Tuning In: What I Learned About Radio Receivers from Al Penney’s Chapter 14
If you’ve been following my ham-radio learning journey, you already know that every week I come away from Al Penney’s RAC Basic class with at least one “aha!” moment. Chapter 14—Radio Receivers—was no exception. In fact, this one took me right back to 1985, when I was sitting at my test bench on the GFC Hammond shop floor with a scope probe in my hand, trying to figure out why a switching supply refused to behave.
Receivers, it turns out, aren’t that different: they’re equal parts physics, architecture, and black-magic engineering. What follows are my notes and reflections on what Al covered—sensitivities, selectivities, superhets, noise sources, and that never-ending problem of intermod and cross-modulation that haunts every urban ham.
What a Receiver Actually Has to Do
I think most non-hams imagine a radio receiver as a kind of magical ear—just “listening” to signals floating around. In reality, it’s a chain of brutally practical engineering steps:
- Capture the RF using the antenna
- Select the one frequency we care about from thousands across the spectrum
- Amplify it without adding too much noise
- Recover the modulation—audio, data, or whatever information was riding on the carrier
- Amplify the audio and send it to a speaker, headphones, or computer interface
Each block in that chain is an opportunity to either improve fidelity or seriously mess things up.
This is why receiver performance is usually described by the “three Ss + D”: Sensitivity, Selectivity, Stability + Dynamic Range [1].
Sensitivity: Hearing Signals That Barely Exist
One of the wildest things about radio is just how small the signals are. At the antenna terminals, we’re often dealing with femtowatts—10-15 watts—yet after running through the receiver chain, we expect clean audio at the speaker.
In modern HF rigs, sensitivity is rarely the limiting factor anymore. Al pointed out that, using SSB, the Kenwood TS-890 reliably hears down to 0.2 µV between 1.7 and 24.5 MHz [1]. That’s astonishing.
But sensitivity ultimately runs into physics:
Below ~30 MHz, the natural noise floor dominates.
In other words, there’s no point trying to design an HF receiver with another 6 dB of sensitivity—galactic noise, atmospheric noise, and man-made noise swamp the front end long before the Low-Noise Amplifier (LNA) does.
A few key measurements I found interesting:
- Noise Figure (NF) – becomes critical at VHF/UHF where sky noise drops dramatically
- MDS (Minimum Discernible Signal) – usually around −120 to −130 dBm for a good HF rig in 500 Hz BW
- SNR and SINAD – important especially for FM receivers
Signal-to-Noise And Distortion (SINAD):
Where
S = Signal
N = Noise
D = Distortion
expressed in decibels (dB).
Al Penney’s material notes:
For a VHF or UHF receiver, SINAD is typically around 12 dB for an input of 0.25 µV (≈ −119 dBm).
This is the standard sensitivity measurement used in commercial and amateur FM equipment.
At VHF/UHF, the internal noise of that first transistor—your LNA—sets the limit. HF? Not so much.
Selectivity: Where the Adults and Children Separate
Selectivity is the receiver’s ability to separate two closely spaced signals. And this, more than raw sensitivity, is what earns premium radios their price tag.
Al made a comment I loved:
Filter skirt steepness is what separates the adults from the children in HF receiver design. [1]
The skirts—the shape of the filter’s passband edges—determine how well a receiver rejects adjacent-channel interference. A typical SSB filter might have:
- –6 dB bandwidth: ~2.3 kHz
- –60 dB bandwidth: ~3.3 kHz
A shape factor near 2:1 at 6/60 dB is excellent.
It’s worth appreciating just how much of our “receiver experience” is determined by these numbers. A crowded contest weekend on 20 m can turn a mediocre receiver into a miserable one.
Digital Signal Processing (DSP) helps—adjustable bandwidth, notch filters, noise reduction—but the basic physics still matter.
Stability: The End of Drifty Radios
Anyone who has ever used an old tube rig on CW knows the pain of “chasing” your signal up or down the band as the oscillator warms up.
Drift simply isn’t a major issue anymore. Modern PLL and DDS synthesizers with temperature-compensated oscillators typically achieve stability in the low parts-per-million (ppm) range [1].
What DDS Does
A DDS system creates RF signals digitally by:
- Using a numerically controlled oscillator (NCO)
- Stepping through a digital sine wave lookup table
- Converting the digital waveform to analog using a DAC (Digital-to-Analog Converter)
- Filtering the output to produce a clean RF signal
This allows the radio’s VFO to tune in extremely fine steps (often <1 Hz) with excellent frequency stability.
Why Radios Use DDS
Compared with older analog VFOs or PLLs, DDS offers:
- Very high frequency stability
- Extremely fine tuning resolution
- Fast frequency changes (great for scanning or DSP features)
- Low drift, because it ultimately relies on a stable reference oscillator
- Ability to implement complex waveforms (FSK, PSK, chirps)
Still, your VFO is only as good as its reference. If the radio’s timebase drifts, so does every displayed frequency. That’s why Al reminded us to check against WWV, WWVH, or CHU periodically.
