Receiver Performance — What Makes a Good Receiver
Why Receiver Specs Matter
On a busy HF band, your receiver might be dealing with signals ranging from barely detectable (−130 dBm) to extremely strong (−10 dBm) — a range of over 100 million to one! A good receiver handles both extremes simultaneously.
Sensitivity — Hearing the Weak Ones
Sensitivity is limited by noise. Every receiver generates some internal noise, and the signal must be stronger than this noise floor to be heard.
The noise figure of the front end is the key specification. It tells you how much noise the receiver adds compared to a perfect receiver.
\( NF_{total} \approx NF_1 + \frac{NF_2 - 1}{G_1} + \frac{NF_3 - 1}{G_1 G_2} + \cdots \)
Translation: the noise figure of the first stage dominates, because noise from later stages gets divided by the gain of earlier stages. This is why a low-noise preamp at the antenna makes such a difference.
Intermodulation Distortion (IMD) — The Strong Signal Problem
When two strong signals hit a receiver, any non-linearity creates phantom signals at new frequencies. The most troublesome are third-order products:
These fall close to the original signals and can't be filtered out!
Real-world example: Two strong stations on 14.200 and 14.210 MHz create a phantom signal at 2(14.200) − 14.210 = 14.190 MHz. If you're trying to hear a weak station on 14.190, it's buried under this IMD product.
IP3 — Third-Order Intercept Point
IP3 is the key specification for strong-signal handling. Higher IP3 = better. It's the theoretical point where the IMD products would equal the wanted signal. In practice, you never reach it — it's just a figure of merit.
Reciprocal Mixing — The Dirty Oscillator Problem
No oscillator is perfect — they all have some phase noise (random frequency jitter). When a strong off-channel signal mixes with this phase noise, it creates noise at the IF that buries weak signals.
Better local oscillator = cleaner reception. This is one reason why more expensive radios perform better on crowded bands.
Dynamic Range — The Full Picture
Dynamic range is the difference between the weakest signal the receiver can detect and the strongest it can handle without distortion. More dynamic range = better performance on busy bands.
A receiver with 90 dB of dynamic range can simultaneously handle signals differing by 90 dB — about 1 billion to one in power!