23 replies to this topic

[Mr.RC-Cam]

There has been reports of GPS reception problems when a 900MHz A/V Tx is installed nearby. To help understand the issue, I horse traded (thanks Gary) for a Racewood 900Mhz 500mW Tx and measured it on a Tektronix Spectrum Analyzer (SA). Here is what I found:

Second and Third Harmonic Test
Test setup: No video applied (unmodulated RF carrier mode). Direct connection, no coax. External 6dB attenuator on SA for protection. The dBm value is normalized to account for the attenuator.

CH1: 910Mhz, +27dBm (500mW).
Second harmonic at 1820Mhz, -3dBm (0.5mW)
Third harmonic at 2730Mhz, +7dBm (5mW)

CH2: 980Mhz, +28dBm (630mW).
Second harmonic at 1960Mhz, +2dBm (2mW)
Third harmonic at 2940Mhz, +8dBm (6mW)

CH3: 1010Mhz, +28dBm (630mW).
Second harmonic at 2020Mhz, -2dBm (1mW)
Third harmonic at 3030Mhz, +8dBm (6mW)

CH4: 1040Mhz, +27dBm (500mW).
Second harmonic at 2080Mhz, +2dBm (2mW)
Third harmonic at 3120Mhz, +2dBm (2mW)

The photos below show the fundamental and second harmonics for CH1.

Attached Thumbnails

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[Mr.RC-Cam]

Spurious Noise Test, Factory Supplied Whip (see photo below).
Test Setup: No video applied (unmodulated RF carrier mode). Hyperlink 3dBi rubber duck antenna on SA, stock whip on Tx. One foot separation. Tx held in metal vise (best I could do for a counterpoise). The dB values are relative to the fundamental frequency's peak amplitude.

CH1:
Spur at 328Mhz, -50dB
Second harmonic at 1820Mhz, -35dB
Third harmonic at 2730Mhz, -28dB
Forth harmonic at 3640Mhz, -50dB
harmonic at approx 4388Mhz, -65dB

CH2:
Spur at 435Mhz, -45dB
Second harmonic at 1960Mhz, -50dB
Third harmonic at 2940Mhz, -38dB
Forth harmonic at 3920Mhz, -55dB

CH3:
Spur at 480Mhz, -50dB
Spur at 507Mhz, -35dB
*Spur at 1517Mhz, -38dB
Second harmonic at 2020Mhz, -42dB
Spur at 2526Mhz, -45dB
Third harmonic at 3030Mhz, -36dB
Spur at 3540Mhz, -55dB
Forth harmonic at 4040Mhz, -60dB

CH4:
Spur at 523Mhz, -28dB
*Spur at 1560Mhz, -35dB, see photo below
Second harmonic at 2080Mhz, -40dB
Spur at 2600Mhz, -32dB
Third harmonic at 3120Mhz, -35dB
Spur at 3645Mhz, -48dB
Forth harmonic at 4160Mhz, -75dB

Attached Thumbnails

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[Mr.RC-Cam]

Spurious Noise Test, Hyperlink 3dBi Whip.
Test Setup: No video applied (unmodulated RF carrier mode). Hyperlink 3dBi antenna on SA and Tx. One foot separation. Tx held in metal vise. The dB values are relative to the fundamental frequency's peak amplitude.

CH1:
Spur at 332Mhz, -40dB
Spur at 1698Mhz, -75dB
Second harmonic at 1820Mhz, -32dB
Third harmonic at 2730Mhz, -16dB
Forth harmonic at 3640Mhz, -50dB
Fifth harmonic at 4550Mhz, -70dB

CH2:
Spur at 490Mhz, -35dB
*Spur at 1470Mhz, -50dB
Second harmonic at 1960Mhz, -36dB
Spur at 2453Mhz, -32dB
Third harmonic at 2940Mhz, -16dB
Spur at 3432Mhz, -45dB
Forth harmonic at 3920Mhz, -50dB

CH3:
Spur at 300Mhz, -75dB
Spur at 480Mhz, -50dB
Second harmonic at 2020Mhz, -32dB
Third harmonic at 3030Mhz, -16dB
Spur at 3680Mhz, -70dB

CH4:
*Spur at 1560Mhz, -75dB
Second harmonic at 2080Mhz, -28dB
Third harmonic at 3120Mhz, -16dB
Fourth harmonic at 4160Mhz, -70dB

[Mr.RC-Cam]

The data marked with a * are the ones that would definitely explain some GPS problems. There could be some more spurs in the SA's noise floor that I cannot readily see as well. It is hard to say, but the strong second harmonic may affect some GPS Rx's too.

