Yeah. It’s a great word, isn’t it?
It may not seem like it, but even an antique radio like this has to overcome a pretty significant
problem before it can become…
a radio.
See, like any radio, I can choose to listen to
different stations by just touching that dial, and just like that the radio is able to zero
in on a single frequency and allow me to listen to…
just that station.
But have you ever stopped to think about how it’s doing that?
Right now the radio is being bombarded by electromagnetic radiation, the vast majority
of which it can’t do anything with. Sure, when it was made, it just had to deal with
AM radio signals for the most part, but soon came FM, Television, microwave transmissions,
cell phones, WiFi, and the list goes on. And yet, it still functions perfectly as an AM radio.
[various stations tuning in and out]
It’s able to toss out all of that nonsense
and focus on just what I want it to focus on.
To do that - and to do that well - it’s being a lot more clever than at first you
might imagine. Obviously it has the ability to tune into one frequency - and thus tune
out the rest. But you might be surprised to learn that it goes beyond being selective
in the frequencies it receives. What it’s actually doing is creating a second frequency
of its own, mixing that with all the incoming signals, and the resulting composite signal
is then filtered and amplified.
That’s what superheterodyne means -
it’s not just a flowery marketing term like "Spectrohydramagnetic,"
it actually comes from supersonic heterodyne. Supersonic means that it’s above the human
hearing range.
Today we’d be more likely to call it ultrasonic
but it was the 1900’s and
radios were getting
CRAZY!
Hetero means different, and dyne means frequency. So it’s a super (ultra) sonic
frequency that’s different.
OK, how does that help?
It helps through creating an intermediary beat frequency. A beat frequency is what happens
when two dissimilar frequencies cause interference with one another. To demonstrate, I’ve placed
these two speakers in front of me, facing each other. This one is playing a 400 hz tone,
and the other is playing a 401 hz tone.
Notice how the sound appears to fade in and out once per second.
[a solid tone with a throbbing undulation]
On their own, they sound pretty
much exactly the same. But together, that slight misalignment of their frequencies is
producing a beat.
And the frequency of that beat is determined by the difference between the two frequencies.
The beat occurred once per second because there was a discrepancy of one hertz.
Hert?
Increase the frequency from 401 to 404,
and the frequency of this speaker could not be found.
I’m just kidding,
Internet humor!
Now, since there’s a difference of four
hertz, that beat frequency becomes four hertz.
[the same tone with much faster undulation]
To see what’s happening, I’ll make the same tones in Audacity and put them next to
each other. If we zoom in on the waveforms, we can see that they’re not quite aligned.
If we were to add them together, at some points they combine to make a larger amplitude. That’s
constructive interference. At others they combine to actually zero out the signal.
That's destructive interference. When I actually combine them, we end up with a new waveform
that is pulsing 4 times per second.
[the same throbbing tone]
The real fun comes when we get into higher frequencies. This speaker is playing a tone
at 5,000 hertz or 5 kilohertz. This one will play at 5,400 hertz, or 5.4 kilohertz.
♫ low-fidelity guitar music ♫
[ an ear splitting tone ]
[ a lower tone appears ]
The principle of constructive and destructive interference producing a new signal at a different
frequency also applies to radio waves. If you inject a signal that is, say, 100 kilohertz
offset from another, it will produce a beat frequency of 100 kilohertz. Can you see how
that might be useful? Let’s go back to the radio.
Say we want to tune to 780 AM, that’s 780 kilohertz.
One way to do that would be to create a filter which can block out every
frequency that isn’t 780 kilohertz.
Now, we can do that, but to design a filter that can shift which frequency it lets through
(and thus allows you to tune the radio) is tricky. It was done before, and until the
invention of the superheterodyne radio receiver in 1917/18/20, that was essentially how all
radios worked. But the trouble was that these filters weren’t that precise, and because
of that the radio wouldn’t be all that sensitive. Competing signals at similar frequencies might
blend together, and the signals themselves would have to be pretty strong in order for
the radio to pick them up. It was always a tradeoff between the precision of the filter,
and the strength of the received signal.
Further complicating things, these early radios also had trouble processing the signals themselves.
The amplification circuitry of the day didn’t like dealing with the (at the time) very high
frequency of radio signals, as it was hard to design a vacuum tube that could effectively
amplify these signals in order to produce audible sound.
But what if instead of trying to design a radio that works in radio’s frequency range,
we take a completely different approach? What if we design a filter that looks for just
one specific frequency, AND we put that frequency outside the normal broadcast range? That would
solve the interference and signal strength problems, and if we were able to lower the
frequency of the signal, the radio could amplify it and make sound more easily. Additionally,
we could make the filter more precise because it is set to a fixed frequency rather than
needing to move around as the radio is tuned.
And that’s what superheterodyne radios do. Take a look at the schematic for this radio,
a Philco 42-PT-7, and you’ll notice it says intermediate frequency: 455 K. C. This stood
for kilocycles, which is another word for kilohertz. I like these old schematics because
they often contain outdated terms, like how what today we call capacitors were then called
condensers.
