Carrier hopping

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jon bondy
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Joined: Sat Nov 17, 2012 2:14 pm

Carrier hopping

Post by jon bondy » Mon Feb 09, 2015 3:46 pm

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.

paul
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Posts: 732
Joined: Mon May 12, 2008 4:19 pm

Re: Carrier hopping

Post by paul » Sat Mar 07, 2015 11:20 am

Jon,

Hmmm, I'm surprised by the frequency hopping and would expect much tighter control also. If you just have one radio I'd like to see you test another before making any determinations. These are very inexpensive radios, mass produced in volume and while we do test them for transmission, and reception, it's a pretty basic test. I believe the receiver ends of the radios have PLL's to pull in errant freqs a bit, but I don't think they'd perform with the bandwidth that you are seeing.

If you're still interested in testing, send us your address and order number and we'll send you another unit to compare with which to compare performance.

Paul

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