Note: This page is intended to document only how I put my system together and a few other miscellaneous items. It should go without saying that there are probably better ways to do these things, that your mileage may vary, blah blah blah...
A while ago, I bought a used HP Z3801A GPS receiver with the intent of providing a handy time/frequency reference. As with most projects, this one is ongoing, with incremental progress dependent on available time and current whim. Nevertheless, it didn't take too long to get it up and running, following the "make it work, then make it look good..." mantra.
Click here for the plots from my receiver, on a web page created by GPSCon. Look at the bottom of this page for recent news/info on the receiver's operation - or lack of it. For a look at the receiver status of others, go to the Web Plots page at the K8CU website.
Uses of a
frequency/time
standard:
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On the status screen of both the HP SatSat program as well as GPSCon (see below) a signal strength indication is given. All the Z3801 manual really says is that this value should be above 20-25 for reliable tracking, and it should be higher than this for best accuracy. In digging around the Motorola Oncore GPS documentation (the receiver used in the Z3801) I determined that this is an indication of "Signal to Noise Density ratio" (S/No) and is shown in db. What does this mean, then? This explanation requires a bit of communications theory - so I'll keep the explanation as simple as practical: Take a relatively weak CW carrier as an example. If your receiver has a bandpass filter 1 KHz wide, let's say that the strength of the that CW signal is the same as total energy in the noise everywhere else in that 1 KHz bandpass. In this case, one has a 0 db Signal to Noise ratio, for the noise and signal are of the same total strength. It would make sense, then, if you were to cut your bandpass filter down to 100 Hz bandwidth, you'd still be intercepting all of your original CW signal, but you'd be intercepting only 1/10th as much noise. In this case, the CW signal is now 10 db stronger than the noise. If you take it the other way, let's say that you widen the bandwidth to 10000 Hz (10 KHz) - the signal/noise ratio is now -10 db. The GPS signal consists of several different signals, all modulated atop the same carrier. If all you want to do is recover the GPS receiver's carrier (but no information) you could (theoretically) use an extremely narrow bandwidth to detect the carrier. For receiving the data, you need wider bandwidth - a bandwidth proportional to the data rate. So, to keep lock on the GPS signal, a C/No ratio of 20-25 db is the minimum required. This represents a pretty "ratty" signal, however, and the amount of noise present makes it difficult to recover the data stream and timing with great accuracy. As you might expect, increasing the the C/No will reduce the amount of noise - and improve the accuracy of the recovered timing. How much signal is "enough?" As it turns out, by the time one gets up to a C/No of 45-50 db, enough of the uncertainty (noise) is gone and relatively little improvement may be had (comparatively speaking) by a further increase - at least for the "typical" L1-only (non-differential) GPS receiver. As it turns out, most receivers don't go through a lot of pains to obtain really good C/No values, so it is more reasonable to take a "sliding average" to get a picture of how well a particular satellite is being received. (I'm a bit skeptical about the receiver's reporting, though: I have a bit of difficulty believing that this parameter is, in fact, accurate when it reports a C/No of >200 db...) An interesting article that appeared in GPSWorld that describes C/No as well as effects of interference on GPS systems may be found here. |
Why does one need a frequency/time standard?
The receiver:
The receiver itself needs several things to operate:
The "PBJ" (Peanut Butter Jar) antenna:
Generally, an amplified (powered) antenna is employed with a GPS
receiver
such as this with the receiver (usually) located at some convenient
distance
- say, in the ham shack. Owing to the very high frequency of the
L1 GPS frequency (centered on 1575.42 MHz) typical coaxial cabling has
high losses. Because these losses directly contribute to receiver
noise figure (and thus, overall sensitivity) it doesn't take very much
coax loss before the GPS signal from an unamplified antenna disappears
into the thermal noise. Having an antenna with a built-in
preamplifier
allows the receive system to tolerate significant cable losses without
degradation of the actual signal - a factor that also translates to
being
able to use inexpensive, smaller-diameter coaxial cable.
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While a wide variety of amplified antennas are commercially available, I decided to "roll my own" GPS antenna. Starting from the article "An Inexpensive External GPS Antenna" (QST, October 2002, Pp. 36-39) I constructed a turnstile antenna, but I departed from the article on several points:
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The upper picture shows how the two turnstile conductors (comprising 4 elements, actually...) are mounted atop the short length of UT-141 coax. While not obvious from the picture, the bottom end of the UT-141 coax goes through the circuit-board disk with the shield soldered on both sides of the board.
On the bottom side of the board is a simple preamplifier (see the next picture) constructed using a Mini-Circuits MAR-6 MMIC (see the schematic, below.) This amplifier has a moderately low noise figure (3-4 db) and reasonable gain (13-15 db or so) at this frequency - enough to overcome reasonable coax losses. This amplifier is powered from the 5 volts superimposed on the coaxial cable: The DC is first decoupled with a coil with the RF being blocked by a chip capacitor, then the DC is bypassed with an electrolytic capacitor. Finally, DC is re-injected into the circuit via another choke. Note that there is no blocking capacitor in the input of the MMIC, so a small amount of DC (about 1.5 volts) is actually present on the antenna - but this isn't any sort of problem.
The output of the amplifier is fed to another length of UT-141 coax which is soldered directly to an "N" connector (refer again to the top picture.) This piece of coax is used to support the antenna and its circuit board, and the entire assembly is "captured" by the glass jar. If preferred, one could use a BNC or even an "F" type connector (if one used RG-6 coax, for example) but in any case, one must assure that the connector is adequately waterproofed.
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The third picture shows the PBJ antenna installed on the roof. In this installation, the coax (about 60' of RG-11) is fed up through the center of the pipe to which another antenna is mounted. The antenna is secured partly by the weight of the coax pulling down, and partly by the black nylon wire ties which tie to a mast clamp below the GPS antenna. It is worth mentioning that while the antenna mast to the right of the PBJ antenna (due south) somewhat blocks the view to the south, it doesn't appear to have any significant effect on the received signals.
As may be seen from the top picture, some holes were drilled in the "lid" (on the bottom) to allow ventilation. After the picture was taken, small pieces of nylon window screening were glued into place (to help keep out insects) and all of the pieces were sprayed with clear lacquer to protect against the possible effects of moisture and condensation. In this case, ventilation is important: When placing a piece of equipment outside, one must either hermetically seal it (usually not practical) or one must provide ventilation to avoid accumulation of moisture due to repeated condensation. A thin layer of very light oil or grease was smeared over the metal lid's threads to prevent any accumulation of rust from "seizing" the lid onto the jar making it difficult or impossible to remove the lid. Finally, electrical tape was applied at the joint between the lid and the jar itself to prevent accumulation of moisture in the lid's threads.
How well does it work? Surprisingly well. The length of RG-11 (I could have used RG-8 type but I had the length of used RG-11 laying around - and the impedance mismatch is, in this application, inconsequential...) offers about 6 db or so of loss at this frequency. Even so, the signal strength peaks at about 180-190 for satellites that are between 60 and 90 degrees elevation and at 50-60 for those satellites that have gained enough elevation to clear ground effects and clutter. RG-6 TV-type coaxial cable will also work well - provided you can somehow put the appropriate connectors on it...
Some eyebrows may be raised concerning the use of the MAR-6 as the
amplifier.
True, there are more modern, lower-noise MMICs around - and if you have
one, by all means, use it! The MAR-6 (also known as the MSA-0685)
is relatively noisy at this frequency - a noise figure of 3-4 db or so
- but because the current generation of GPS satellites have such strong
signals, a full-sized antenna that is in the clear gets plenty of
margin.
Also, owing partly to aperture, a full-sized turnstile antenna such as
this is likely to intercept a little more signal than a
dielectrically-loaded
(i.e. smaller) patch antenna - something that can help make up a bit of
lost signal.
In looking at the track map and statistics, the tracking ability of
the receiver/antenna system appears to be limited only by horizon
visibility.
To the southeast and west/northwest, visibility is somewhat limited by
some tall trees - but in other directions (due south, due east,
southwest)
only mountains (elevations of <7 degrees, typically) limit the
view.
I have seen the GPS receiver track satellites as low as 4 degrees - but
this is rare because there are usually enough satellites at higher
elevations
and the receiver rarely has to try to track them...
Comparison of the
homebrew
antenna with a commercially-made antenna:
For several weeks (ending 3/10/04) I used a commercially-made GPS antenna instead of the homebrew. The antenna used was a Magellan OEM-type of antenna (the round, white one.) The results were observed to be as follows:
Putting this antenna
on the roof:
The best location for any GPS antenna is one with a clear view of the eastern, western and southern horizons (or northern horizon, if you are down 'unda, or both if you are nearing the equator.) In North America and Europe, for example, the inclination of the GPS constellation causes the minimum elevation to be fairly high. In most cases, therefore, the preferred location for mounting the antenna is on the roof.
What are the mounting options? A few come to mind:
Unfortunately, none of these are viable options on my roof::
These ballast mounts are also available for the smaller direct-to-home satellite dishes - although one may have to mail order it, as they aren't always found at local sources. For the GPS antenna, one of these types of mounts - along with the requisite number of cement blocks - is overkill and you are likely to lose your shingles before the mount blows away...
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You could also make your own mount from a metal plate sized to accommodate two side-by-side cement blocks. Using angle brackets and a piece of pipe, one can make a suitable mount for small antennas. One would also use one or two pieces of metal as a "strut" not only to stabilize the pipe but to make the pipe sit vertical to accommodate the pitch of your roof. Under this plate, one would put the protective mat, and atop the plate, the two cement blocks: Two are recommended as they form a "square" footprint, aiding in mechanical stability.
