1. 1 5 2 9 8 4 3 7 99 6 10 HARP-B Local Oscillator
Gunn oscillator ( GHz)
Local oscillator ( GHz) 1 5 2 9
Gunn Oscillator
Crossguide Coupler
Level Modulator
x3 Frequency Multiplier
Phase Ref (350 +/- 6 MHz)
Gunn Bias ( VDC)
MRAO PLL
Phase Detector
Gunn (-3 to -25 dB)
Mixer IF plus Bias 8
Gunn (-27 dB) 4 3
Harmonic Mixer
Diplexer
YIG oscillator (8-11 GHz) 7 99 6
YIG plus Mixer IF
Frequency Synthesizer
10 MHz Standard
YIG Oscillator
Observatory Reference 10
bmw April 16, 2002

2. 1. Gunn oscillator
o Generates frequency in the range of 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
o Output power tuning is mechanical (backshort adjustment via
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 /
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
MHz. Instead, set it to 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.
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.