A system for automatic monitoring of a communication signal in the RF or IF spectrum utilizes a superheterodyne receiver technique with a VCO to select and sweep the frequency band of interest. A first memory is used to store one band sweep as a reference for continual comparison with subsequent band sweeps. Any deviation of a subsequent band sweep by more than a predetermined tolerance level produces an alarm signal which causes the band sweep data temporarily stored in one of two buffer memories to be transferred to long-term store while the other buffer memory is switched to its store mode to assume the task of temporarily storing subsequent band sweeps. Each alarm signal alternates the roles of the two buffer memories. In a second embodiment, the roles of the two buffer memories are switched at the end of each heterodyne receiver band sweep for display of successive band sweeps at a faster rate and for automatic long-term store of any band sweep producing an alarm.

A receiver time delay calibration device designed for low cost retrofitting of airborne equipment and more specifically DME equipment. In this system, the output of a VFO is applied to the RF input port of the receiver to be tested. The VFO is swept through a frequency range of the receiver until the receiver provides an output, which occurs at the operating frequency of the receiver, f.sub.t. The receiver's output at f.sub.t is used to stop the sweep of the VFO and maintain it at f.sub.t. The output of the VFO is then pulse modulated and the delay between the RF pulse applied to the input of the receiver and the corresponding video output pulse from the receiver is measured to determine the receiver delay time.

A digitizer circuit for converting analog waveforms displayed upon a CRT screen into digital signals suitable for storage in memory employs a microprocessor to determine, from user-specified parameters, the times at which the output of the CRT vertical amplifier is to be sampled. The user has great flexibility in defining sample resolution and the location of the sample window on the displayed waveform in which the samples are to be taken and is permitted to intensify the sample window on the CRT display before any samples are taken in order to view the sample window in relation to the remainder of a displayed waveform.

This discloses a pattern generator having a programmable product cycle timer in which a pulse train, i.e., the pattern generated can be repeated or switched from a first pulse frequency to a second pulse frequency without the usual transient switching periods between pulses. The invention accomplishes this by providing the generator with a cycle timer using a clock operating in conjunction with a down counter so that at a pre-selected time interval, before the end of the pulse is achieved, a test is made to determine if a required condition needing a different pulse frequency exists. If such a condition does not exist the present pulse frequency is reinitiated so that at count 0 it is repeated without delay. If the required condition does exist loading of the needed pulse frequency is initiated so that upon termination of the presently existing pulse at count 0, the newly selected pulse will be introduced into the product being tested without delay.

This discloses a pattern generator having a programmable product cycle timer in which a pulse train, i.e., the pattern generated, having a time raster measurable to one nanosecond can be repeated or switched from a first pulse frequency to a second pulse frequency without the usual transient switching periods between pulses. The invention accomplishes this by providing the generator with a cycle timer using a ten nanosecond clock operating in conjunction with a ten nanosecond down counter so that a pre-selected time interval, before the end of the pulse is achieved, a test is made to determine if a required condition needing a different pulse frequency exists. If such a condition does not exist the present pulse frequency is reinitiated so that at count 0 it is repeated without delay. If the required condition does exist loading of the needed pulse frequency is initiated so that upon termination of the presently existing pulse at count 0, the newly selected pulse will be introduced into the product being tested without delay. A programmable cycle timer is provided to permit the implementation of pulses which has a time raster that is other than a ten nanosecond multiple.