There is definitely a need for a low pass filter on the Tx's RF port. I prototyped a 7-pole using monolithic components and it was working well. But during testing it was damaged. So, I need to build another and try again. It's simply a matter of finding the spare time to do it. Photo of the prototype low pass filter is shown below (installed on Tx).

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[headhunter23]

So stick with 1 and 3 if your using onboard gps?

[Mr.RC-Cam]

Not that this is new information to many of you, but the stock whip antenna is not a good choice since it has low efficiency on the 900Mhz/500mW Tx. Plus, there have been reports that it makes the GPS problem worse. In addition, it causes a bit more Tx heat due to the burden on the internal LM7808 VReg. For better results, I recommend using a decent quality 2 or 3 dBi after-market antenna on the Tx. Or build something sane.

But if you must use the stock antenna, then you can hack it for an optimized 1/2 wavelength as shown in the photo below. Be sure to observe the length dimension, which includes the SMA connector. If yours is too long, then you can cut it shorter. If too short then replace the whip section. You can use a heatgun and the plastic sleeve at the eyelet can be released. Inside you will find that they crimped some cat whisker wire to the eyelet. Just cut and drill it out and install some piano wire (use the largest diameter practical) in its place. You can dress it up with heatshrink and no one will know it was customized. Lastly, it does not work as well if the articulation joint is bent, so keep it straight.

In case you might ask, the answer is Yes; I've tested the modified antenna on the spectrum analyzer. That is how the length was optimized. Tx case temperature was reduced (now about the same heat level as the L-Com/Hyperlink HG903RD +3dBi rubber duck). But keep in mind that the stock antenna is a simple monopole and needs a decent size counterpoise, which is something that the video Tx's tiny metal case does not provide effectively. So, I recommend upgrading to a dipole design like the HG903RD.
.

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Edited by Mr.RC-Cam, 03 March 2009 - 06:03 PM.
Clarified mod.

[Mr.RC-Cam]

So stick with 1 and 3 if your using onboard gps?

Because of the variables, probably the best thing to do is to try each of the RF channels. Then find the one (if any) that works best with your GPS. However, in the USA only CH1 (910Mhz) is allowed under the ham rules. So, for compliance a law abiding license holder would be restricted to using only it.

[Old Man Mike]

The data marked with a * are the ones that would definitely explain some GPS problems. There could be some more spurs in the SA's noise floor that I cannot readily see as well. It is hard to say, but the strong second harmonic may affect some GPS Rx's too.

There is definitely a need for a low pass filter on the Tx's RF port. I prototyped a 7-pole using monolithic components and it was working well. But during testing it was damaged. So, I need to build another and try again. It's simply a matter of finding the spare time to do it. Photo of the prototype low pass filter is shown below (installed on Tx).

Thanks for posting the spur data. It appears that the issue (at least for Chan 1 users which is probably most users) is not really spurs but rather a noise floor issue. I assume that your SA's noise floor is too high to measure the noise floor of the 900 Mhz transmitter.

OMM

[Mr.RC-Cam]

It appears that the issue (at least for Chan 1 users which is probably most users) is not really spurs but rather a noise floor issue.

The SA has a dynamic range of -70dBm to +30dBm, so if there is stuff in the mud I would have a hard time seeing it.

From looking at the harmonic data, the obvious issue is that the Tx does not have a low pass filter on the RF output. It is something that the design should have included (shame on them). Additional mfg cost would have been minimal, perhaps a few cents.

