FSK SER on the same symbols

[kellogs]

Out of 192 symbols received at 19.2 kbd, two of them, towards the end of the packet, are always incorrectly received. The same two symbols.

Changing the antenna fixes the problem. Turning on the transmitter volume / moving it around the receiver does not solve the problem. FWIW, the PCB antenna is a bowtie while the commercial one is a dipole, both electrically short.
Is this just SER due to a bad antenna or is there something else to it?

 

[Papabravo]

You should try to correlate your received levels with the receiver threshold. Changing the antenna may have lifted the received levels enough above the threshold to make the reception more reliable.

 

[Tony Stewart]

How much ISI & SNR do you have? send 011011011... or 6DB6DB6.... measure SERDES jitter.

 

[ahsrabrifat]

Out of 192 symbols received at 19.2 kbd, two of them, towards the end of the packet, are always incorrectly received. The same two symbols.

Changing the antenna fixes the problem. Turning on the transmitter volume / moving it around the receiver does not solve the problem. FWIW, the PCB antenna is a bowtie while the commercial one is a dipole, both electrically short.
Is this just SER due to a bad antenna or is there something else to it?

This is most likely an antenna-induced SER issue. The PCB bowtie is probably detuned or mismatched enough to cause symbol distortion or SNR degradation.

 

[kellogs]

Can SER cause the exact symbols to be received incorrectly each and every time ?

Some news:
- no idea what I have done, it is now receiving many more incorrect symbols, some two thirds I would say
- changing antenna place also changes which symbols are being incorrectly decoded
- tried changing the power source to even batteries; still having the issue

I was and still am using the antenna in a spot where SWR measures 31 +/- 3. I did take the PCB inside my car where it will eventually live and got SWR readings between 2 and 4. Tried it there too, it has received same percentage of symbols incorrectly if not even more...

Papabravo do you mean getting RSSI out of the receiver chip in the middle of a receiving a package with both antennas ? It has a 3-wire SPI interface on which I can only write but could not make it read back from the IC.

Tony Stewart I only have a nanoVNA, woud not know how to measure ISI and SNR. As for SERDES, I would not even know where it is being used in my case: two sub GHz radios: transmitter and receiver pair, integrated circuits.

Thank you!

 

[Papabravo]

Without a schematic I can't tell you precisely what I am talking about. Any analog receiver chip will have specifications related to thresholds for various signal states. When signal levels are degraded for whatever reason the reliable detection of signals also decreases. At some point reliable decoding becomes impossible.

 

[rjenkinsgb]

What receiver are you using? Is is a dedicated data receiver or a general purpose one?
What data encoding, if any - if you are sending UART serial, it may well be DC offsets with different combinations of characters.

 

[kellogs]

What receiver... oh, sorry! It is the MAX41470
Manchester code, no UART involved; I am saving the symbols in MCU's EEPROM.