•
All other input and output connections have low pass filters to keep the
interference outside and limit the frequency response to what is absolutely necessary.
•
All control inputs have 10-ms LP-filter time constants.
•
Each circuit function on the board is specifically designed to be insensitive to
frequencies outside its normal operating range. This is particularly important for
operational amplifiers which have a tendency to demodulate rf-signals and, as a result,
produce erroneous dc output.
Dynamic testing
The different rf-circuit and filter functions of the BPM signal processor
require alignment and dynamic testing during development and in the final production
phase. Conventional rf circuits have special coaxial test connectors. Considering the high
packing density on our printed circuit, this was not a desirable solution. Instead, after
some initial experimenting, we developed a special miniature rf test set. On the board, this
required simply two plated-through holes at a distance of 0.1 inch, with one of them
connected to the ground plane. The coaxial test probes have two corresponding test pins,
consisting of gold plated spring loaded contacts. For use with a network analyzer
(Hewlett Packard HP8753D), a matching calibration set (open, short and 50 ohm load)
permits to establish the correction constants, which take care of the small residual
discontinuity of these connections. The test set is used for measuring impedance and
transfer functions (Fig. 3) for frequencies up to 3 GHz.
Testing the transient response of feedback (agc) and phase locked loops uses
the same type of test connections, but requires a different instrument set-up. Generally
speaking, the method consists of injecting a square wave modulated stimulus, most often
a dc current, in a suitable summing junction of the loop under test. The transient response
is observed with an oscilloscope at another test point and can be optimized by tuning the
different elements of the loop filters or phase correction networks.
A spectrum analyzer (Anritsu MS2601B) is used to measure the signal at different
points of the signal path. It can determine the 1-dB gain compression point and the
saturation limit for every location along the signal processing chain. Spectral purity and
stability of the local oscillator is verified with the spectrum analyzer.
A special test generator has been built to simulate fast pick-up signal pulses
with repetition rates up to 15 MHz. This allows to test the BPM signal processor by
simulating different filling patterns and single bunch mode. The peak amplitude of the
generator is at present limited to 22 V and therefore insufficient to test the upper limit of
the dynamic range in these modes.
An automated test bench for final testing and certification of the BPM signal
processor is at present under development. It uses a programmable cw signal