There's also evidence of significant conducted noise through the Tx's A/V cable and beyond. In other words, the GPS issue isn't all radiated noise, it also includes the content from the conducted paths as well. For example, I am currently testing a inline GPS cable filter for the common mode noise that has made its way to the GPS's wiring. It has allowed some beta users to reliably use their GPS. Gary Evans talks about his success here: http://www.rcgroups....ad.php?t=931202

[Mr.RC-Cam]

I repaired the prototype low pass filter. I was worried that it was electrically damaged by bad antenna connection etiquette, but it was more basic than that. It seems every time I screwed and unscrewed it from the Tx (which I did many times), the SMA's middle pin would unexpectedly torque over. That eventually broke a component (but the damage was not visible).

So far, I'm excited by the results. The filter might actually help reduce some GPS interference issues. But, I'll reserve that comment a bit until it is tested in the field.

At 910MHz, its insertion loss is ~0.7dB, which is not bad. It attenuates the second harmonic about -27dB and also reduces noise in the GPS band (1.5GHz). BTW, at 1040MHz, insertion loss is ~0.9dB and the second harmonic is reduced by -30dB. For all RF channels, the third harmonic is now -60dB below the fundamental, whereas without the filter it is obnoxiously strong.

I expect that if I move the crude construction over to a custom PCB, the performance will significantly improve and yield much steeper out-of-band attenuation. So, the next step is to do that. I should have new data to share in 3-4 weeks. Then it's off to testing (I'll invite a couple of the existing beta EMI filter users to try it).

[elossam]

I am currently testing a inline GPS cable filter for the common mode noise that has made its way to the GPS's wiring. It has allowed some beta users to reliably use their GPS. Gary Evans talks about his success here: http://www.rcgroups....ad.php?t=931202

Will be this product available? when?

[Mr.RC-Cam]

If you are interested in the cable filter then just "PM" me via the forum. Please keep in mind that this special GPS host cable filter is optimized for the EB-85A (FV-M8) GPS module. It is intended for installations that have already tried the usual solutions to cure the 900Mhz interference problem.

The Filter is still under evaluation. So far, it has done a great job of getting desperate 900Mhz wireless video users back into the air with improved GPS performance.

[Old Man Mike]

The data marked with a * are the ones that would definitely explain some GPS problems. There could be some more spurs in the SA's noise floor that I cannot readily see as well. It is hard to say, but the strong second harmonic may affect some GPS Rx's too.

There is definitely a need for a low pass filter on the Tx's RF port. I prototyped a 7-pole using monolithic components and it was working well. But during testing it was damaged. So, I need to build another and try again. It's simply a matter of finding the spare time to do it. Photo of the prototype low pass filter is shown below (installed on Tx).

A new solution to improving the Range Video Antenna issue has been posted here:

http://www.rcgroups....68#post10721267

OMM

[Mr.RC-Cam]

That looks like a nice antenna modification. Thanks for the link.

Your improved GPS operation with your notch filter offers good evidence for my low pass RF filter project. I think it will be just the ticket to help solve some of the 900Mhz RFI issues for a wide variety of antennas. Custom filter PCB's are on their way and my fingers are crossed!

[Old Man Mike]

That looks like a nice antenna modification. Thanks for the link.

Your improved GPS operation with your notch filter offers good evidence for my low pass RF filter project. I think it will be just the ticket to help solve some of the 900Mhz RFI issues for a wide variety of antennas. Custom filter PCB's are on their way and my fingers are crossed!

Thanks,

While the coaxial notch provides substantial improvement with all the 900 Mhz transmit antennas I tried (RV 1/2 wave, Tuned 1/2 wave and Vertical Dipole), my experiments tend to indicate that radiated interefernce is only part of the issue. You would expect that a 25 dB notch would be enough to eliminate all intereference but there is still some . For example, I can tell a difference between the IF GPS with the extra SAW filter vs the ET GPS which does not have a front end filter. Apparently some GPS modules simply desense from the a nearby strong signal and/or have LOs with high noise floor.

It would be interesting to try one of your low pass filters for comparison.

OMM

[Mr.RC-Cam]

my experiments tend to indicate that radiated interefernce is only part of the issue.