Dynamic Range
Although it doesn’t get listed alongside Sensitivity and Selectivity, the Dynamic Range of a receiver is every bit as important. It describes, in dB, the span of signal levels over which the radio can still do its job—pulling out the signals we actually care about without collapsing under the load. On the ham bands, this really matters because we routinely deal with whisper-weak signals sitting right beside blowtorch-strong ones, and a receiver with poor dynamic range simply can’t cope. There are a few different ways to measure it, but the rule of thumb is simple: the higher the dynamic range, the better the receiver will behave in real-world conditions.
Cross-Modulation and Intermod: The Curse of Strong Signals
Urban hams know this pain well: you’re trying to listen to a clean signal on 146.97 MHz, but the local taxi dispatcher or paging transmitter blasts into your receiver anyway.
Two related but distinct demons cause this:
Cross-modulation
A strong AM signal overloads the front end, causing its modulation to appear on your desired signal. It shows up when:
- You’re close to a high-power AM broadcast tower
- Your front-end LNA is being driven into nonlinearity
Attenuators help. So do front-end filters.
Intermodulation (IMD)
Two (or more) strong signals mix inside a nonlinear junction—an RF transistor, a mixer, even a rusty fence. This produces:
- f1 ± f2
- 2f₁ ± f₂
- 2f2 ± f1, etc.
On 2 m and 70 cm FM, IMD is especially brutal when driving through downtown due to the density of transmitters [1].
A high third-order intercept point (IP3) is your best defence.
A high IP3 is one of those specs that separates a decent receiver from a truly capable one. It’s a measure of how linear the receiver is—basically, how well it can deal with strong nearby signals without creating its own intermodulation junk. When multiple strong signals are floating around just outside your passband, a receiver with good IP3 will stay composed; a poor one will start generating intermod products that show up as false signals and make life miserable.
The higher the IP3, the more signal the front end can tolerate before nonlinearities kick in and begin creating interference that wasn’t actually there. In real operating conditions—especially in busy urban RF environments or during contests—high IP3 performance is your best defence against overload, distortion, and those phantom signals that clutter up the band. It’s a key part of keeping the receiver clean, stable, and able to pull in the signal you actually care about.
Explanation of Third-Order Intercept Point (IP3)
- IP3 is a theoretical power level where the power of the third-order intermodulation products theoretically equals the power of the desired signal.
- It defines the linearity limit of a receiver or amplifier; beyond this point, nonlinearities create distortion.
- A higher IP3 indicates better linearity and less distortion at high signal levels, which are crucial for maintaining signal integrity in crowded or high-signal environments.
Importance of high IP3 in Radio Receivers
- Receivers with high IP3 are better at handling strong signals from nearby transmitters or multiple signals simultaneously without generating spurious signals.
- This reduces false readings or signal interference, which is critical in applications such as amateur radio, communications, and radar.
Noise, Noise Everywhere
Noise sets the absolute bottom limit of what we can hear. Al divided it into two broad categories:
Natural Noise (QRN)
- Galactic background
- Atmospheric static
- Lightning
Lightning bursts are wideband, energetic, and brutal—but noise blankers can help.
Man-Made Noise (QRM)
This is the stuff we fight all the time:
- LED lightbulbs
- Switching power supplies
- Thermostats
- EV chargers
- Solar inverters
- Grow lights (ask any ham living next to a basement gardener…)
Al even mentioned Chinese HF radar systems—those rapid-fire pulses that light up the waterfall like a machine gun. We can’t fix those, but we can recognize them.
The only real cure for local noise? Track it down and eliminate it at the source. A portable receiver is your best friend here.
A Beautiful Example: The Humble Crystal Radio
Al finished with a walk-through of the simplest possible receiver: the classic crystal set like the one I built as a kid. No power supply. No amplifier. Just:
- A resonant LC circuit
- A diode detector (originally galena with a cat-whisker)
- High-impedance headphones
The entire audio output is derived from the energy in the received RF signal. That still blows my mind—especially knowing how much we depend on active devices today.
Crystal sets are a reminder that, at its core, radio is elegant physics. Everything else—DSP, PLL synthesizers, roofing filters—is refinement.
Final Thoughts
Every chapter in this course has filled gaps in my understanding, but this one tied everything together: noise, filters, amplifiers, mixers, all working in a carefully balanced dance. And as someone who spent a good chunk of the ’80s elbow-deep in analog circuits, it’s incredibly satisfying to revisit the fundamentals with fresh eyes.
Next week, we move into transmitter architecture. I’m looking forward to it—and I’m already expecting another “aha!” moment.
References
[1] A. Penney, Radio Receivers (RAC Basic Course – Chapter 14), Radio Amateurs of Canada, 2025.
[2] A. Z. Peebles, Communication System Principles, 4th ed. Reading, MA: Addison-Wesley, 1998.
[3] American Radio Relay League (ARRL), The ARRL Handbook for Radio Communications, 100th ed. Newington, CT: ARRL, 2023.
[4] B. Finio, ‘Build Your Own Crystal Radio’, Science Buddies. Accessed: Nov. 18, 2025. [Online]. Available: https://www.sciencebuddies.org/science-fair-projects/project-ideas/Elec_p014/electricity-electronics/crystal-radio
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