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Mr.RC-Cam

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About Mr.RC-Cam

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    R/C, FPV, Embedded Programming, Electronic Design.

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  1. Version 2.1 firmware has been released. Get it here: Click Me! V2.1 adds a Mini-Circuits BW-S40W2+ 40dB attenuator to the Attn profile list. This affordable attenuator (~$35 USD) has good accuracy on all FPV RF bands. But more importantly, it extends the DiY RF Meter's maximum allowed power level so it can handle all FVP transmitters (even a high powered vTx). It can be purchased directly from MiniCircuits.com and occasionally one will show up on eBay too.
  2. I expect that simply adding another attenuator will introduce an error that overbears the sensor datasheet's 0.2dB. Instead, for FPV hobby measurements I would accept the 1dB error that a AD8317 may have with the -4dBm input since I think it would be the lesser of two evils. Also, this larger error span conforms to the manufacturer's advertised ±1dB error (over 55 dB range) specification. But as JoostB stated, the data sheet shows the error can be reduced to ±0.2dB by limiting the input level. But to achieve that low error the measurement system cannot default to the data sheet's -22mV/dB typical Slope value. Both Slope and Intercept values would need to be precisely determined for each RF band using a lab quality RF signal source. By the way, the meter project discussed here uses the AD8318 sensor (the AD8317 is supported, but is not used). The AD8318 chip can tolerate a slightly higher input level than the AD8317. For example, the -4dBm signal level from your 5.8G 400mW vTx would have ~0.2dB typical error on a AD8318. So if your home brew meter is using the AD8318 then a high quality 30dB attenuator would be fine to use with the 400mw vTx .
  3. Ok, it's now 2017 and another five years has passed since the last post. And 10 years since this discussion began. Time to reflect on how things have progressed. What can I say; So much has changed since we began talking about multirotors. Even the little things are different -- Back then calling them "drones" was a insult to most R/C hobbyists. And to add a twist to that, Drone Racing is now the official name of a professional sport that's broadcast by ESPN. Reflecting back on the progress / evolution of multirotors can all be summed up in one word: Amazing.
  4. @Joost, Thanks for stopping by! It's great to see that your RF meter project is still active. When I have some spare time I'll have a look at your new firmware release. Please feel free to adapt my VSWR code feature in your meter project.
  5. Joost's Arduino program is simple and a good starting platform for doing your own thing. I came across his work a couple months ago and noticed the last activity was back in 2015. But a few days ago he released a new firmware version, so his project is still alive. Maybe there's something new in it that will help your development.
  6. That is correct, the Arduino software is an adaptation of Joost's (his work is credited in the source code). His full project is published at https://goo.gl/P5wXkM
  7. Well, that about wraps this project up. I think you'll find it to be a good performing meter that can be built at a bargain price. If you build the DiY RF Power Meter then please share your feedback and/or improvements. If you have any technical questions then post them here in the public forum so that others can learn too.
  8. Go ahead and explore the menus to become acquainted with the meter's functions. The features are simple enough that I didn't feel it necessary to write a formal instruction manual. But here's some basic tips: [1] Menu navigation uses five context sensitive keypad buttons (see photo below): 1 = Select / Enter / Change Freq Band & Attn / Reset Min & Max 2 = Left / Previous 3 = Up / Next 4 = Right / Next / Edit Attn Value 5 = Down / Previous [2] You can change the Frequency and Attenuator settings by pressing the Select (Enter) key while in the main RF power mode. This mode is identified by the toggling FREQ: and ATTN: status messages on the LCD's top line. [3] The Attn. Profile menu includes some pre-defined attenuators that are characterized for all five RF bands. The *USER DEFINED* profile uses flashed data values that are defined (by you) in the config.h's SER_ATT_PROFILE[ ] array. Lastly, the Custom profile allows you to manually enter a fixed attenuator value using the Keypad. To edit the Custom entry begin by pressing the Right (Edit Attn Value) Key. This will turn on the custom attenuator edit mode. [4] The main RF power mode uses data averaging. It's the recommended mode for measuring analog RF power (typical FPV vTx). In case you are interested, data is sampled at 200Hz and 200 rolling values are collected for averaging. [5] The Instant Power mode is for Instantaneous (non-averaged) measurements. This is the recommended mode for measuring digitally modulated RF signals (Wi-Fi FPV and long range UHF R/C). Keep in mind that digital signals aren't continuous RF so reliable measurements are difficult to achieve. The display is limited to a 5Hz update rate for legibility. [6] The Min / Max Power measurements are held in a rolling buffer that automatically resets after 10 seconds. Or you can manually reset the buffer at any time by pressing the Enter Key.