Anyway, what this means is that, in a way, this radio is always tuned to 455 kilohertz.
Now that may seem pretty useless, but it’s actually genius. See, this tube here, which
is a pentagrid converter tube, is able to generate its own sine wave output at whatever
frequency we like. This component here, called the tuning capacitor, works with the tube
to change the frequency it generates as the plates move in and out (which therefore changes
its capacitance value). By turning the tuning knob, the capacitance value of this component
changes, and this in turn increases or decreases the frequency being generated by the oscillator tube.
If we want to tune to 780 AM, we can take all of the incoming signals, mix them together,
and inject a sine wave at 1,235 kilohertz. The interference between our desired frequency,
780, and the frequency we’re injecting, 1,235, will produce a beat frequency at 455
kilohertz. And, since this tube here, the intermediate frequency amplifier, is tuned
to only pass through signals at 455 kilohertz, this newly created composite signal will pass
right through it. It then gets rectified by the detector tube, amplified by the output
tube, and with the help of some other smoothing capacitors, the resulting output gets sent
to the loudspeaker, and we get audible sound.
See how cool this is? We don’t have to change the frequency we’re looking for in order
to change which frequency we receive. By modifying the frequency that we inject into the mix,
we can single one out because the combined signals will produce beat interference which
will pass right through our intermediary filter.
It’s science!
It’s math!
It’s fantastic!
But, this isn’t to say that we can just inject a signal and everything is fine and dandy.
There’s one slight problem.
See, if we inject our 1,235 kilohertz signal without
doing any other filtering, we’ll actually get beat frequencies from two incoming frequencies.
One from 780 kilohertz, and the other from 1,690 kilohertz. Both of these are 455 kilohertz
away from the frequency we’re injecting. So, the radio will typically need some other filtering
on its input to block out one of these frequencies,
and thus prevent the second signal from coming through.
That second signal is often called an image, because it’s analogous to a mirror
image of the intended frequency - flipped across the axis of the injected frequency.
This first filter doesn’t need to be precise, but it does need to move with the tuning dial.
Otherwise, the radio couldn’t receive the full frequency range. If you take a look at
the schematic, you can see that the tuning condenser (slash capacitor) takes a role in
two places - it functions both as a filter on the antenna and as a generator of sorts
for the oscillator tube. But to be clear - that first filter is really sloppy. It just has
to block out frequencies that are some 400 kilohertz or further away from the local oscillator’s
frequency.
Now some of you might be asking, how can we change the frequency of the signal without
affecting the sound? Well, that has to do with the way the signal is actually encoded.
Remember, the frequency of the radio signal is simply a carrier. For amplitude modulation,
it’s the overall intensity of the signal over time that encodes sound. So we can shift
the carrier frequency up and down, without actually changing the information that it
carries. The beat frequency generated by the mixing of the signals will follow the same
exact pattern as the original carrier wave.
The superheterodyne radio receiver solved one of the fundamental problems of radio in
a very clever way. It proved to be such a good method of isolating frequencies that
it became the de facto tuner design for many decades, continuing into the television age
and through the transistorization of radio,
though these days tuning is largely done with software.
It wasn’t perfect though. The presence of a local oscillator in the radio meant that
it created a bit of electromagnetic radiation of its own, which could interfere with other
radios nearby. To be fair, nearby usually meant within the same room, but it’s only
fair to say that there were downsides to this approach. Still, thanks to the Superhet as
it's sometimes called, tuning to your favorite station became a piece of cake.
Thanks for watching, and I hope you enjoyed this video. I’ve been sitting on it
for a long time, as my earliest videos dealt with the history of artificial sound.
You can watch them if you like, but be warned--they are pretty cringetastic.
And that’s my assessment!
As always, thank you to everyone who supports the channel on Patreon, especially the fine
folks that are scrolling up your screen. With the generous support of people like you,
Technology Connections has gone from my hobby to my job!
And I’m very grateful for your support.
If you would like to support the channel and get perks like early video access, behind-the-scenes
footage, as well as the inside scoop on the latest projects, please check out my Patreon
page. Thank you for your consideration, and I’ll see you next time!
♫ incomprehensibly smooth jazz ♫
It was always a trade-off betreen...
See you can’t think that things are going well
because as soon as you think that, it falls apart.
Nope. I missed a “The”
I love these old schematics because they often
contain outdated terms, like how we today call
bweeeehhh, bweeeehhh
Take a look at the schamat…
mouth noises!
We need to restart.
...and one is playing a 400 Hz tone, and the other is playing a 401 Hz tone.
The wires aren’t hooked up yet so you’re gonna know I’m faking this!
Eurghhh!
Because there was a discrepen….
I mixed my tenses
[clears throat, inhales as if about to start speaking]
Oh yeah, let’s move this over.