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Build an I/O Services Sidecar for the Z3801A

In this article Daniel Nelson describes the design of his I/O Services accessory box that attaches sidecar style to the rear of the Z3801A.




I began my adventure with the Z3801A via an Ebay purchase giving me the box and Magellan puck antenna with free shipping. The purpose of the purchase was to provide a self-calibrating 10MHz clock signal for various test equipment on my bench. From this need, I have developed the Z3801A GPS Interface detailed below. I hope it may help you with your project. Feel free to use any of the information and please contact me with your observations and questions at djn@ieee.org for Daniel Nelson.

My GPS Interface project requirements:

- Do NOT modify the Z3801A - everything MUST be added as a 'sidecar' that bolts onto the existing unit.

- Isolated 10MHz sine wave outputs to simultaneously feed several test instruments on my bench. Spare outputs must be available for future use.

- 120VAC to 48VDC Power supply for the Z3801A

- RS-422 to RS-232 conversion to hook up my computer

- F antenna connector for easy use of commercial cabling

- TTL 1PPS outputs with 50% duty cycle to drive 50 ohms

- TTL 10MHz 50 Ohm square wave output - just in case

The block diagram below shows the circuits to convert the Z3801A outputs to my circuits. After reading the various web pages and sites and looking over the available parts, the following strategy was conceived:

  • An aluminum chassis can be bolted onto the rear of the Z3801A using the existing mounting nuts and bolt holes. Attachment of my unit to the receiver was found to be tough enough without adding any new holes.

  • No metal chassis is available for purchase that is the same size as the rear of the Z3801A. I settled on a 3" tall, 10" wide Bud box. This is slightly shorter and several inches wider than the receiver, however, it is not as wide as the front rack flange and so will allow insertion into a cavity built to accommodate the original unit (depth of the new box allowed for).

  • Power supplies would be needed to run the Z3801A and the required circuitry.
    Z3801A - 54V @600mA max
    + 5 and - 5 @ 250mA for Maxim distribution amps
    + 5 for other circuits
    A Cincon 48V 60W switcher offered by Mouser has enough excess current to supply both the Z3801A and the other needs. So I configured the power system by converting 120VAC to 48VDC (adjustable to 54VDC) and then using DC-DC converters to make the +-5 and +5 supplies from the 48VDC.

  • 10MHz sine wave outputs take advantage of the article by DL8JT "10MHz Video Distribution" by Wolfgang Sauerwein on this site. In this he details use of a Maxim 4135 Evaluation Kit to make 6 independent outputs. A truly wonderful observation was that 1:1 isolation transformers are available on obsolete network cards, along with the isolated BNC connectors. The combination of the Maxim board, transformers and connectors satisfy the isolated distribution needs.

Maxim 4135 Evaluation Kits:

I decided on two of these for a total of 12 outputs. Later, I noted that one would have been enough, but there it was, sitting there, money spent, so I went ahead and put both into the 'sidecar'. These are available for direct purchase from the MAXIMIC.COM website.

My box is exactly 3" tall - outside. The MAXIM boards are exactly 3" tall so they had to be sanded .030" on the top and bottom edges to remove enough to fit inside the chassis. Tedious work with a file, but worked out OK.

The Z3801A has ONE 50 ohm output at 10MHz. In order to have as much amplitude as possible and to feed both the MAXIM units, I decided to modify the MAXIM boards. I removed R7 a 75-ohm surface mount resistor and replaced it with a 1/8 watt 100 ohm resistor (done after photo taken). Two boards in parallel then presents 50 ohms load to the Z3801A +- reflections due to split feeds to the 2 MAXIM boards. Works fine.

I removed the DIP-switch from the MAXIM kit as it ended up in a poor place in my construction. Besides, I intend to run all the outputs all the time and not try to use the DIP-switch at all. After removal, the contacts on the board were jumpered so all six outputs are enabled. The MAXIM Kits require both +5 and -5 supplies. By my calculations, two of the kits will work fine when supplied by a single 300mA source.

I have inserted 2-pin male headers into the MAXIM board for +5, -5 and ground to allow plugging the supply harness into them after assembly. These boards are well bypassed and so the power supply wire lengths inside the 'sidecar' are not at issue.

The MAXIM kits have 1 + 6 SMA connectors for the input and 6 outputs. I was able to purchase about (20) 6" SMA M-M cables on Ebay to use for interconnects between the kits and the output board. See the photo for details. Wiring was straightforward. Each completed group is bolted to the rear of the 'sidecar' using the BNC connector mounting rings.

Output Isolation:

As noted in the DL8JT article, isolated outputs are needed for each of the 10MHz sine wave connections. Cables that connect equipment to the distribution amplifier must not, at the same time, connect the equipment to each other. Coax connections of perhaps several feet can produce different 'ground' potentials across the group.

As suggested, I was able to obtain about 20 old network cards on Ebay for under $25, delivered. From these I scavenged the isolated BNC connectors and the 1:1 isolation transformers. With these each output is isolated from all others and not referenced to ground at the distribution amplifier.



I mounted the isolated BNC connectors 3 in a row to a strip of breadboard, connected the 1:1 transformers and the end of the SMA cables. Two of these breadboards and SMA cables connect to the modified MAXIM kit to comprise a package of 6 isolated outputs.


Chassis Preparation and Stuff !

The first thing was to put the holes into the box. I created a drawing for the rear label and sent it off for manufacture. This label was used to mark the hole centers after sticking it to the box wall. The hole for the power entrance was actually cut with an X-ACTO saw blade and then filed to final size. The other side was drilled for attachment to the Z3801A. Finally, the power supply was mounted on one end.

A single breadboard was created to mount most of the remaining parts. It has both the +5/-5 and +5 DC-DC converters on it, RS-422 to RS-232 and pseudo-ECL converters. I put a number of .1" male headers on the board for connections. Details are below.

For the antenna, a cable with an N connector on it was cut off and then soldered the F jack mounted to the rear. For the computer, a DB-9 female connector was pre-wired bolted to the rear wall. I also pre-wired a DB-25 male connector with the wires for the RS-422 , 1PPS and 10MHz outputs. Wiring was routed to the breadboard card.

I found a led in a chassis mount package with dropping resistor rated for 12 volts. This was wired across the + and - 5V supply. The 120V entrance was wired and grounded at one of the mounting bolts holding the 'sidecar' to the Z3801. All else is on the breadboard.


Breadboard Card - Conversion Circuits:

This card is the core of the project. Here I mounted circuits for power supplies, pseudo-ECL conversion, RS-422 to RS-232 and conditioning for 50 ohm outputs. I will treat each separately. DC-DC CONVERTERS: The Maxim distribution amplifiers, loaded to 50 ohms and driven with 1.8Vpp inputs from the Z3801A are slated to draw a total of 250 mA from both a +5 and -5 volt supply. I picked out a DC-DC converter from Mouser, 418-EC3A34H made by Cincon. This is a 3 watt converter and provides regulated +-5V @ 300mA from a 32-72 VDC supply. Obviously it is tied to the 48V DC supply.

However, I was concerned that the +5 supply would be overloaded when the 1PPS and 10MHz TTL outputs were loaded to 50 ohms. Over this, I have added a second DC-DC converter, also from Mouser, Cincon 418-EC3A31H. Probably one dual voltage DC-DC converter above will work fine, especially if only one Maxim kit with 6 outputs is used. Again, the stuff was purchased, sitting here, so I put it in.


I used the schematic right off this website for this using the Max232 and SN75179BP chips. For clarification, I have shown the again schematic below. Note that ground on the DB-9 connector is power supply ground.


Pseudo-ECL to 10MHz TTL Output:

After studying several articles on the definition of Pseudo-ECL, the following working understanding was reached:

To review: ECL uses a -5.2VDC supply with open emitters that are terminated in maybe 50-100 ohms at the far end of the signal path. Signals were below zero with logic 1 at -.9V and logic 0 at -1.8V.

Pseudo-ECL is the same as the above but uses a +5V supply, so the signal levels are positive with logic 1 at +4.0V and logic 0 at +3.3V approx. What is of interest here is that the Z3801A signals are differential and fed with two wires. As it turns out, these signals are well inside the voltage levels for RS-485 transmissions. The differential swing is high enough to be compatible also. I employed a 75176B RS-485 chip to convert the Z3801A signals to TTL. This is a single chip solution that in addition has the drive ability to drive a 50-ohm output directly. Works great!


