RFM95W Module Question

This message was edited 1 time. Last update was at 04/12/2014 07:14:55

Hi,
I have two RFM95W-915Mhz LoRa transceivers I purchased a while back. They are now assembled in a long range, one way rf actuator of my own design. When I first got the devices they were not configured for 915 Mhz, it was some other frequency, not sure what. My micro software drives the rf to 915 mhz on start up, so all is OK.

Is it normal for these parts to be shipped at something other than 915 Mhz?
Dick

This message was edited 3 times. Last update was at 02/12/2014 02:00:11

I've not used the radios of which you inquire, but the following applies to most all of HopeRF's modules/boards.

There are band-specific RF filter components (434,868,915) MHz bands on the board. The boards are marked as to which band these components are tuned. When soldered on to a board, this marking may be obscured, so you have to have otherwise marked them yourself, or you have purchasing documents/receipts that show which band you ordered.

The radios are tuned by software command; they're not tuned by internally fixed constants (e.g., firmware configuration).
If your software tunes a 915Mhz product to 868MHz, the penalty will be small, about 10%, so I've read.
Tuning this to 434MHz, or 315MHz, has a prohibitive degradation in transmitter power and receiver sensitivity - e.g., 20dB.
Vice-versa, tuning a 434MHz radio to 915Mhz.

In North America, the "915" MHz band refers to the ISM band spanning 902-928MHz, with 915Mhz being a default.

that's what I know.

Rick may have other insights.

steve

This message was edited 1 time. Last update was at 04/12/2014 07:16:03

Hi Steve,
Thanks for your comments. You're correct that the radios had markings on the bottom. I looked at them briefly before installation but can't recall the i.d. However, if something was amiss I probably would have noticed???

Here is what led to my question: I have a second radio system which is identical to the one in question, but uses Hope LoRa 915 Mhz transceivers that I got from D6 Labs. These radios are most definitely 915 Mhz. It is the only freq. sold by D6 Labs and is the default configuration.

When I built a receiver using the Anarduino module it would not pick up a signal from the D6 transmitter. I had assumed that the Anarduino radio was 915 Mhz by default. When I reconfigured the Anarduino receiver to 915 it did work correctly and picked up the transmitter signal.

This led me to question what kind of part I got from Anarduino. Is there anyone at the company who could check and confirm what kind of radios were shipped to me?

Dick Ivers

This message was edited 1 time. Last update was at 03/12/2014 19:19:47

did you keep your receipt to show the part number ordered?

Hi again,
Update on the issue regarding my RFM95W-915 Mhz modules:

I now know for sure that the radios shipped to me are not 915 Mhz devices. I was able to read the default frequency settings on the parts I received. I did this by reading registers 06, 07, and 08 through SPI and then writing the values in those registers to my microprocessor's eeprom. I then read the eeprom using a my pic programmer. The results are as follows:

06 MSB = 6C hex
07 MIB = 80 hex
08 LSB = 00 hex

Collecting the results together gives a sum value is 7,110,656 decimal

Multiply this number by the freq resolution of 61.035 gives a final result of 434 Mhz

Needless to say this is a very upsetting finding. The parts are way off from the intended frequency. Also, they are soldered to my boards making replacement difficult if not impossible.

Is there anyone at Anarduino who can explain this situation? My emails go unanswered.

Dick Ivers

Hi,
Update #2
Upon further review of the RFM95W data sheet it appears to be normal for this radio to have a default rf frequency of 434 Mhz.

The module has two frequency bands; a lower band below 525 Mhz and a high frequency band above 860 Mhz. The default value is the low band. The default freq. is 434 Mhz. To get to the high band and to tune to 915 Mhz requires re-configuration via SPI and that's what I have done (see post#1). So I think all is OK with my radio link..

It is odd that the radio can operate over such a wide freq range without a change in the values of the rf filter components. Does the switch over in bands also involve electronic switching of filter components?

Dick

I don't recall that HopeRF has a non-volatile memory update that's meaningful. User must do all settings (which RadioHead does do, with many alternatives).

The radio per se is the same for 433 and 915MHz; the key difference is in the filter components on the PCB external to the radio chip.
I've noted that if you tune a radio for one band to the other band, and adjust the antenna length accordingly, the filter diminishes the signal strength about 10-15dB. That's fine if you have lots of excess signal strength due to using high power and/or have a short or clear path, or have 6dB or better gain antenna(s)

This message was edited 2 times. Last update was at 14/12/2014 21:02:48

An internet search yielded the following information about dual band circuit design for the SX127x radios:
http://www.semtech.com/images/datasheet/AN1200.19_SX127x_RefDesign_STD.pdf

Hello,

seems I can confirm that something strange happening with frequency on LoRa MW.
I have topic with detailed information abut this issue - http://forum.anarduino.com/posts/list/0/82.page#2832

in short - yesterday I have 2 anarduino+LoRa(RF96) that works on 915Mhz - 6,7,8 registers on both were - E4, C0 and 0 so it is == 915Mhz
today - one of MW start to work on 433Mhz! registers are - 6C, 80, 0 which mean 433Mhz.

ok, got it - I realized that I use different version of libs for client and for server.

for Client I use libs from Moteino site - ver.1.6, for server - last version from RadioHead site, ver 1.8

and what we see in RH_RF95.cpp files?

setFrequency(915.0); moteino 1.6
and
setFrequency(434.0); radiohead 1.8

great..

It's up to your app to set the correct/desired bands and frequency within the band.
The RH libraries have defaults but the library cannot know what band the hardware is designed for... the radios are identical. The external RF components (inductors, capacitors) are band-specific.
The part number you ordered has a part number portion.
Unlikely that Anarduino shipped a PCB with external RF components for a band other than what you ordered.

A test might confirm:
Put to identical boards on bench-top, about 1m apart. Line of sight between them.

Load firmware in both, compiled for 915MHz ( or 868MHz in the EU). Run and note the RSSI reported by each.
Load firmware in both for the other band (e.g., 433MHz). If the RSSI is (on average) over 10dB better, then this is the correct band. If 10dB or so poorer (more negative RSSI), the hardware filters are for the other band.

Note that in the US and other regulatory regions, the transmitter on-time (duty cycle) is very restricted in the 433MHz band. It will have about 4dB of advantage over the 902-928MHz band due to reduced line of sight path loss. But 4Db is a nit in link budget.

This message was edited 1 time. Last update was at 30/10/2015 12:08:45

Hello,

ok, I've found the way how to check RSSI - I run driver.lastRssi();

and with 434Mhz I have results from -69 to -73
with 915Mhz results are -32 - -39

so I suppose correct band is 915MHz ?

This message was edited 1 time. Last update was at 13/01/2017 01:12:56

Hello, I have a problem. I am using rfm95w and try to send data, but can't. I set Lora - 869Mhz, fill fifo, switch Tx, but nothing happend. I even see how pull-up PllLock and TxDone DIOs, registers said - ok, your data sended. But I see nothing. I used RTL-SDR antenna and cymometer and have seen zero on both. Could you help me?

UPD Solved, RFM95W couldn't work, if PaSelect reset. When PaSelect set rf96 use for transmit another pin, wich not connected.

The radios are tuned by software command; they're not tuned by internally fixed constants (e.g., firmware configuration).
If your software tunes a 915Mhz product to 868MHz, the penalty will be small, about 10%, so I've read.