Z3816A Modification

Description: The Z3816A is a GPS disciplined timing source manufactured by HP in the mid-1990s, intended for use in cellular telephone equipment. It provides ten output signals through SMA connectors on the back panel. The first four connectors on the left output a 1 PPS signal, which is based at 0 V. Every second, a 4 V positive-going square pulse is generated into a 50-ohm load (5 V open circuit), with a pulse width of 20 µs. The device also outputs 19.6608 MHz signals, which can be easily divided down to decimal fractions of a second or standard baud rates. The Z3816A is primarily utilized to generate time bases for test equipment, with the 10 MHz signals being particularly useful. However, the 19.6608 MHz signals are not directly applicable. A modification is proposed to convert the 19.6608 MHz outputs into additional 10 MHz outputs while maintaining the square wave outputs from the four connectors.

To implement this modification, the front panel must be removed by loosening or removing the screws and nut on the main power switch. After removing the main board and unplugging the antenna wire, the SMA connector nuts and Torx screws must also be removed. Upon examining the main board, it is noted that several wire jumpers are present, with two located in the area of modification. There is uncertainty regarding whether other Z3816A units will have the same factory rework, necessitating individual inspection to determine circuit wiring for modification. A 19.6608 MHz crystal is identified as the source of the 19.6608 MHz output, which is believed to be locked to a basic 5 MHz OCXO synchronized with GPS satellite signals. The locking mechanism of the 19.6608 MHz frequency remains unclear.

The 74F74 chip processes the 19.6608 MHz signal to generate the 1 PPS signal for the four outputs. The upper left corner of the board is responsible for generating the 10 MHz signals, with key test points indicated for access to these signals. A small coax connector (J5) near the center provides a square wave 10 MHz output, while TP14 is noted as a clean 10 MHz signal source. The 74ACT04 chip distributes versions of the 10 MHz output, and each square wave output (four at 19.6608 MHz and four at 1 PPS) is driven by a 74ACT04 buffer chip. The factory jumper wire assists in distributing the 19.6608 MHz signal, connecting the source to the output buffers and the input for the F74 chip required for 1 PPS output generation. The same circuit configuration appears to be used for both the 19.6608 MHz and 1 PPS square wave outputs.

The Z3816A's design incorporates various components for generating precise timing signals, essential for cellular communication systems. The ability to modify the output frequencies enhances its utility in test equipment applications, allowing for greater flexibility in signal generation. The careful examination of the circuit board and its components is crucial for successful modifications, ensuring that the integrity of the timing signals is maintained while adapting the device for specific operational requirements.The Z3816A is a GPS disiplined timing source that was manufactured by HP in the middle 90`s. It was intended for use in cellular telephone equipment. It provides 10 output signals through sma connectors on the back panel. 1 PPS - from the 4 connectors on the left. This is signal is based at 0 V. Every second a 4 V positive-going square pulse into 50 ohms (5V open circuit), 20 uS wide is generated. The 19. 6608 MHz signals are easily divided down to decimal fractions of a second or into standard baud rates. For my purposes, I planned to use the Z3816A mainly to generate time bases for test equipment. The 10 MHz signals are useful for that purpose, but the 19. 6608 MHz signals are not directly helpful. I decided to see if I could figure out a way to modify the Z3816A to convert the 19. 6608 MHz outputs to additional 10 MHz outputs. I assumed these four connectors would need to remain as square wave outputs, but that is ok. Remove or loosen the front panel. Remove the four screws. Loosen the nut on the main power switch. If this nut is left attached but very loose, the panel can be pulled off of the LED`s and rotated. If the power switch nut is removed, the front panel can be completely removed. Remove the main board. Unplug the antenna wire from the main board. Remove all the nuts on the sma connectors on the back panel. Remove all the torx screws holding the main board and lift it out. When looking at the main board, I found that there are a few wire jumpers applied to the board. As we will see later, two of these jumpers are right in the area that I ended up modifying. I don`t know if most Z3816`s will have this same factory rework. If you want to apply this modification to another unit that does not have the same factory rework, you may have to do your own snooping to see how your circuits are wired and adjust the modification.

Some of the results of sniffing around with a scope and continuity tracer are shown in the labels. In the bottom-right you can see that there is a 19. 6608 MHz crystal. This is apparently the source of the 19. 6608 MHz output. I assume that this must be locked somehow to the basic 5 MHz OCXO which is locked via the GPS satellite signals. I never found how this locking of the 19. 6608 MHz might happen though. If anyone knows more, I`d like to hear about it. The (74)F74 chip in the upper right seems to take the 19. 6608 MHz signal and gate (or otherwise generate) the 1 PPS signal that drives the four 1 PPS outputs.

It can be seen that there are two factory jumper wires in the area of this chip. The 10 MHz signals seem to be generated in the upper left corner of the picture. The labels show two of the key test points that provide 10 MHz signals. The small coax connector (J5) near the top center provides a square wave 10 MHz, but the TP14 seems to be one of the earliest and cleanest 10 MHz signals. The 14-pin chip to the left of J5 is a 74ACT04 that distributes versions of 10 MHz. There is a resistor pack (labeled Output R-Pack in the image) for each of the square wave outputs of 19.

6608 and 1 PPS. More details on these circuits later. Each of the eight square wave outputs ( 4 - 19. 6608 MHz and 4 - 1 PPS) has a 74ACT04 buffer chip as seen across the top of the image. Each of these chips provides the drive for one of the square wave output signals. More on these circuits is described later. The factory jumper wire, shown slightly to the left of center in the image, turns out to be part of the distribution of the 19. 6608 MHz signal. The left side of the wire is connected to the source of 19 MHz. The right side of the wire connects to a net that feeds each of the 19. 6608 MHz output buffers, but also feeds the input to the F74 chip that is needed for 1 PPS output generation.

The same general circuit seems to be used for each of the square wave outputs of 19. 6608 MHz and 1 PPS. A single 74ACT04 chip drives each of the outputs. The R-pack chips