Foundations of Amateur Radio #205:
May 11, 2019
Add a comment about this article!
SDR Sample Rates: How fast is fast
If you measure the voltage at the base of an
antenna and record the readings, you end up
with a collection of numbers that represent
the voltage over time. These numbers, or
samples, can be used to represent the antenna
signal inside a computer.
An antenna system voltage is an example of an
analogue signal, continuous over time, the
recorded readings, the samples are an example
of digital, discrete and intermittent.
It's possible to reconstruct an analogue
signal from digital samples and that's
exactly what Software Defined Radio or SDR is
The process of sampling essentially converts
a continuous signal into an intermittent one.
As recording separate samples implies, there
is loss of information in this conversion.
For example, if you sample once a minute,
you'd represent a continuous signal as 60
samples per hour, probably enough to
reconstruct where you've driven in your car
along the highway, but hardly enough to
reconstruct the route through the middle of
the city, let alone represent an antenna
signal that varies millions of times per
So, how often do you need to record a sample?
Turns out that if you sample at least twice
as fast as the highest frequency you're
representing, you're good to go.
So, for sound, the human ear can hear about
20 kHz, so more than twice that, explains
some of why a CD is sampled at 44 kHz.
If you want to represent the 20m band, up to
14.350 MHz, you need at least a sample rate
that's double that, or 28.7 MHz.
As an aside, there are other ways to look at
this problem. If you managed to move the 20m
band down to 0, then you'd only need at least
a sample rate of 700 kHz to do this.
Let me say that in a different way.
The width of the 20m band is 350 kHz. So
sampling it would require at least twice
that, or at least 700 kHz. Moving frequencies
around is something that we've been doing in
traditional radios for a long time. The
technique uses one or more frequency mixers.
This means that combining some traditional
radio tools with an SDR gives you even more
Truth be told however, this idea of moving
the band with one or more mixers is becoming
less important as technology improves and
there are plenty of reasons not to use this.
I'll talk about that at another time.
So, the first takeaway is that to sample a
continuous signal and be able to represent
that signal accurately requires a sample rate
that's at least twice as high as the highest
frequency in the continuous signal.
Without going into the actual proof of this,
consider a sine wave that oscillates at 1 Hz.
If you sample it at anything less than 2 Hz,
you'll end up with some cycles being sampled
only once, which isn't enough to represent
the sine wave. If you sample it at exactly 2
Hz, you'll have two samples on every cycle,
but if you happen to sample when your signal
is 0, all you'll ever measure is 0. By
sampling at a rate greater than 2 Hz, you
overcome that limitation.
I'll make brief mention of another
phenomenon, that of over sampling. An
interesting thing happens if you sample
twice, three times or more than the minimum
sample rate. In short, the higher sample rate
improves the dynamic range, noise performance
and filtering, all very useful when you are
processing radio signals. Cheaper and cheaper
hardware are making this very attractive and
it explains some of the reasons why SDR
manufacturers are using sample rates that far
exceed double the highest frequency being
sampled, for example, the Flex-6600 samples
at 245.76 Mega Samples Per Second, or Msps,
even though the maximum receive frequency is
between 30 kHz and 54 MHz.
In case you're wondering, yes, I'm leaving
out a lot of detail here, one thing at a
The opposite, under sampling, has its uses as
well, but I'll also leave those for another
The second takeaway is that higher sample
rates are used to reduce cost, increase
performance and reduce component count.
Some of what I've talked about can be
explored with the popular RTL-SDR USB dongle
which is actually a mass produced commodity
digital television receiver, made in the
millions and accessed directly thanks to the
combined efforts of many different people. If
you'd like to start to play, $25 should get
you a dongle and most of the software you can
start to experiment with is free. Check out
rtl-sdr.com to get started.
If you'd like to get in touch, please do,
I'm Onno VK6FLAB
TL;DR This is the transcript of the weekly
'Foundations of Amateur Radio' podcast - for
other episodes, see http://vk6flab.com/
There are no comments on this article: