (or perhaps 14), and the feeling was to wait until they can take this full data rate.

Data Recording and Display

Both the decoder and the Radar Interface dump their data to memory buffers. The Master Control Program routes these buffers across the VME bus to client devices, such as a tape or disk, the Array Processor, or display devices. Buffers can be sent to the ethernet, and possibly directly to the SBus of the observer Sun workstation for processing and display.

Other issues

Some other misc. issues were discussed. We need to monitor the leakage from the transmitter, and to sample the outgoing code for random codes. There's 22 dB of available attenuation before the fiber coming downstairs, which may be enough, but it's not certain. Also, the specifics of sending an arbitrary code to the transmitter are not set.

Bill wants to know where to inject the signal for "closed loop tests". Presumably, the earlier the better, but the only real restriction is that it should be before any changable devices, like filters. It would be much more work to inject the signal upstairs, and it would be most convenient if it could be in the 260 -> 20 IF box. He will discuss this question with DC.

Notes added after a discussion with Mike Davis 1995 November 14

At this point (1996/03/25) several of the issues we discussed are now moot, as the relevant hardware does not (and will not) exist.

The "old" CW system and the "Sampler" both seem to need the "old" RI. Can these be switched in a reasonable amount of time in an observing session?

The meeting was called at least in part to discuss what changes to the Downstairs IF/LO were required to do this project, because Eddie Castro and Bob Zimmerman want to close off vague "little changes" to that system, as they get expensive at this point. We discussed using one of the MUX outputs, but there was no concrete discussion of what was to happen and who was to do it, so this goal may not have been met.

We may be being bad frequency-citizens by square-modulating with the code, and could we filter the modulation to keep our bandwidth more contained? It would require a different filtering in the decoder, I think, if it's true. Note that the transmitter has a fairly narrow bandwidth, so it may not be too bad.

The half-band filters give an extra 1 bit per octave, so do we really need (presumably expensive) 12 bit A-D?

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Last updated 1998 June 26