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Carrier Hopping

Added by jon_bondy about 5 years ago

We are using the standard RF12 demo software.

We are having reliability problems with the RF12B transceiver, which is causing arbitrary delays. When we view the transmissions using an HP8920A Communications Test Set using the spectrum analyzer feature, we see that the radio transmissions occur over a wide range of
frequencies. The carrier frequency ranges between 433 and 435 MHz.

We expected all transmissions to occur at one fixed carrier frequency, rather than having the carrier hop around without our asking it to. Would you expect the carrier to be stable?

We can imagine a system where the transmissions hop around to prevent them from being absorbed by some multi-path situation, but this would imply that the receiver had an extremely broad band. We can find no information about the theory of operation of this
system (RF12 hardware combined with the demo software).

When we use the “o” feature to create frequency offsets, the offsets are accepted by the software, and the new carrier frequency is displayed, but this does not affect the carrier frequencies that we are seeing on the spectrum analyzer: they remain erratic and do not shift as the offset is modified.

We also expected the transmitter to transmit repeatedly until an ACK was received from the receiver. We do not see this at all.


Replies (3)

RE: Carrier Hopping - Added by JohnO about 5 years ago

jon_bondy wrote:
> When we view the transmissions using an HP8920A Communications Test Set using the spectrum analyzer feature, we see that the radio transmissions occur over a wide range of
> frequencies. The carrier frequency ranges between 433 and 435 MHz.
Might there be other interfering devices?

> We expected all transmissions to occur at one fixed carrier frequency, rather than having the carrier hop around without our asking it to. Would you expect the carrier to be stable?
I too am surprised you are seeing this - I believe the carrier frequency is stable at the value set using the offset command (‘o’).

> We can imagine a system where the transmissions hop around to prevent them from being absorbed by some multi-path situation, but this would imply that the receiver had an extremely broad band. We can find no information about the theory of operation of this system (RF12 hardware combined with the demo software).
It is not coded to frequency hop.

> When we use the “o” feature to create frequency offsets, the offsets are accepted by the software, and the new carrier frequency is displayed, but this does not affect the carrier frequencies that we are seeing on the spectrum analyzer: they remain erratic and do not shift as the offset is modified.
Could you try holding down the ‘t’ command in RF12Demo to stream packets at what in my experience will be the correct frequency. Does your HP see anything different than before?

> We also expected the transmitter to transmit repeatedly until an ACK was received from the receiver. We do not see this at all.
The RF12Demo software was created to display capabilities, when using the ‘a’ or ‘t’ command the request for ACK will be included in the packet. A second node equipped with RF12Demo would respond to the packet. The RF12Demo software is not currently coded to wait for a returning ACK but will display one if it is received.

RE: Carrier Hopping - Added by martynj about 5 years ago

jon_bondy,

Don’t be a tease - where are the screen scrapes?

RE: Carrier Hopping - Added by jcw about 5 years ago

> We also expected the transmitter to transmit repeatedly until an ACK was received from the receiver

At the lowest level, the RF12 driver does not do this, but there are “rf12_easyInit”, “rf12_easySend” wrappers which add this functionality on top - see this post.

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