Presentation on theme: "HARP-B Local Oscillator"— Presentation transcript:
11 5 2 9 8 4 3 7 99 6 10 HARP-B Local Oscillator Gunn oscillator ( GHz)Local oscillator ( GHz)1529Gunn OscillatorCrossguide CouplerLevel Modulatorx3 Frequency MultiplierPhase Ref (350 +/- 6 MHz)Gunn Bias ( VDC)MRAO PLLPhase DetectorGunn (-3 to -25 dB)Mixer IF plus Bias8Gunn (-27 dB)43Harmonic MixerDiplexerYIG oscillator (8-11 GHz)7996YIG plus Mixer IFFrequency Synthesizer10 MHz StandardYIG OscillatorObservatory Reference10bmw April 16, 2002
21. Gunn oscillatoro Generates frequency in the range of GHz byexploiting the Gunn effect (dc-bias a slab of galliumarsenide or indium phosphide to create rf oscillations).o Course frequency tuning is mechanical (vary internalcavity size using computer controlled, motorized micrometer)o Fine tuning via bias voltage where the Gunn becomes thevoltage-controlled oscillator (VCO) part of a phase-locked loop(PLL) servo system.o Power level tuning is mechanical (backshort adjustment viacomputer controlled motorized micrometer)o Enabled by turning on 13 VDC power supply to PLL Gunn Driver(actually using #8c which is computer controlled via BMI card)o Available status: none directly although a buffered versionof the Gunn bias voltage is available from the Gunn Driver (#8c)2. Multipliero Varactor-diode based frequency triplero Input power tuning is mechanical (backshort adjustment viao Output power tuning is mechanical (backshort adjustment viao Power level adjustment could be via bias voltage if isolatorwas used but since the LO output level is controlled by ferritelevel modulator, multiplier bias will likely be set and forget.o Available status: Multiplier bias voltage and current3. Harmonic mixero Mixes the 12th harmonic of YIG oscillator with the phaseand/or frequency varying Gunn oscillator signal then puts themixing products back on the same YIG line. The portion of thatsignal that is of interest is the difference between the 12thharmonic and the Gunn oscillator and should be a signalcentered at about 350 MHz.o Available status: None directly although the Mixer biasvoltage is available from the Limiting Amp (#8a)4. Diplexero Like a coupler with a low pass filter on one leg.o The YIG signal passes thru to the mixer where its harmonicsand a portion of the Gunn signal are combined (and fed backinto the diplexer).o The low pass filter in the diplexor only passes the mixingproducts below 1 GHz onto the PLL.o Available status: None5. Ferrite level modulatoro Controls power from Gunn oscillator but with 3dB signal loss(i.e. attentuation variable via computer control from 3-25 dB).o Also acts as an isolator to minimize multiplier effect on Gunn(provides about 18 dB of isolation back to the Gunn oscillator)o Bias current applied through coil around ferrite rod in waveguidecontrols signal attenuation.o Bias drive current should switch to new setting very rapidlyo Available status: Modulator bias current6. YIG oscillatoro Computer controlled frequency synthesizer.o YIG means Yttrium-Iron-Garnet resonatoro Placing YIG in variable magnetic field allows tuning 8-11 GHzo Slower to lock to new frequency than the PTS-500 synthesizero Generates mixer reference in the range of 8-11 GHz whichcorresponds to Gunn osc. output divided by 12th harmonic.Actually the difference between YIG 12th harmonic and theGunn frequency should be nominally 350 MHz (same asnominal synthesizer output).LO /YIG =12However since the resolution of the YIG is 500 KHz, the YIGis set as near as possible to get a 350 MHz difference thenthe synth is set to equal that. For example, if you wanta 372 GHz output, the ideal YIG frequency would beMHz. Instead, set it to GHz (generates anIF of 352 MHz) and set the synth to 352 MHz.o Uses an external 10 MHz reference (Observatory ref)o Available status: YIG oscillator lock status7. Frequency Synthesizero Computer controlled (remote mode) frequency synthesizero Can also be controlled via front-panel knobs (local)o Why is this adjustable? See #6 YIG details.o Generates phase pure 350 MHz PLL reference signal.o Range of 1MHz to 500 MHz (we'll require 350 MHz +/- 6 MHz)o Normally uses an external 10 MHz reference (Observatory ref)o Can use very stable internal oven controlled crystaloscillator as reference when Obs. ref. not availableo Tuning resolution of 100 Hz but only need 1 KHzo Uses direct analog synthesis meaning the output bandis covered soley by arithmetic operations on a numberof standard frequencies derived from the reference(using mixers, filters & switches). This makes frequencyswitching very fast.o Available status: None8. MRAO Phase-lock loop:o Increases frequency stability.o Reduces phase noise.o Enable/disable loop [run in closed loop or open loop]o MRAO design includes 3 modules:a. Limiting Ampo Amplifies low level mixing products and passes IFsignal at ~0 dBm to Wideband Phaselock.o Available status:Locked IF frequency signal at -20 dBm (not used)Harmonic Mixer biasb. Wideband phaselocko Phase/frequency comparator that compares theamplified mixing products from the Limiting Amp withthe phase pure 350 MHz reference (nominal) andoutputs 350 MHz plus DC component proportional tothe phase/frequency difference.o This error signal is low pass filtered to about 40 MHzthen goes thru a loop filter where loop gain/bandwidthis set.o This signal (about 2 MHz bandwidth) is sent to theGunn Driver.Phase lock error voltage.Phase lock status.c. Gunn Drivero Wide band power op amp.o Gunn bias tuning signal (PLL error signal)o Available status: Gunn bias voltage9. Crossguide couplero Like a splitter except the signal is reduced by 27 dBin order to:a. Isolate mixer products from RF output.b. Minimizes loss of RF output.10. Observatory referenceo 10 MHz signal used to phase stabilize YIG oscillatorand frequency synthesizer and hence Gunn frequencyand ultimately the LO output.