A few comments:
Needless to say, these are suggestions: It is up to you to do the appropriate research and take appropriate safety measures!
The power supply:
I recently replaced the simple voltage-doubling unregulated transformer-type supply with a cast-off "48 volt" 2 amp switching-type supply from a telephone switch. A simple modification was made to raise its voltage from 50 to 56 volts (the addition of a parallel resistor which was done to keep the Z3801 happy.) The supply was mounted in a metal box and "brute-force" L/C EMI/RFI filtering was added on both the input and output to keep its noise out of my receivers. I am still evaluating the efficacy of this supply as compared to the old one. (It does run cooler...)
There is one "gotcha" about using switching supplies: The Z3801A itself has a switching-type supply - and like typical switching supplies, it has its own start-up issues:
The solution to this problem? Actually, the fix is a very old one: Delayed start. If you power up the supply - then connect it to the GPS receiver - it may do just fine. This "delay" may be done with an old-fashioned relay and delay circuit, or one could take a solid-state approach and use a transistor (bipolar or MOSFET) switch.
Power supply RFI:
As mentioned before, the Z3801 uses a switching supply to generate its internal voltages. Actually, it uses several...
One of my "sub-hobbies" is LF/VLF listening. With winter arriving - and the summer static subsiding - I decided to test my LF receive setup once again and was greeted with a noise floor about 30 db higher than I was expecting.
Presuming that the antenna itself was having some problems, I checked further and no, there wasn't a problem with the antenna at all - but that it was an external noise source. It was now time to start shutting things down and/or disconnecting things.
The "Ah Ha!" moment came when I disconnected the antenna from the GPS receiver and the noise dropped 15-20 db. Shutting the receiver off (something that I hate to do...) returned the LF receive antenna system to its expected performance levels...
The Fix:
At this point, I knew what I had to do: Add a power supply filter in the Z3801.
Removing the top cover, the obvious place to mount it was against the back panel - next to the power connector. I carefully cut a piece of circuit board material to the maximum size that would fit in the space and mounted it with two screws. (Note: When drilling holes in the Z3801 case, turn the receiver upside-down so that all filings will not fall onto the circuit board. Then, before righting it again, de-burr the holes with a larger drill bit and vacuum all the filings from the tabletop and the lip of the chassis. Failure to do this will likely result in hair-pulling intermittents or overall failure of the receiver!)
Then, the power connector was removed from the chassis and disconnected from the input on the power supply board. I then carefully "un-crimped" the "board-end" of the internal power cable and soldered longer wires to it - although I could have simply cut and spliced. Doing this gave me a long enough run of wire to go from the power supply input to the (to be added) filter.
Effective filtering at VLF/LF/MF frequencies isn't as straightforward as it might seem - especially when higher currents are involved. Too often, I have seen hams simply slap ferrite beads on QRM sources - only to be disappointed whendoing so wasn't particularly effective. Furthermore, simple ferrite beads and snap-on chokes only have effects above several MHz anyway and will do absolutely nothing at VLF/LF and MF frequencies.
The Filter:
L1 was salvaged from a computer power supply. Typically, these chokes come in two flavors: A bifilar-wound toroidal choke, or one that looks like a small transformer with two distinctly separate windings side-by-side. In either case, both windings are extremely tightly coupled together and any common-mode energy passing through them is suppressed significantly. If you have a means to do so, make certain that this choke has quite high inductance. The choke that I used measured about 1.5 mH (that's Millihenries) per winding and thus has enough inductance to offer significant reactance at LF frequencies (nearly 100 ohms at 10 KHz.) If you don't care about frequencies that low, then a choke with lower inductance may be used. Remember: We are trying to keep LF energy from the switching supply inside the GPS receiver from being conducted onto the power cable!
As mentioned before, capacitors C3-C7 re-assert common-mode signals and ground them out. By this time, LF energy is mostly quashed by L1 and is then routed to "ground." Both electrolytic and ceramic capacitors are used to help guarantee that both low and high frequencies are bypassed.
L2 can be either a choke similar to that of L1, or it may be two smaller inductors. Their primary purpose is to keep HF out of the receiver and thus, the inductance of each winding (or inductor) may be in the 10-100 uH range (or higher, if you wish.) Finally, C8 is used to make any energy entering (or exiting, for that matter) the GPS receiver's power supply common-mode. Resist the temptation to connect a capacitor to ground at this point because any RF energy entering on the power supply lead will find it as a low-impedance path to the chassis: Let L2 do its job and choke it out, first!
Finally, make certain that all capacitors that you use are rated for the voltage: Ratings of at least 100 VDC are recommended on all capacitors. If you don't see a voltage rating plainly printed on the component, don't use it!
