Spectracom 8160A NBS Frequency Standard Receiver overhaul
Top view of the 8160A before adding the acquisition module.
The output frequency after power-up climbs up slowly, but when the acquisition aid circuitry is activated, lock is obtained within 2 minutes.
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Like so many other off-air receivers the loop bandwidth of the 8160A is way too wide, with poor Allan deviation and feedthrough from the transmitter keying as the result. The 8160A locks quickly to the source, but the downside is a less than desirable performance. Suggested modifications include a reduction of offsets in the phase detector and the loop control amplifier, and a reduction of the loop bandwidth.
As a first and straightforward attempt to improve my 8160A I reduced the loop bandwidth with a factor of 10, and replaced a number of components to tame the offset and drift. The manual with schematics, available on the Internet, came handy. This sums up the changes :
A reduction of the bandwidth that would have a significant impact on the performance will increase the acquisition time beyond acceptable limits. With the modification above the 8160A now needs about half an hour to obtain lock, which at least for my purposes is tolerable. However, should I decide to reduce the loop bandwidth even more, the acquisition time would become a real nuisance. The 8160A employs the method of increased gain before lock is detected to speed up the acquisition, but this remains a poor substitute for a real aquisition aid circuitry, which I eventually decided to add. The acquisition aid circuitry consists of an auxiliary PLL with a wider bandwidth than that of the main PLL of the 8160A, and a frequency difference detector, similar to the experimental design of the acquisition aid circuitry for the True Time 60-TF. The circuitry is powered by +/- 12 V and + 5 V from the main board, and receives the 60 kHz signal from the output of RF amplifier module. A 1 MHz clock is taken from U5C pin 6. The output of the acquisition aid module is connected to test point E2. The Spectracom 8160A features a pair of cast metal frames with holes, perfect for adding custom circuitry, and actually something to value by anyone with a desire to experiment with off-air receivers. The picture on the left shows the acquisition aid circuitry mounted on the right cast metal frame. By pressing the RESET button on the front panel for more than 2 seconds, the acquisition circuitry becomes active and will start to nudge the main PLL into place. It will turn off automatically when the detected frequency difference is within the lock range of the main PLL. Thanks to this circuitry, the lock time is reduced from more than half an hour to about 2 minutes. |