  9. The DiY RF Power Meter can measure antenna SWR (VSWR). A RF Directional Coupler is needed to use this feature. Mine is a Krytar 0955-0098 that is rated for 2-8GHz applications. Despite it's 2GHz rated lower end, I get good VSWR results down to the 900MHz band. The VSWR function is simple to use. Here is an example involving a 910MHz FPV antenna. 1. Apply power to your DiY meter. Select the appropriate RF band and ATTN setting for your equipment. Then use the Up/Down keys until you see the VSWR: [SEL] TO START menu. Then Press the Enter (Select) Key to select the VSWR function. 2. Prepare to install the Directional Coupler, vTx (video transmitter), attenuator, and antenna (DUT). For best results do NOT use coax cables to connect them; SMA-SMA coupler adapters are recommended instead. 3. See photo below for Forward measurement setup. When ready, press the Enter Key to perform the measurement. 4. See photo below for Reverse measurement setup. When ready, press the Enter Key to perform the measurement. 5. The SWR measurement results will be automatically calculated and displayed. See photo below for example:
  10. All the missing details will be posted as I find time to do it. No promises, but perhaps in a week or so. Aug-23-2017 Edit: Arduino code link has been posted.
  11. Now that the 900MHz slope value is calibrated we can move on to the RF sensor's Intercept calibration. These instructions are for the 900MHz band. PART 2: Intercept Calibration (900MHz) The items you'll need are as follows: 1 each, 910MHz vTx (100mw to 800mW). 1 each, trusted RF Power Meter (to be used as a Reference Meter). An ImmersionRC RF power meter is used in this example. 1 each, 30dB fixed attenuator. MiniCircuits' VAT-30W2+ is used in this example. 1 each SMA-SMA adapter or very short coax patch cable. 1. Connect the attenuator to the Reference Meter. In this example I used an ImmersionRC Power meter along with its included 5cm long SMA patch cable and 30dB attenuator. 2. Apply power to Reference Meter and configure it for the 900MHz band. Set the Attn value to 29.7dB (nominal value for VAT-30W2+). 3. Connect the 900MHz vTx to the attenuator and apply power. Allow it to warm up for a few minutes. Do not allow the vTx to overheat (use a small fan if necessary). 4. Measure / record the vTx's mW and dBm values. Example shown in photo below. 5. Apply power to your DiY RF meter. Confirm that it is still set to the 900MHz band and Attn value is 29.7dB. 6. Remove power from the vTx and move it (with the attenuator) to your DiY RF meter. Apply Power to vTx and allow it to warm up again. 7. Measure / record the vTx's mW and dBm values. Example shown in photo below. 8. Using your recorded measurements, subtract the Reference Meter's dBm value from the DiY meter's measurement. If the difference value is more than 0.1dBm then the RF sensor's 900MHz Intercept value will need adjustment. This requires editing / reflashing the Arduino code. 9. Calibrating the intercept value begins by opening the config.h file. Find the CAL_INTERCEPT array directly below the AD8318 SECTION BEGINS HERE text region. The six comma separated intercept values are ordered by RF band (433, 900, 1200, 2400, 3300, 5800). The second value is the 900MHz band's intercept data. Identify this value -- it is the one you will edit. 10. If the dBm measurement was too high then decrease the intercept array value by subtracting the difference value from it. If the value is too low then increase the intercept value by adding the difference value. After editing the value, save the file, then flash the Arduino board. 11. Confirm the DiY meter's 900MHz RF power measurement is now within 0.1dBm of the reference meter. Your meter's 900MHz band is now fully calibrated. Repeat the slope and intercept calibration steps for the other RF bands you intend to use.
