The Hewlett Packard Z3801A is a GPS based frequency standard that tracks global positioning satellites to get accurate timing data to adjust the long term frequency of an internal oven oscillator. It was originally used for synchronizing CDMA cellular land network wireless base stations. It provides highly accurate timing. If a satellite signal is lost, the receiver automatically switches to holdover mode, which ensures system synchronization for up to 24 hours with reduced accuracy. You get the best of both worlds -- crystal oven oscillator short term stability, and GPS long term stability.

This type of frequency standard is roughly comparable to a Rubidium standard, without the maintenance issues of the Rubidium lamp. The frequency accuracy is specified at less than one part per billion, on a one day average. This is exceptional for a home work shop and ham station. These units have an HP double oven oscillator, the HP 10811D/E, with a frequency output of 10 MHz.

This is a necessary component in an amateur radio station equipped for weak signal detection at VHF frequencies and above, such as low power EME work. If you aren't listening on exactly the right frequency, you can't pull a weak signal from below the noise level. The 10 MHz output drives an external frequency synthesizer that generates the radio's operating frequency. This frequency standard is also useful as a home work shop reference for test equipment maintenance. I use it to drive a Tektronix 2901A time mark generator that makes handy marker signals up to 500 MHz.

GPS frequency standards are expensive if purchased new. QST had a remarkable article by Brooks Shera, W5OJM on making your own GPS standard using a surplus commercial oven oscillator, Motorola GPS boards, and a custom designed embedded PIC controller.[1] I considered building one of these, but I was able to purchase a ready made HP GPS frequency standard at a cost comparable to the home made version.

These GPS receivers are surplus equipment. I'm not aware of the specific technology change that has placed them into this category. Since CDMA (a digital spread spectrum method pioneered by Qualcomm) requires each cell site to have an individual GPS receiver, perhaps a network upgrade or revision has released these receivers. This suggests that many of these receivers are becoming available.

The HP Z3801A receiver is a basic OEM unit and has only a simple front panel since detailed control is provided using a rear panel RS-422 serial control port. Since your PC uses RS-232 for serial control, a simple RS-422 to RS-232 converter may be built, or the Z3801A must be internally modified for RS-232 use. I chose to use the external RS-422 interface. Most vendors allow return and exchange of a unit that is not performing. Since there is a chance the unit will be returned, I did not want to modify the receiver. Modifying and soldering components on a new receiver prior to acceptance testing just doesn't make sense.

The Z3801A main board has a set of uninstalled headers marked RS-232 and RS-422. These headers are located inside the receiver near the serial connector on the rear panel. The Z3801A photo below shows this location near the rear panel of the receiver. Install jumper headers and plugs or selectively solder jumper wires to convert to RS-232. It is then necessary to turn the board over and remove five Zero ohm jumpers which hard wire selected the original RS-422 interface option. When these surface mount jumpers are removed, the new headers (or solder wires) will select either RS-422 or RS-232 [5]. The center row of pins is common. Jumper from either marked side of the PC board to the center for your selection.

A two page schematic gives the details on a home made power supply and RS-422 to RS-232 cable converter. Three different options for the RS-422 to RS-232 converter are shown. An external power supply (usually 54 volts DC nominal) and external outside GPS antenna are required. HP supplies a program called SatStat that is the front panel and control interface for the receiver. Other software offerings are available, and some offer significant advantages over HP Satstat.

Home made RS-422 to RS-232 Interface

PC Com Port (DB-9M)	Z3801A Serial Port (DB-25F)
2 (Rx)	2 (Tx)
3 (Tx)	3 (Rx)
5 (Gnd)	7 (Gnd)

Suggested serial port settings are 19200, ODD, 7, 1.

The HP Z3801A has these rear panel connectors: • One 25-pin female DB25 connector. This connector provides two 1 PPS time outputs, two 10 MHz frequency outputs,and an RS-422 serial interface port. • 10 MHz output BNC connector. • A remote antenna N-type connector. • Power input connector [2].

The Z3801A Voltage Converter assembly is 4.75" x 7" in size, and is located on top of the main circuit board. Keyed connectors from the main 54 volt DC input, the oven oscillator, and the main circuit board are easily disconnected. After removing five Torx T10 screws, the complete power supply assembly is ready to remove from the receiver cabinet.

