Frequency Synthesizer YIG Oscillator Observatory Reference Diplexer Gunn Oscillator Harmonic Mixer Crossguide Coupler Level Modulator x3 Frequency Multiplier MRAO PLL Phase Detector Local oscillator (330-370 GHz) Gunn oscillator (110-124 GHz) Gunn (-3 to -25 dB) Gunn (-27 dB) YIG plus Mixer IF Gunn Bias (8.8-9.6 VDC) Mixer IF plus Bias 10 MHz Standard Phase Ref (350 +/- 6 MHz) YIG oscillator (8-11 GHz) HARP-B Local Oscillator bmw April 16, 2002
1. Gunn oscillator o Generates frequency in the range of 110-123.5 GHz by exploiting the Gunn effect (dc-bias a slab of gallium arsenide or indium phosphide to create rf oscillations). o Course frequency tuning is mechanical (vary internal cavity size using computer controlled, motorized micrometer) o Fine tuning via bias voltage where the Gunn becomes the voltage-controlled oscillator (VCO) part of a phase-locked loop (PLL) servo system. o Power level tuning is mechanical (backshort adjustment via computer 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 version of the Gunn bias voltage is available from the Gunn Driver (#8c) 2. Multiplier o Varactor-diode based frequency tripler o Input power tuning is mechanical (backshort adjustment via computer controlled motorized micrometer) o Output power tuning is mechanical (backshort adjustment via computer controlled motorized micrometer) o Power level adjustment could be via bias voltage if isolator was used but since the LO output level is controlled by ferrite level modulator, multiplier bias will likely be set and forget. o Available status: Multiplier bias voltage and current 3. Harmonic mixer o Mixes the 12th harmonic of YIG oscillator with the phase and/or frequency varying Gunn oscillator signal then puts the mixing products back on the same YIG line. The portion of that signal that is of interest is the difference between the 12th harmonic and the Gunn oscillator and should be a signal centered at about 350 MHz. o Available status: None directly although the Mixer bias voltage is available from the Limiting Amp (#8a) 4. Diplexer o Like a coupler with a low pass filter on one leg. o The YIG signal passes thru to the mixer where its harmonics and a portion of the Gunn signal are combined (and fed back into the diplexer). o The low pass filter in the diplexor only passes the mixing products below 1 GHz onto the PLL. o Available status: None 5. Ferrite level modulator o 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 waveguide controls signal attenuation. o Bias drive current should switch to new setting very rapidly o Available status: Modulator bias current 6. YIG oscillator o Computer controlled frequency synthesizer. o YIG means Yttrium-Iron-Garnet resonator o Placing YIG in variable magnetic field allows tuning 8-11 GHz o Slower to lock to new frequency than the PTS-500 synthesizer o Generates mixer reference in the range of 8-11 GHz which corresponds to Gunn osc. output divided by 12th harmonic. Actually the difference between YIG 12th harmonic and the Gunn frequency should be nominally 350 MHz (same as nominal synthesizer output). LO / 3 - 0.35 YIG = --------------- 12 However since the resolution of the YIG is 500 KHz, the YIG is set as near as possible to get a 350 MHz difference then the synth is set to equal that. For example, if you want a 372 GHz output, the ideal YIG frequency would be 10304.167 MHz. Instead, set it to 10.304 GHz (generates an IF 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 status 7. Frequency Synthesizer o Computer controlled (remote mode) frequency synthesizer o 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 crystal oscillator as reference when Obs. ref. not available o Tuning resolution of 100 Hz but only need 1 KHz o Uses direct analog synthesis meaning the output band is covered soley by arithmetic operations on a number of standard frequencies derived from the reference (using mixers, filters & switches). This makes frequency switching very fast. o Available status: None 8. 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 Amp o Amplifies low level mixing products and passes IF signal at ~0 dBm to Wideband Phaselock. o Available status: Locked IF frequency signal at -20 dBm (not used) Harmonic Mixer bias b. Wideband phaselock o Phase/frequency comparator that compares the amplified mixing products from the Limiting Amp with the phase pure 350 MHz reference (nominal) and outputs 350 MHz plus DC component proportional to the phase/frequency difference. o This error signal is low pass filtered to about 40 MHz then goes thru a loop filter where loop gain/bandwidth is set. o This signal (about 2 MHz bandwidth) is sent to the Gunn Driver. o Available status: Phase lock error voltage. Phase lock status. c. Gunn Driver o Wide band power op amp. o Gunn bias tuning signal (PLL error signal) o Available status: Gunn bias voltage 9. Crossguide coupler o Like a splitter except the signal is reduced by 27 dB in order to: a. Isolate mixer products from RF output. b. Minimizes loss of RF output. 10. Observatory reference o 10 MHz signal used to phase stabilize YIG oscillator and frequency synthesizer and hence Gunn frequency and ultimately the LO output.