–n
This topic comes up repeatedly on this list. When you have a radio
“tuned” to a specific frequency, there is
nearly-always a certain amount of residual frequency error.
Synthesized LOs (local oscillators) have a frequency
offset that is proportional to the PPM tolerance of the reference
oscillator that they’re using.
Let’s say that the oscillator has a 10PPM tolerance (which is typical, I
don’t, off the top of my head, know what the
PPM spec is for the XO in the USRP1). So, that’s 10Hz for every MHz
of crystal frequency. That error “carries through”
the PLL synthesizer. In this case, we have an LO frequency of
2.4GHz, so, that 2.4GHz/10PPM which gives us a potential
frequency offset error of 24KHz–let’s be charitable and assume that
means anywhere +/- 12KHz.
For wideband modulations (higher data rates), a small frequency offset
is generally mostly-harmless, since most of the
modulation energy falls within your passband, even if the edges of
your passband don’t fall where you think they fall.
But for narrowband modulation schemes (low data rates, or narrow OFDM
buckets), frequency offset can be disasterous,
since an offset in the band-edge causes most of the energy to fall
outside of your passband.
In “real” data receivers, particularly narrowband ones, there’s
generally a feedback mechanism that tugs on the LO circuit
a little bit to try and zero-in on the correct frequency. In the
analog world, FM stereo receivers have a so-called AFC
circuit that uses noise estimates to steer the LO. Television does
the same thing (so-called AFT).
In the world of software-defined radio, it’s easy to forget about these
things, because, hey, it’s all digital, and therefore
perfect, right? Wrong.
So, an SDR-based analog data communications system needs to be able to
deal with this. There are a couple of ways of doing this:
1) lock your PLL Synthesizer to a high-quality reference clock,
usually improving the PPM error by an order of magnitude or more
On the USRP2/N2XX/E100 there are explicit inputs for an
external 10MHz reference clock, and UHD makes it easy to
enable this feature when you create the UHD source/sink.
2) Have your demodulator provide feedback to the frequency-setting
code to tweak the actual LO frequency (or DDC frequency,
which is usually faster). This is the most general approach,
since it makes your code work well even on a platform that doesn’t
have a high-quality external reference. Note this is on the
receive side. No sense in tweaking the transmitter when you have
potentially-many receivers. The conventional thing to do is
to have the receiver track wherever the transmitter is right now.
This class of problem is in no way unique to SDR hardware. Ham radio
operators on the list will tell you about many adventures
(particularly in the old days) of tweaking the LO performance on
their VHF receivers to allow “full quieting” reception of the local
FM repeater, and as I observed, FM radios and televisions have had
some kind of automatic-fine-tuning for many many decades.
–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
