FYI: XN297 vade retro satanas

If you already noticed by yourself, or read some last testings of new toyquadcopters, all of them now are suffering of a new disease: a poor control range out of the box. For example: Cheerson CX-10C, WLtoys Q282, V686G (2nd revision), JJRC H11D, H20 et many more. What are their common point ? All of them are using the XN297 RFchip, a new chinese and cheap model in theory compatible with popular nRF24L01 model. This move for the XN297 have been done by most of the toy compagnies 6 months ago approximatively. The former chip used was a Beken model, i.e. the BK2423 perfectly 100% compatible with the nRF24L01.

Here is one of the packaging found for the XN297
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Here is the main tech caracteristics of this chip
http://www.panchip.com/en/products_show.aspx?cid=63&id=333

The most important lines are:

Transmit Power: -11~11dBm
RX Sensitivity: -88dBm@1Mbps -85dBm@2Mbps

If you look datasheet of a BK2423

Transmit Power: -10~5dBm
RX Sensitivity: -91dBm@1Mbps,-84dBm@2Mbps

One thing to know, for most of toyquadcopters, the datarate is fixed in general to 1Mbps (except the last syma protocol fixed to 250Kbps)

What does these numbers mean ?

The RX sensitivity is the lowest energy of the signal which allow to decode correctly the received data packet. Below this value is not possible to keep the radio link. Values are expressed in logarithmic scale but expressed in natural scale, the lowest value is 1/10^(-91/10)= 1258925411 times smaller than the 0dBm reference signal (impressive)

The effect of the Transmit Power (from the transmitter side) permits to increase the RX senstivity indirectly. Here is equation bilan for the receiver side

If we assume than max power is transmitted, we have

XN297: -88dBm – (+11dBm) = -99dBm of new RX sensitivity

Let’s see for the BK2423

BK2423: -91dBm – (5dBm) = -96dBm of new RX sensitivity

So it means here that the XN297 should be even more sensitive than the BK2423 (if max power is set of course).
It’s strange, coz I said exactly the opposit few lines earlier ? What is the trick ?. The trick is the usage of a PA+LNA stage in the TX module (PA for Power Amplier, LNA for Low Noise Amplier)

Look the photo of a nRF24L01 transmitter bundle

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With this configuration, the emitted power of the nRF24L01/BK2423 is set to 0dBm then send the signal to PA+LNA parts.
The final output power is now 20dBm !!!! (100mW). Have a look again to the equation

BK2423 (PA+LNA): -91dBm – (20dBm) = -111dBm of new RX sensitivity

So as you can read you have 111-99 = 12dBm of difference and you have to know that every 6dBm, in theory the distance control is doubled. It means that the range of BK2423 is at least multiply by a factor 4 !!!!! and if we admit that max power have been set to the XN297.

To conclude, I think the main problem of these new toys is the absence of a PA+LNA stage in the transmitter module. Can this one be added ? I don’t know yet…

EDIT: In fact, the MJX X600 have a daughter RF chip including a LNA+PA stage. It explains why this model offers a very good range

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How to improve a bit the radio range. Since the range is divided by a factor 4, a must to do is now to optimize the antenna positionning. Since IMHO, you are more flying in the X-Y plan than the X-Z one, I strongly advice to install antennas vertically. Another radical solution is to fly your quadcopter with a Devo controller if the protocol have been hacked. In this case, the TX emmits 20dBm

Most of these toy’s compagnies hire an external subcontractor to design the RF part… In order to decrease production costs, they offer the XN297 w/o PA+LNA without clearly taking into account the consequences. Who to blame ? For me, more than subcontractors, it’s more the main compagnies who wanted to reduce costs at ANY PRICE…. It’s now time to the market to show them they were wrong to downgrade performances.

Categories: FYI

8 Comments

  • onclefly says:

    Ah super toutes ces infos, mêmes si je dois relire plusieurs fois pour comprendre tout (cause mauvais encore en Anglais et en électronique) 😉

    Mais si j’ai bien compris, la porté du quad est vachement réduit ?
    De plus cela n’est marqué nul part, il faut démonter pour savoir.

    Merci, merci, pour ce super cours Mr le professeur. Lol..

  • onclefly says:

    100mW en2.4G cela fait quelle distance de porté a ton avis

    • SeByDocKy says:

      It’s not only a question of Power but also of the impedance matching, antenna positionning, algorithm to encode/decode packet data …
      FrSky with the same power can achieve up to 2.5km of range in LOS conditions

  • Bill says:

    Hi SeByDocKy! I have the MJX X600 and the Tx you have pictured. I also just bought a Devo 7E.

    Can I do the nRF24L01 mod to the 7E using the chip and antenna out of the MJX Tx, rather than buying the nRF24L01 you specify in your tutorial?

    • SeByDocKy says:

      The MJX radio have a XN297 and is in practice less efficient/powerful and cherry on the cake is not supported by deviationTX. Only the nRF24L01 is … so I strongly advise to pick up one. I don’t know where you are located (probably US), but I am sure you can find a local eBay seller. Be sure to take one with the “PA + LNA” part to emit up to 100mW and enjoy full radio range

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