520 ohm loading resistors were picked for low power to terminate the incoming differential lines. A 520-ohm resistor loads the 75176B output to kill ringing and maximize signal quality.

1PPS TTL Output:

I used the 75176B RS-485 receiver section as noted above to translate the 1PPS signal to TTL levels. The 1PPS signal is a narrow positive going pulse. So, to provide a 50% duty cycle, I used an NE-555 timer to generate a 50% duty cycle square wave. The NE-555 has enough current capability to drive a 50 ohm load directly.

The NE-555 is triggered by the rising 1PPS pulse and then generates a logic 1 on its output for ½ second. The timer then waits to be triggered on the next 1PPS rising edge. The ½ second time is defined by: t=1.1RC. Be sure the actual parts selected for the RC time constant equal .454. Using a 1M resistor, a .454 uF cap would be needed. Select a combination of resistor and capacitor that is close to this value for a 50% duty cycle.

Final Notes:

I have enclosed a parts list by functional block. I am pleased to report that after assembly, all the circuitry worked fine without adjustments, so I expect that anyone can breadboard up whatever bits of this project that seem appropriate. If you have questions or observations, please feel free to contact me at Daniel Nelson djn@ieee.org. You are free to use this information for your project, pass it on to others.

My Z3801A is up and running great! I placed the antenna on a Radio Shack15-891B eave mount just proud of the center ridge. As recommended in the literature, I also installed a Radio Shack 16-1156 In-Line Amplifier to boost the antenna strength. Currently, I have SS (Signal Strength) numbers reported in the 75 - 175 range and no complaints from the system.

Finally, I purchased the GPSCON software offered by K8CU and really like it. This program runs in the background on my computer logging data and providing me with a visual picture of how everything is working.

Download all drawings as high resolution PDF file


Parts List:

Quan Description

Misc and Chassis
1 In-Line Antenna Amplifier Radio Shack 16-1156
1 Adapter BNC-F to F-M To adapt antenna to amplifier
1 Antenna lead-in 75 Ohm RG-6/U Coax with F connectors. Length as needed.
1 Mount - Antenna - Whatever works for your house!
1 Universal Power Cord
1 DB-9 Serial Cable Connect GPS unit to computer
1 DB-9F Solder Cup
1 DB-25M Solder Cup
1 F-F Bulkhead Connector Solder
1 8" Coax cable N-M one end.
1 Label, GPS Local label maker
1 Box, Aluminum 3 x 5 x 10 Bud AC-404 Mouser 563-AC-404
1 Cover, 5 x 10 Bud BPA-1591 Mouser 563-BPA-1591
1 Plug, 3 pos Amp Mate-N-Lok 1480700 Mouser 571-14807000 For DC to Z3801A
2 Socket Pin, Socket Amp 3506891 Mouser 571-3506891

Isolated Distribution
For (1) 6 output distribution amp
6 6" Coax cable SMA-M one end. Ebay or surplus store
1 6" Coax cable SMA-M to BNC-M Ebay or surplus store
6 Isolated BNC connector similar to BNC-LNRD-BPNA From old NIC cards
6 1/2"-28 Nut Mounts BNC connector to chassis
2 Triple 1:1 10MHz Isolation Transformer LanKorn LP-160C or Bothhand TA100-05B
1 Maxim MAX4135EVKIT-SO Dist Kit - Order from Maxim 1-888-629-4642
12 #2 Lockwasher - used to solder BNC connectors to breadboard
2 .062"x1.1"x2.9" Pad Per Hold Breadboard
4 4-40 x 3/8" Hex spacer
8 4-40 x 1/4" screw

For (2) output distribution amps
2 List of items above
2 100 Ohm 1/8W Carbon Film resistor - Change R7 on the Maxim board
1 BNC Tee


Power Supply
1 48VDC 60W Switcher CinCon CFM60S480 Mouser 418-CFM60S480
1 3 pos housing Molex 09-50-3031 AC Plug

Copyright © 2004 Daniel Nelson

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