Receivers for. FFRF Tutorial by Tom Clark, NASA/GSFC & NVI Wettzell, March 19, 2009

Transcription

1 Receivers for VLBI2010 FFRF Tutorial by Tom Clark, NASA/GSFC & NVI Wettzell, March 19, 2009

2 There is no fundamental difference between the receivers for PRIME FOCUS & CASSEGRAIN Except for: the beamwidth of the feed, the sense of circular polarization, issues of access, maintenance, etc Therefore we can talk about GENERIC receiver architecture. One of the prototype 12M ALMA antennas at the VLBA site. There are many new receiver developments relevant to VLBI2010: The ATA Hat Creek with 42 6M dishes covering GHz, Several SKA projects like Meerkat, Sandy Weinreb s Cryo LNA s, The NASA/Haystack VLBI2010 Prototypes, NRAO s E-VLA upgrade etc

3 The Old Generic S/ Mark-3/4/5 Geodetic Receiver Chain Phase & noise CAL CAL MHz IF Baseband Converters Dual Freq Feed LNAs S LO H-Maser Clock Formatter Recorder Small Cryogenic Refrigerator Analog Digital

4 A typical, old standard Cryo S/ Receiver Fairbanks, Alaska 1984

5 The Next Generation: VLBI2010 Our old, analog hardware has become very difficult to maintain The Unobtanium Problem - Expensive Geodesy wants more precise measurements Need new, fast telescopes with wider bandwidths Many technology advances have come from the Radio Astronomy community Arrays (ATA, SKA, EVLA) - New PHEMT LNAs Wideband Feeds - Mark-5 & E-VLBI FPGA developments Bad RFI S-band= GHz) Technology advances (especially digital) have redefined the best approach from 25 years ago!

6 The ATA is a Showpiece for the technology relevant to VLBI M Antennas Microwave Fiber Optics direct from feed to control room Dual Polarization GHz feed ibob FPGA modules used as PFB (Polyphase Filter Bank) & Correlator

7 The New Haystack/GSFC Receiver CAL CAL H-Maser Clock COTS µλ RF/fiber/RF DBC = Digital Baseband Converters Dual Pol n 2-15 GHz Wideband Feed V H LNAs T T R R UDC = Up/Dn Conv UDC = Up/Dn Conv ADC ROACH ADC Mark 5 & evlbi Cryogenic Refrigerator Analog Digital

8 The New Haystack/GSFC Receiver

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10 The NRAO EVLA Receiver NOISE CAL Digital Fiber R T Master Time/Freq Clock Several Dual Pol n Feeds V H LNAs Ana alog Up/D Down Conv verter ADC ROAC H ADC PFB PFB T T R R Cryogenic Refrigerator EVLA Correlator Analog Digital

11 A Few Comments about Phase Cal Consider a set of very narrow pulses (i.e. delta functions) spaced t in the time domain: t In the frequency domain, we will see the Fourier transform of these pulses, a set of rails in the frequency domain, with a spacing f = 1/ t f

12 Receiver Dynamic Range & RFI From Thermodynamics &Boltzman: P=kTB Where T is Temp in ºK, B is bandwidth in Hz & Boltzman s k = 1.38 x W/(K*Hz) Engineers find a more convenient form is Power in dbm = dbm + B in dbhz + T in dbk For Sandy s Cryo LNA with 100 ºK input over 10 GHz P IN = dbhz + 20 dbk = -78 dbm The LNA has ~35 db gain, so P out is ~-48dBm Sandy indicates the amplifier has -5 dbm output Hence we have only ~38 db of headroom before we experience overload!

13 The Tunnel Diode has a negative resistance region Current I ~200 ~300 mv Voltage V When a tunnel diode is driven with a sine wave 5 MHz), this yields a bipolar series of pulses in the time domain: t

14 We feed the pulse train directly into the front end of the receiver along with the Quasar s RF energy. In the time domain, we can think of these pulses as constituting VLBI s reference clock which is used to time tag the observable geometric delay. We must line up the phases at different observing frequencies in order to determine the group delay for the radio source. The pulses must to be unipolar and they must have fast rise time if the rise time is 20 psec, then the RF spectrum will be useful up to ~10-12 GHz. Alan Rogers invented the first suitable pulse generator in the 1970 s using a Tunnel (Esaki) Diode extracted from a Tektronix Sampling Oscilloscope. Some groups have used Snap or Step Recovery diodes, but they have a high temperature sensitivity problem.

15 The raw pulse train is bipolar (which contains only odd harmonics!!). So it is desirable to get rid of one polarity. Also we have normally reduced the rate by discarding pulses with a series microwave switch. For a 5 MHz input, the gated unipolar pulse rate is 1 MHz t

16 The Phase Cal Problem -- Microwave Tunnel diodes now seem to be made of that rare metal known as unobtainium The Tunnel Diode mounting fixture had to be tweaked by Alan Rogers by hand. The Tunnel Diode pulsers don t work well above ~22 GHz So we wanted a new approach.

17 The New Digital Phase Cal About 1½ years ago, I learned that Hittite had announced a new line of 13 Gb/s digital logic. I was interested in the HMC-672 and/nand gate and suggested this scheme which produces unipolar pulses: The prototype circuit board resulted in a phase cal that looked like this:

18 Unfortunately, the output died around 11 GHz. So Alan Rogers married the old design (with a series µλ switch) to the digital logic, and got good performance, which Chris describes next. However, Hittite has just introduced a new 50 Gb/s=25GHz logic family. I'm intrigued with the new HMC-C065 module, details can be seen at Rather than being a logic chip, the HMC-C065 is a fully packaged, connectorized module which apparently costs $4275 in small quantities. So, my all digital Phase Cal using COTS logic parts still has a chance!