HP Z3801 - GPS Disciplined Frequency Standard

This is a kind of cool toy (because it's not like I have a particular use for this kind of accuracy, today, at least).

I got mine at http://www.buylegacy.com/, which is a place that primarily deals in telecom gear (switches, T1, DS3, etc.), but wound up with a pile of these receivers. They also sell an antenna from Magellan (about which more, below) that works with the receiver. The real gold mine of information is at K8CU's web site: http://www.realhamradio.com/, where there's pictures, articles, a bulletin board, etc.

My modification to the receiver for RS232

The K8CU site had a description of how to remove 5 jumpers (zero ohm SMT resistors on the back side) and install 3 rows of headers to switch to RS232. When I pulled the board out of my receiver, I noticed three sets of pads next to the 5 SMT resistors. A few minutes work with the multimeter, and I had confirmed that I didn't need the headers (which was good, because I didn't have them in the junk box), and could just jumper the pads.

Power Supply

The K8CU site had a schematic of a simple transformer, bridge, capacitor power supply. It called out a 36Vrms transformer, which I didn't have, but I did have a 12.6 and a 24, so I wired them in series. A small bridge, 5000 uF in filter and I was done. Except... it didn't work very well. The transformers were rated at only 1 Amp, and were cheap imports, with, as it happens, a LOT of series impedance. Not only did they get hot (with the roughly 1/2 amp load of the receiver), but the output voltage was only about 38VDC. And, when the a/c compressor came on, the line voltage dropped enough to reset the receiver.

I rebuilt the PS using a pair of 22VCT transformers in an old Micom power supply, and now, under load, it puts out 56V.

The antenna

There is a fancy HP active antenna originally used for these receivers with a fairly high gain LNA. I got the inexpensive Magellan antenna offered by buylegacy. It works well enough, but, it has a 20 foot cable on it made of RG174. By the time I added 100 ft of RG58 to get from roof to receiver, I couldn't lock any satellites. I replaced the 100ft cable with a 50 ft cable, and now I can lock to satellites, but, the signal strength is pretty low (20-30 most of the time, when it should be 50+) and I drop lock occasionally.

A bit of browsing shows that RG174 has a loss of 45 dB/100 ft at 1 GHz, and we're operating at 1.5-1.6 GHz, so it could be even worse. That 20 feet is probably on the order of 10-15 dB, which is a really big hit. So... I got out the wire cutters, soldering iron, and heat shrink tubing. BIG difference in signal strength:

Before chopping the cable After chopping the cable (a week later)
Satellite Status __________________
Tracking: 2        Not Tracking: 6 
PRN  El  Az   SS   PRN  El  Az
  3  43 287   27     9  17  87
 21  63 313   39   *14  Acq .
                   *15  Acq .
                   *17  Acq .
                   *18  56  29 
Satellite Status _____
Tracking: 4        Not Tracking: 1 
PRN  El  Az   SS   PRN  El  Az
 11  48 283   83   * 1  13 253
 14  35  48   53 
 20  24 308   78 
 25  70 132  117
 

 

Software

The receiver comes with (and can be downloaded from the web) a program called SATSTAT. I got the receiver up the first time with good old hyperterm. There are a pile of programs that do fancy stuff like logging EFC codes, tracking satellite positions, etc. Being basically cheap, and not wanting all that stuff, I figured I could whack out a quick BASIC program to send the few messages you need and just log the output to a file. I did this, and it works reasonably well, and I will put the programs up here in a few days.

It would be nice to periodically log the tuning signal and to monitor the number and quality of satellites.

 

Choke ring antenna dimensions: http://facility.unavco.org/project_support/permanent/equipment/antennas/ant_cals.html

Acknowledgments

Thanks to Jan Tarsala, WB6VRN, who came into my office at work and offered this up as a suggestion on how to spend a small cash award I got at work.

radio/z3801.htm - 28 September 2002 - Jim Lux
radio home page - (Jim's home page)