[USRP-users] Amplifiers for USRP Transmission

Marcus Müller marcus.mueller at ettus.com
Mon Jul 6 17:31:02 EDT 2015


Hi Evan,

I'm a bit confused over your use of a microphone, though. I'm making a
sketch, and would ask you to correct me if I got anything wrong:
Drawing of the System

I'm presuming you're actually tring to use the microphone to emit sound,
thus making it a speaker?

There's a bit of a problem, here:
The energy the antenna, even under perfect circumstances, will pick up
is very little. That's a good thing, in principle -- your antenna is not
very selective, and if the power coming from your wire would be
sufficient, all speakers that unconnectedly lie around would instantly
receive a lot of signals from the spectrum around you. If you look at
the classical AM radio, you will notice that people (for much lower
frequencies) used large antennas, which they were able to /tune/, very
much like you tune the string of a violin, to emit and receive a
specific frequency. This made the antenna work good as a receiver for a
small bandwidth, the channel they wanted to receive, and bad for other
channels. This way, you could listen to a station from Paris in London,
even though the BBC of course had an overwhelmingly large signal power
-- but at a different frequency, so by means of frequency selectivity,
people were able to pick up the "right" channel only.

The fact that your receiver works well if you put the transmitting and
the receiving antenna very close to each other doesn't rely on this
principle of reception: Whilst all the radio waves picked up by your
wire are still very weak electrical signals in the wire, if you put both
antennas sufficiently close to each other, they form a capacitor.
Capacitors are very good conductors at high frequencies, so almost all
energy produced by the SBX transmitter amplifier (it already has one;
these 100mW don't come from nowhere :) !) is directly fed into
microphone-used-as-a-speaker. That's not really radio -- almost all
audio amplifier you typically see have capacitors between the actual
amplifiers output and the speaker connectors (mainly to protect speaker
and amplifier).

What then happens is that a small amount of the energy passed through
the speaker to ground actually moves the coils in your microphone (or
the crystal, if you're using a piezo mic/speaker), because these
mechanical things have high mass and thus large inert mass and low-pass
character. In principle, if your microphone was a theoretically perfect
thing, you would hear nothing -- for example, let's assume your radio
wave has a frequency of 500 MHz, and you want to hear a 500 Hz tone --
in the time the membrane should have made a full oscillation, one
million oscillations of the electric current induced in the antenna
would have happened. If you take an average about whole oscillations,
it's zero, no matter what you do. What you need to do is actually build
what we call an envelope detector:

Envelope detector
<https://commons.wikimedia.org/wiki/File:Amplitude_modulation_detection.png#/media/File:Amplitude_modulation_detection.png>
As you can see in C, the audio signal is the envelope of your RF signal.

So to make the best of your experiment, we have to do three things:

a) we need to build something that selects a specific bandwidth equal to
the bandwidth of your audio signal, so that you don't receive a mixture
of all the signals that your antenna picks up, and
b) we need an envelope detector that "removes" the radio frequency from
your signal, and
c) build an amplifier, so that the small electrical signal from the
antenna can actually move the membrane.

Point c) is the only point where you could "cheat" with more output
power (I really don't recommend doing that; legal, health and
neighborhood issues would be the consequence); in fact, some early
radios with small earphones could do completely without an amplifier,
but since this is 2015 and not 1905, the amplifier is actually the
easiest to get part.

a) is called a filter. You can build a filter by tuning your antenna
(which really doesn't make much sense, since you're not doing the low
frequencies usually associated with AM), or by actually using a normal
antenna like yours and having a filter further down the signal
processing chain; either directly after the antenna, or after a only
slightly filtering radio frequency amplifier has already increased the
voltage of the signal. Easy filters can be built by using an
inductance/capacitor circuit (google "LC resonant circuit"), but it's a
bit hard to make the antenna work well directly with these; but
basically: if you play around long enough, you'll build a filter that
works. You're lucky, because you can just build a filter, and then use
the USRP to transmit at a lot of different frequencies, until you hit a
frequency that your filter lets through well. If you then adjust the
parameters of your components, you can tune your filter to the desired
frequency. That's a variable filter, and these are painful to get very
exact. Still, it's what is used in very simple radios.

Modern Radios often use a different approach: They do what is called
"mixing". That's basically taking the Radio signals (all radio signals
picked up by the antenna) and moving the whole spectrum down (and up,
but that component can be eliminated easily) by an adjustable frequency,
so that fixed filters can select a channel.

b) Hit Wikipedia about "Crystal radio". The trick here is that you
simply need a rectifier to make the average-zero signal an
average-varying one -- a simple diode (given it's fast and sensitive
enough) will do.

c) now that's just a normal amplifier. Use something with a high input
impedance -- a microphone preamplifier might do well, or a guitar amp or so.

All in all, building a crystal radio is a fun project -- but the signal
theory behind and the physical basics of why the individual components
(electromagnet waves, antennas, filters, mixers, detectors,
semiconductors like diodes, amplifiers ...) work are something that
electrical engineers study for years, literally. I've only selected a
very small subset of the physical and engineering problems that you
might meet.

Don't let yourself be discouraged by initial hardships -- Marcus Leech
has already pointed you to the ARRL handbook, I think. Although I'm in
Europe, I also happen to have one of these -- they really introduce what
we learned "the hard way" at university [1] much more from a radio
"technicians" point of view: What does make the radio work? How can I
built a simple one myself?

Best regards,
Marcus

[1] (e.g. from linear algebra and differential equations->linear
electrical networks->field physics->wave physics->wave
propagation->antenna theory and from stochastics&integrals->integral
transforms->signal theory-> communication theory->receiver mechanics in
a 5 years degree)
On 07/06/2015 09:32 PM, Evan Chavis via USRP-users wrote:
> Hello USRP users,
>
> I've been working with a USRP1 to transmit an amplitude modulated
> signal to a very basic improvised receiver, basically just a wire of
> length corresponding to the frequency I'm transmitting at attached to
> a microphone such that the microphone is picking up the signals on the
> wire as it would audible noises.  I'm using an SBX daughterboard
> (which can produce 100 mW) and a log periodic antenna (with 5-6 dbi
> gain).  I can get a decent transmission at a range of up to about 25
> cm, although it is best at a distance of just a few cm.  I've got the
> gain set with GnuRadio as high as possible without causing distortion,
> and I'm looking to improve the range of possible transmission.
>
> My questions are, what do I need to consider when looking for an
> amplifier to use with a USRP, are there any special problems a
> beginner would be likely to miss when trying to use those 2 things
> together?  Also, if an amplifier's gain is listed at 20 dB, would that
> be 20 dB on top of the 31.5 the daughterboard is capable of
> outputting, or would the amplifier be useless in that it is capable of
> providing less power than the daughterboard itself?  And third, how is
> a device such as this
> one: http://www.minicircuits.com/pdfs/ZX60-V63+.pdf powered?  The
> data-sheet gives the numbers 5 V and 69 mA but I don't see an obvious
> way to power it aside from just soldering wires onto the marked bumps
> that say +5 and ground, is that the actual intended way to do it?
>
> Many thanks for any input and have a great day
>
>
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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