The main 54 volt power input has both leads fused with two small Pico fuses. These are small, solder-in fuses that appear like 1/4 watt resistors. F1 is rated at 10 amps and F2 is a 3 amp fuse. These have no visible indication of a blown condition, and must be tested with an ohmmeter.

Two large DC to DC power converter packages are located on this circuit board. One is a Lucent CW025ACL-M that has input of DC 36-75V@.92A with outputs of DC 5V,5A and plus and minus 15 V at .8A each. The other converter package is a Datel UWR-5/4000-D48 that converts 48 VDC to 5V 4A.

Some discrete active power circuitry is located on the board. A LT1270ACT 10 Amp switching regulator at U104 is used for the temperature regulated outer oven supply. The input to this regulator is fused by another small pico fuse F101 rated at 5A. Also, a IRF530 FET switch is used at Q1.

Separate grounds are used in this circuit. The input power ground is isolated from the output DC voltage grounds. The diagram of the Z3801A power supply shows typical operation DC voltages. If the box color is Blue, the reference ground is TP5 Common Ground. If the box color is Red, the reference ground is the minus lead of the input 54 VDC power. A number of voltage test points are available from the component side of the power supply assembly.

Connector P2 to Main Board Pin Wire Color DC Voltage Reading 1 +5RAW Red + 5 2 +5RAW Red + 5 3 DGND Black 0V 4 DGND Black 0V 5 +15RAW Yellow + 15 6 AGND Black 0V 7 -15RAW Blue - 15 8 DOVEN-ON Red 2.1V 9 DOVEN-MON Green + 8.1

Connector P3 to Oven Pin Wire Color DC Voltage Reading 1 Gray 2.5V (from thermistor) 2 Gray Gnd 3 Red + 14V (regulated by U104) 4 Red + 5 (heater return) 5 n/u n/u 6 n/u n/u

Voltage readings are made with all connectors in place and operation stabilized. The outer oven controller as implemented by HP is a standard temperature bridge with the internal oven thermistor as the variable. The circuit consists of U101 (high accuracy +5 volt reference), a bridge unbalance error amplifier op-amp at U102, and a low impedance switch-mode power regulator at U104 (LT1170) acting as the heater power driver for the outer oven. This circuit will stabilize the outer oven temperature, and the control voltages will vary depending upon the specific state of the outer oven control. In particular, the readings at TP-102 (op-amp error output) and TP-104 (switch-mode regulator output to the outer oven heater) will vary. The values listed are nominal readings measured after extended warm-up.

Z3801A HP 10811 Dual Oven Oscillator

J21 Inner Oven Connector Pin Voltage Reading The oscillator and oven sections are internally isolated. 1 0V Agnd 2 +12V for oscillator; ~25 mA 3 0V Agnd 4 -4V oven health monitor (referenced to frame ground) 5 + 12V oven power 6 - 12V oven return   Oven P3 Pin resistance reading between pins, oven disconnected from circuit. 1 to 2 Gray-Gray 110K @ 25C.   3 to 4 Red-Red 18 Ohms

Frequently Asked Questions:

Question - What antenna do I need to make this receiver work? The Motorola antenna (ANT62301A/B) of the time when the Z3801A was built included a preamplifier with 24 dB gain, a noise figure of around 2.5 dB and expected 6-10 dB of cable loss. If you can get the original Motorola antenna, use it. Make sure the antenna you use has a pre-amp that will run from the 5 Volt supply the Z3801A provides on the antenna coax center lead. An outside antenna mounted in the clear with a good view of the sky (preferably in all directions to the horizon) will give you the best results. I have had good success mobile Magellan Mobile GPS antennas. These mobile antennas work perfectly and have an "O" ring seal to keep out the weather. I mounted this external mobile antenna (with internal pre-amp) on a PVC pipe mount that clamps to an outside roof pipe on my home. I used a six-inch square aluminum plate as a ground plane to simulate the vehicle's metal roof. I spliced a length of RG-58/U coax to the smaller cable on the mobile antenna to give me enough length to bring the coax into the work shop. Cable length or coax attenuation isn't too critical. The GPS receiver system expects about 10 dB of loss in the cable. The L1 frequency band used is around 1.5 GHz, so take care if you are using lossy cable or a very long cable length. Try to keep the cable loss between 6 to 10 dB. I used 20-feet of RG-58/U with no problems [3]. With my particular antenna mounting method, the receiver always reports a minimum of at least four to six separate satellites that are actively being tracked.