Results:
The results of adding the filter were good: No detectable
noise
on LF, MF, or HF emanates from the GPS reciever anymore.
The RS-422 interface:
One popular scheme is to utilize the Z3801's built-in RS-232 circuitry. Although installed, it is not connected by default - a process that takes some minor surgery to complete. Because this procedure may be found on the K8CU page (see below) it is not detailed here.
There are also numerous schemes to convert RS-422 levels to RS-232 levels - also found on the K8CU page. If you are in a real hurry, however, you can do a faux conversion using just a single (optional) resistor:
Because of the differential nature of this scheme, it is possible to run RS-422 signals for miles - the distance being limited mostly by the high frequency response of the wire pair limiting the data rate. A note here: Even though RS-422 doesn't use the "ground" wire for signaling, per se, one is strongly recommended to limit the swing of the common mode signals on the wire pair to a level that the receivers can handle.
Another important reason for using RS-422 over RS-232 is that '232 has in inherently slow slew rate - that is, it can't swing very fast. While RS-422 is capable of carrying signaling rates of several megabaud (for short distances) this simply isn't the case with RS-232. What this also means is that if you want to use the GPS receiver for absolute timing accuracy with the 1PPS output, you don't want to use the RS-232 output unless you take into account the "slowness" of the RS-232 hardware - and the likelihood that this reference is likely to drift around a bit as thresholds in the RS-232 hardware change with time and temperature. This isn't likely to be too much of a problem for the vast majority of users, as the difference is only likely to be a fraction of a microsecond, anyway...
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Several years ago, I was hiking with a group of hams, and we happened to be using a simplex frequency of 146.52 MHz. I also happened to have my GPS receiver with me (back then, a Magellan GPS-4000 - non-XL version) turned on. I couldn't help but notice that it didn't seem to be working very well at all: It was constantly losing lock on all satellites. When we stopped for a break, I happened to let my HT (a Yeasu FT-530) do it's timed auto power-off: Since our group was all together in one place, there was no need to leave the radio on. I then noticed that my GPS receiver was working normally. Putting two-and-two together, I surmised (and later verified) that the '530 was, in fact, jamming the GPS receiver. Here's how: At 146.52, the FT-530 uses low-side injection for its local oscillator. With an IF of 15.25 MHz, this places the LO at 131.27 MHz - the 12th harmonic of which is 1575.24 MHz - almost exactly on the L1 GPS frequency of 1575.42 MHz. Apparently, when the GPS receiver and the HT are within a few feet of each other, there's enough LO energy present to "jam" the GPS receiver! The cure for this problem? We now use a 147 MHz simplex frequency - which isn't as busy, anyway... |
Use as a time reference:
It would be nice to be able to, say, drive a clock and supply GPS information to a host computer. Being that there is only one serial port on the Z3801, one would have to multiplex both systems in a manner that the Z3801 can support. I'm currently working on a PIC-based controller that will do the following:
Additional projects:
One problem that I need to address is receiver jamming. When I operate 2 meter SSB, for example, the receiver loses lock on all satellites. Why is this? There are several possible explanations:
As it turns out, I answered the question (at least partially) by substituting the homebrew antenna with a commercial one. Because the commercial antenna was unaffected by the presence of a 2 meter transmitter, the "theory" that weak harmonics are the cuplrit was discounted. It could still be that the amplifier and/or the receiver itself are being overloaded by the presence of the local 2 meter signal.
| Status of the GPS receiver at KA7OEI:
- 3/08: Not much to report, really: The "PBJ"
antenna and the Z3801 continue to work. Note that the computer
connected to the Z3801 is not used very much, so the graphs on this web
page are not updated vary often. This change was done as part of the "permanentizing" of the receiver installation. Much work remains to be done... |
What if it is the second situation? A simple bandpass filter stuck inline (a relatively tightly-coupled 1/4 wave cavity filter) would take care of the problem. The "awkardness" here is that I'd have to feed the DC (for the amplifier) around the filter. (I'll probably just build a GaAsFET preamp and avoid the problem...)
What about the last one - weak harmonics of the 2 meter signal? Because I don't think that this is likely to be too serious a problem, I haven't given it much thought. In reality, the occasional nature of the SSB operation - and the fact that the receiver seems to recover nicely from "holdover" (resulting from loss of GPS lock) may make it so that I'll just live with the problem...
Now, I wonder how things will work when I fire up my 23cm transverter???
This page will be updated as time goes on - check back later...
For more info on the Z3801A and other GPS
receivers
used as time/frequency standards, go to K8CU's page at:
http://www.realhamradio.com/GPS_Frequency_Standard.htm
Any comments or questions? Send an email!
This page copyright 2004 and maintained by
Clint
Turner, KA7OEI and yes, I know the background is for the MedFER
beacon...
This page last updated on 20080307