Background

A separate, and completely out-of-date, version of the LUT's is burned onto an EPROM in B3's micro. The latter version will be invoked if one runs B3 directly via its micro rather than via the normal VAX commands.

The Gunn oscillator has a tuner and a backshort micrometer, and is followed by a quadrupler which has input and output backshort micrometers and a bias voltage setting. The output is the local oscillator (LO) frequency, which is subsequently mixed with incoming external signals to produce upper and lower sideband responses.

The tuning process proceeds roughly as follows. The Gunn tuner micrometer is set to the position demanded by its LUT and then is adjusted slightly until it achieves a lock. The actual frequency of the lock is not verified by the system as correct. The Gunn output backshort is set to a value given by its LUT, and the output passed to the multiplier (x4, and hence a quadrupler), for which the input and output backshort micrometers and bias voltage are set to LUT values which give the maximum output.

Current LUT's

The present LUT's are shown in the following figures. The continuous lines are the polynomial curves which result from the application of the LUT's, and the symbols indicate either measured best values (circle-symbol) or ranges ("error bars") over which good performance was obtained during manual tuning tests. The LUT's are derived from these measured values, and since there is often more than one value of a backshort position at a given frequency, we have tried to choose the best of the possibilities in such cases. The LUT's are quite satisfactory for the Gunn tuner and the multiplier backshorts and bias voltage, but we are less satisfied with the Gunn backshort. In addition, overall we are concentrating our efforts at improvement as time permits on the extremities of these curves, where LO power is likely to be in short supply. The "blank" forms are intended for any continuing efforts in determining LUT's on site.

• Gunn tuner
• Gunn backshort
• Multiplier input backshort
• Multiplier output backshort
• Multiplier bias voltage
• Blank LO tuning form (Postscript)
• Blank multiplier tuning form (Postscript)

The LUT's in general should have some physical significance; that is, one can generally expect smoothly changing values with frequency, and perhaps more than one parallel curve. In general this is what is seen, and at the extremities, where there are no data, it has been assumed that the fit obtained will continue for a short distance before becoming non-optimal. In the case of the multiplier bias voltage, we have found that a low value is the best, and that the maximum output is still not reached in many cases down to the point at which the interlock is activated (2 volts).

When tuning the receiver manually, one should be aware of the range over which each of the four adjustable micrometers may be set. The present values are given below. Going outside these ranges will drive the micrometer in question into a hard limit, and may burn out the motor if a prolonged attempt to go beyond the appropriate limit is attempted.

Device	Lower limit	Upper limit
Gunn tuner	1.0	98.0
Gunn backshort	1.0	93.0
Multiplier input backshort	1.5	80.5
Multiplier output backshort	1.0	67.5

There are also vestigal mixer backshort LUT's stored in the system. However, since the mixers were replaced with tunerless versions, the hardware and software were retained in case the current spare tuneable mixer ever needs to be pressed into service. The actual values of the vestigal mixer backshorts are set to 20 across the range of the receiver operation.