Question - I have a question about the Magellan GPS antenna mentioned in your article. I am curious about how our weather affects the antenna efficiency. From your antenna photo, I can imagine your antenna having one foot of ice setting on top of it. Please let me know how the weather affects the operation. Like most microwave signals, GPS L1 band at about 1.5 GHz is attenuated by snow, ice, and even birds sitting on the antenna. Using the mobile mount Magellan, I have been able to be pretty successful, even with a couple inches of snow. The holdover mode of the Z3801A will keep a reasonable frequency at the 10 MHz output during periods of signal loss. Professional outside permanent GPS antennas are designed to shed snow and ice and deter birds. They look like inverted funnels with the tip pointed skyward. Motorola offers one called the "Timing2000".

Question - There are six diagnostic LEDs on the main printed circuit board below the power connector. Does anyone know how to interpret them? One of mine is flashing. The six LEDs are: 1. 'Boot code' if ON, Normal if Flashing, 'broken' if OFF 2. EPROM failure 3. CPU failure 4. RAM failure 5. UART failure 6. QSPI, FPGA, interpolator, GPS, power failure In normal operation all should be off except #1 which should be flashing. Above is from page 6-29 of the Symmetricom 55300A User's Guide.

Question - I just got my unit. My receiver appears physically okay, it powers up correctly, the antenna is probably good and is connected, but the Z3801A never reports a GPS Lock on the front panel LED, even after hours of power on time. What's wrong? The Z3801A receiver thinks it is still at the location where power was last turned off. This may have been thousands of miles away. You must initiate a Survey command to the receiver. This instructs the Z3801A to determine its present location. From this point, the unit will probably function properly. The Survey command is issued using the software Satstat. Issue this command to the receiver: :PTIME:GPSYSTEM:POSITION:SURVEY ONCE Hint: after you first hook up your receiver, the GPS location reported to Satstat is the OLD location where the unit was in service. Write these numbers down before the receiver determines its new location. Its interesting to see where your unit came from and its old elevation. My particular unit lived the first four year's of it's life in a cell telephone site in Florida along highway A1A. I determined the receiver's age by the internal component date code markings. The receiver internal memory told me it's exact location when it was last turned off. You can also easily determine how long your new receiver has been powered up. Using Satstat or similar, issue the command  :DIAG:LIFETIME:COUNT? which returns the total time that the GPS Receiver has been active. The value returned by the GPS receiver is in increments of 3 hours. So if you get a number like 1000, this means your receiver has been powered-on for 3000 hours. Mine showed a value of about 3.5 years when I first got it.

Trimble's look-alike GPS receiver Information on the Z3816A GPS Receiver Helpful suggestions made by other successful users  Home made 10 MHz video distribution amplifier by DL8JT View worldwide GPS receiver performance Web-Plots Look inside a HP 10811 Oven Oscillator with G8LMX  Discover Z3801A EFC and DAC circuitry with KO2Y Build Richard Bailey's inexpensive Z3801A battery backup Read details on the Datum System 2000 rubidium GPS   Take a first look at the HP 58540A GPS Receiver Build Steve Smith's simple distribution amplifier Build a simple power supply for your Z3801A Add a TAC-32 software interface to your Z3801A Learn about adjusting the Z3801A crystal oven turning point Distribute accurate time across a network using a Z3801A Add battery backup to keep Z3801A satellite almanac data Correct for Z3801A controller oven confusion! Some Z3801A's sing loudly at you. Here's how to fix it. Build this handy 'sidecar' I/O services interface Build the G4FRE Z3801A digital clock - no PC needed

Notes

1. Brooks Shera, W5OJM "A GPS-Based Frequency Standard", QST, July 1998 .

2. J4 power connector is a three terminal AMP MATE-N-LOCK connector, Mouser Electronics part number is 571-7700181. Two socket pins are also required, p/n 571-7702513.

3 .R. Dean Straw, ed., The ARRL Antenna Book, 18th Ed. (Newington:ARRL, 1997), includes on floppy disk a useful transmission line loss calculator named TL, written by Dean Straw, N6BV.

4. A 48 volt UPS made by Lucent, and new in the box were available from Ebay. They weighed about 50 pounds. These units have perhaps more capacity than normally required, but would provide extended outage protection.