Controlling a YIG oscillator from a power supply
Created: April 2013.
Used YIG oscillators are available on the internet (e.g. ebay) at reasonable prices. The most common frequency range is 4-8GHz probably because they were cannibalized from HP/Agilent 4GHz sig-gens such as the 8648D. However, the controller board is harder to come by and is often priced beyond amateur means. Moreover a microcontroller board has to be developed for the serial communication with the controller board (see KE5FX's "Notes on Stellex YIG Synthesizer Performance and Control"). Even without the controller board, the free-running YIG oscillator is still useful in applications that do not require frequency stability. The oscillation frequency can be controlled using a constant current source, such as a DC power supply set to the current limiting mode. Controlling the YIG oscillator with a power supply is definitely not novel and is actually quite trivial to those familiar with YIG oscillators, but may be useful information to those who had thought the controller board is mandatory for operation. Additionally, the value of this work lies in documenting the HP/Agilent 5087-7084 YIG oscillator's characteristics for the public domain because its specifications are not published by Agilent.
Figuring out the pinouts of an unmarked YIG oscillator
Where the YIG's pinout is unknown, the two coils can be differentiated by their DC resistances - for example, the 5087-7084 YIG's main coil measures ~10R and the FM coil is ~2.5R. (The expected range of resistance for these two coils in other YIG oscillator models can be found here: B. Kaa, "A simple approach to YIG oscillators", VHF Communications, vol. 4/2004). The electronic section consists of +15V and -5V supplies. For an unmarked YIG oscillator, the two supplies connections can usually be identified using a multimeter in the 'diode' mode. As the YIG's positive supply terminal, e.g. +15V, is internally protected by a reverse connected shunt diode, it will read -0.5V on the meter. The -5V supply terminal has reading around -0.7V.
Fig. 1 The connections to this barrel-shaped HP/Agilent 5087-7084 YIG can be easily figured out from the labels silk-screened on the attached PCB.
Fig 2 An eBay offering, this barrel-shaped HP/Agilent YIG comes without the interface PCB. Some detective work is needed to figure out the connections. The earth connection is the easiest to figure out because the pin is directly connected to the chassis.
Fig. 3: Two differently shaped YIG oscillators covering the 4-8GHz range. The barrel-shaped Agilent 5087-7084 on the left has a 5x2 DIL connector, whereas the box-shaped Microsource MC00407-217-02 on the right has a 8pin SIL connector.
The oscillation frequency is varied through the current supply to the main coil, e.g. between pins M+ and M-. The FM coil can be left unconnected in this application. The results below are obtained from an Agilent 5087-7084 YIG oscillator with a specified 4-8 GHz range.
Fig. 4: The almost straight line relationship between frequency and current simplifies interpolation
Given that the output power at 8.0GHz is higher than at 7.4GHz, it is probable that the actual upper frequency limit lies far above the specified 8.0GHz. It will be interesting to find out how much higher the frequency can go beyond 8GHz before the power output is significantly reduced.
Fig. 5: As the amplitude varies as much as 4.9dB over the specified 4-8GHz frequency range, some form of amplitude stabilization will be required for wide-band sweeping. However, the amplitude variation is a tolerable 0.2dB if the frequency range is restricted to 5.2 - 8.0 GHz.
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