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The datasheet of SYN531R suggests 13.51783 MHz crystal.

I found available available 13.52127 MHz on an online store.

It seems very similar to me, however in reality would it work and if yes, would it make a big difference or not? Is it of a so big importance the crystal to be exactly the same one as on the datasheet or not?

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  • \$\begingroup\$ What's on the other side (the transmitter)? Is it something you are building as well? This receiver seem to be pretty sensitive to frequency differences. \$\endgroup\$
    – Eugene Sh.
    Mar 23, 2021 at 15:27
  • \$\begingroup\$ On the other side is a transmitter based on SYN115( datasheet: rhydolabz.com/documents/33/… ) \$\endgroup\$
    – Kris
    Mar 23, 2021 at 15:28
  • \$\begingroup\$ That transmitter is a joke. OBW < 1000kHz. Oh how nice, except the band isn't even 1000kHz wide! Also so funny how these parts list harmonics & emissions always at like -38dBm. With the legal limit being some -37.5dBm. "No, we aren't trashy at all, look, you got several microvolts/m in margin!" \$\endgroup\$
    – Lundin
    Mar 23, 2021 at 15:32
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    \$\begingroup\$ @Kris Oh, I was just assuming you might want to use this radio legally in Europe, North Africa, Middle-East, Australia or South America. If you only intend to use it in some exotic country I know nothing about, then OOK away by all means. \$\endgroup\$
    – Lundin
    Mar 23, 2021 at 15:39
  • \$\begingroup\$ @Lundin yes I do. As i replied to you on your answer(thanks for your time) synoxo is a european company so I do not understand why you say its illegal to use those components... \$\endgroup\$
    – Kris
    Mar 23, 2021 at 15:42

2 Answers 2

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In the datasheet there's this formula:

Fosc = Frf / (32 + 1.198/12)

OFFICIAL QUARTZ CRYSTAL:

When using the official quartz crystal you get a carrier frequency of 433.92 Mz

13.51783 MHz = (433.92 / (32 + 1.198/12)) MHz

UNOFFICIAL QUARTZ CRYSTAL:

When using the unofficial quartz crystal you get a carrier frequency of 433,9967 MHz

You are off of by 70 KHz and the receiver might go blind.

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  • \$\begingroup\$ the incoming signal is usually 25 KHz or less, for this kind of receivers, by law. What law? I can receive all I want as long as my transmissions stay within the allowed frequencies and power. The SYN531 has an IF bandwidth of 330 kHz (page 3 of the datasheet), for receiving OOK signals that will work fine. \$\endgroup\$
    – Bimpelrekkie
    Mar 23, 2021 at 16:09
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    \$\begingroup\$ @Bimpelrekkie The law is called EU 2006/771/EC unless national law is different, then national law takes precedence. For 434.04-434.79 EU-wide, you either need to keep 10mW ERP 10% duty cycle, or 1mW ERP 13 dBm/10 kHz power density for bandwidth modulation larger than 250 kHz, or 10mW ERP 25kHz channel spacing. Channel spacing applies to the receiver as well as the transmitter. \$\endgroup\$
    – Lundin
    Mar 24, 2021 at 7:22
  • \$\begingroup\$ In either of the three cases, I don't see how 1MHz occupied band 10mW ERP fits the bill. \$\endgroup\$
    – Lundin
    Mar 24, 2021 at 7:25
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According to the manual that you linked: F OSC = F RF /(32 + 1.198/12). This is the ref oscillator they expect. And yes it will make a huge difference which one you pick because this oscillator determines how stable your frequency is. Professional radio designs will use a TCXO here. Depending on the transmitter, you don't necessarily have to place your carrier on 433.92MHz however. Use the formula they gave you and see if you end up within the license free radio band.

However, the part you linked is highly problematic to use on the 433MHz band, where ASK and OOK simply aren't feasible due to the high amount of other radios present on that band. Sensitivity and image rejection are mediocre. Always avoid parts that don't list adjacent channel sensitivity/rejection, because that always means it's poor. The IF bandwidth of 330kHz also makes it useless in practice. This is a skunky part and I don't see how you can design a legal, working radio with it, at least not if you intend to use it in Europe.

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  • \$\begingroup\$ which part is problematic? receiver or transmitter? Synoxo is a european company so i guess its not illegal.. what makes it illegal? \$\endgroup\$
    – Kris
    Mar 23, 2021 at 15:41
  • \$\begingroup\$ @Kris The 433MHz band differs a bit from country to country in Europe. EU has tried to standardize it and they divide the band in two: one from 433.050 to 434.900 where you are only allowed 1mW ERP or 10mW ERP with 10% duty cycle. Alternatively you can use 434.040 - 434.900 with 10mW ERP. Typically you need to be narrowband <25kHz. At the very least you need to fit inside the f***n' band, which rules out OCB of 1MHz. -> \$\endgroup\$
    – Lundin
    Mar 23, 2021 at 15:48
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    \$\begingroup\$ @Kris In plain English: you are allowed to drive on the public highway if you have a drivers license and an inspected car. You are however not allowed to drive a monster truck without plates, far wider than the whole highway, it's wheels tearing up all public property on the sides of the highway. You also end up blocking the highway for everyone else, so they get very annoyed at you. And as soon as any little car happens to drive in front of you, you have to wait for them to leave the highway, since your massive monster truck is so easy to block. \$\endgroup\$
    – Lundin
    Mar 23, 2021 at 16:04
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    \$\begingroup\$ @Kris Rather, you should download ETSI 300 220, see what parts that may apply to your product and ask a question here if you don't understand certain parts. If your product conforms to 300 220 you can sell it pretty much any country that has a license-free 433MHz band. \$\endgroup\$
    – Lundin
    Mar 24, 2021 at 9:13
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    \$\begingroup\$ My remarks about ASK and OOK being unsuitable has nothing to do with legislation though, I just know from experience that they simply won't work on this band. For example Volvo cars made a simple wideband 433MHz radio for their car keys at one point, with some questionable modulation method. Then everyone who happened to own a Volvo often couldn't lock or unlock their car in case they happened to use their car in a city. In cities, other transmitters will be present. \$\endgroup\$
    – Lundin
    Mar 24, 2021 at 9:14

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