I agree. For example, there is some conducted noise that needs attention too. I created a companion common mode filter for the 900Mhz Tx's A/V cable that I think will aid in resolving some of those issues. The PCB for it will arrive with the low pass filter boards.

[Mr.RC-Cam]

Antenna Filter Project:
The PCB for the 900Mhz low pass antenna filter arrived. The bench tests show it works well. Insertion loss at 910 Mhz is about 0.4dB and it provides >-25dB of signal attenuation at the GPS's RF band. The second harmonic is >-55dB below the fundamental. The third harmonic has been suppressed and I can no longer see it on my SA. Some field testing will begin in a few days and hopefully the antenna filter helps those 900 MHz installations that suffer from persistent GPS problems.

Here is a photo of the assembled antenna filter:


Here it is mounted on the 900 MHz Tx:



Early this morning I found that even with the stock wire antenna (which I consider a worst case antenna), the GPS was decently immune to the Tx. I could get the Tx within five inches of the GPS module (I did not try any closer due to the fear of GPS damage) and could not see any decrease in HDOP or satellite count. With the filter removed, the GPS lost all satellites in seconds.

I repeated the test several hours later and the results were quite different. With the same setup, within three minutes I lost most of the satellites with the filter installed. I had to move ten inches away before it could maintain the full satellite fix. With the filter removed, I lost a couple satellites within a minute. I was surprised that the results were so different from this morning. It must be related to satellites that are in view at the time. Or evil spirits.

Same subject, different required solution:
As I mentioned before, not all the interference is coupled through the air. In addition to the coupled noise, some installations seem to experience significant EMI/RFI noise that is conducted through the cables. So, the antenna filter may need some additional help to fully resolve hardship situations.

Toroids are helpful (every installation should include them!), but they are not always the complete fix. The GPS host cable filter that was created a few weeks ago was meant to cure the common mode noise. But, it seems to me that it might be more practical to filter the A/V cable on the 900 MHz Tx instead. So, as an experiment I created a companion Tx cable filter board that has LC filters for power, video, and audio. It is designed to reduce the common mode noise that travels out of the module via its wiring.

I haven't had time to test out the Tx cable filter, but here is a close up photo:


Here it is mounted on the 900MHz/500mW Tx module:

[Mr.RC-Cam]

The antenna filter was used on a 900Mhz video equipped model that suffered from satellite fix problems and it worked very well. That is welcome news. Once I get it back from the test site it will be sent to another fellow that has a model with lingering GPS issues.

I finished the bench tests with the Tx cable filter. Next step is to field test it, along with the antenna filter. To create a more noise-vulnerable installation, I added a microphone on long leads (used a dpcav Tiny-Mic). There's solder pads on the filter board for a mic, which is convenient. Even the mic leads are LC filtered, so the extra wires should not increase the RFI noise {if all goes according to plan}.

Here is the Tx cable filter, with a fancy rubber band holding it on (ran out of duct tape ).

[Werneck]

where can I get that filter?

I am having problem with my system, the TX is interfering on the radio receiver and I lost control. I have crach 2 times.

I have hobby wirelees 900mhz and futaba 6ex 2.4 ghz,

[Mr.RC-Cam]

I am having problem with my system, the TX is interfering on the radio receiver and I lost control. I have crach 2 times.

At this point the various filters are not being sold as upgrade parts. But more refined versions will be available as part of a special 900Mhz A/V system that will debut in a few weeks at dpcav.com.

Maybe old man mike's notch filter solution will help you out: http://www.rcgroups....ad.php?t=937068

Some things I recommend to go along with the antenna filter solution you use:
(1) Change the simple wire whip (monopole) that is provided with the A/V Tx and replace with a good dipole.
(2) Add a toroid to the A/V Tx's cable. Install 1-2 inches from the Tx. Use ten (10!) or more tight wraps of the cable, wound neatly through the bobbin. Do not use a split Toroid, one piece type is mandatory.
(3) Maximize distance between ALL A/V equipment and ALL R/C equipment.
(4) The A/V Tx antenna must be as far away from the R/C Rx as possible.
(5) Do not share R/C power with A/V power.