  12. As mentioned, each RF band that you intend to use will need to be calibrated. In this example we'll calibrate the meter's 900MHz band using a 910MHz FPV video transmitter (vTx). PART 1: Slope Calibration (900MHz) The items you'll need are as follows: 1 each, 910MHz vTx (100mw to 800mW). 2 each, 30dB fixed attenuator. MiniCircuits' VAT-30W2+ is recommended. 1 each SMA-SMA adapter or very short coax patch cable. 1. Apply power to the meter and wait a few seconds for the main measurement screen to appear. 2. Press the Enter key. Use the Up/Down keys and set the meter to the 900MHz band. 3. Press Enter again. Use the Up/Down keys and set the meter's Atten Profile to match your attenuator's value. For example, if using a VAT-30W2+ then select that entry. Otherwise use the Custom entry and set the correct dB value for your attenuator. Press Enter one more time to exit the setting mode. 4. Use the SMA-SMA adapter and connect the 900MHz vTx and one (1) attenuator to the meter. 5. Apply Power to vTx and confirm you see a dBm / mW value. 6. Now use the Up key and select the Relative RF Diff mode. Press the Enter key to zero the value. See the photo below. 7. Remove power from the vTx and install the second attenuator. The second attenuator will be measured and used to calibrate the meter. Reapply vTx power. See photo below: 8. The measured attenuation value will be shown on the meter's display. IMPORTANT: ALL attenuators have frequency dependent tolerances. That is to say, the actual dB value will vary depending on the measured frequency. In this example I am measuring a VAT-30W2+ attenuator and its datasheet shows that typical 900MHz attenuation will be 29.7dB to 29.8dB. The measurement in the photo example (see above) shows the correct value for my "30dB" attenuator. That's because this is a calibrated meter. If the measured value is incorrect then the RF sensor's 900MHz slope value will need adjustment. This requires editing / reflashing the Arduino code. I won't explain how to use the Arduino IDE to edit the code because there are endless YouTube tutorials that will do a better job of teaching you the basics. 9. Calibrating the slope measurement begins by opening the config.h file. Find the CAL_SLOPE array directly below the AD8318 SECTION BEGINS HERE text region. The six comma separated Slope values are ordered by RF band (433, 900, 1200, 2400, 3300, 5800). The second value is the 900MHz band's slope data. Identify this value -- it is the one you will edit. 10. If the displayed measurement is too large then decrease the slope value. If the value is too low then increase the value. The exact amount of the change should be small, perhaps try ±0.0005. After editing the value, save the file, then flash the Arduino board. Repeat these slope calibration steps until the meter accurately measures the attenuator's expected value.
  13. The AD8318 RF sensor's data sheet advises performing a board-level calibration to ensure measurement accuracy. Here is the datasheet, see Device Calibration section: AD8318.pdf Calibrating the meter will ensure accurate RF power measurements. However, if you only intend to use the meter's VSWR function then calibration is NOT necessary. But no doubt you'll want to measure RF power too, so plan on performing the calibration procedure. What does this all mean? Editing the calibration data in the Arduino source code is needed for each RF band you intend to measure. The Arduino config file has two data arrays that holds the sensor's Slope (signal gain) and Intercept (signal offset) calibration information. The two data arrays are found in the config.h file, as follows: slope data array: CAL_SLOPE [ ] intercept data array: CAL_INTERCEPT [ ] Each array has six values that relate to the meter's six supported RF bands. The data values are arranged in this order: 433, 900, 1200, 2400, 3300, 5800. The calibration data began as values determined from the datasheet that were further optimized by using a trusted RF power meter and attenuator. Fortunately I have other RF power measuring equipment that is accurate, so I used it to determine the final calibration data for my DiY meter. You'll need to do a similar meter measurement comparison to find the best calibration values. So plan on hunting down a FPV buddy that has a reliable (calibrated) RF meter and attenuator. Beg/bribe and borrow it. For example, the ImmersionRC's power meter is fine for this task.
  14. Great! I think you'll enjoy building & using the Vid Cal Tool.
  15. Yes, that's how I do it. For serious video work it's important to have a scope with TV-sync capabilities. I also rely on a calibrated test pattern generator during testing to provide the precise 1Vpp video signal. However, the recently published DiY FPV Video Test Cal Tool would be useful solution for FPV'ers on a limited hobby budget. It provides a low cost way of accurately checking an FPV system's video levels. https://www.rc-cam.com/forum/index.php?/topic/4126-diy-fpv-video-calibration-tool-low-cost/
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