Phase Noise in Ham Radio: Reciprocal Mixing and Dirty Transmitters

When a very strong station is on the band a few kilohertz away from the one that you want to hear, the noise floor of your receiver rises as if somebody had turned up a hiss control, and the weak signal disappears. The strong station is not overloading the receiver in the usual sense. Nothing is clipped, the AGC is behaving, and the filters are doing their work. The cause is a quieter and more subtle defect of every oscillator in the radio: phase noise. It limits how well a receiver can work next to a strong signal, and the same defect in a transmitter spreads a faint skirt of noise around the carrier that can be heard hundreds of hertz or several kilohertz away. This guide explains what phase noise is, why it matters in contests and at the field day site, how to interpret the figures in dBc/Hz and what you can do about it with a normal budget.

What Phase Noise Is

An ideal oscillator makes a pure sine wave at a single frequency. A real oscillator is not perfect: its phase wobbles by tiny random amounts from one cycle to the next, because of thermal noise in the transistors, noise in the control voltage and imperfections in the reference crystal. The Wikipedia article on phase noise describes it as the frequency domain representation of the rapid, short-term, random fluctuations in the phase of a waveform. In a spectrum display the result is not a single line but a line with skirts on both sides, strongest near the carrier and falling away with distance.

The skirts are measured in decibels relative to the carrier in a bandwidth of 1 Hz, written dBc/Hz, at a given offset. A figure of -120 dBc/Hz at 10 kHz means that at 10 kHz from the carrier the noise in each hertz is 120 dB below the carrier power. The better oscillator has the more negative number. A very good HF transceiver may reach -140 dBc/Hz or better at 10 kHz, while a simple design with a poor synthesizer may show -110 dBc/Hz. The figures on a data sheet are usually given for several offsets, such as 1 kHz, 10 kHz and 100 kHz, because the skirt is not flat.

Where It Comes From

A modern transceiver does not use one fixed oscillator. It uses a synthesizer that generates the local oscillator, or LO, at any frequency in the band, usually with a phase-locked loop that compares a voltage-controlled oscillator with a stable crystal reference. The noise of the reference, the noise of the phase detector, the design of the loop filter and the noise of the voltage-controlled oscillator all add to the total. Direct digital synthesis can give a very clean output at close offsets, but its spurious lines and its own noise floor far from the carrier need attention. The designers of the radio balance these effects, and the result is a number that is quoted in the specification and measured in reviews.

The same oscillator is used in the receiver and in the transmitter. In a superheterodyne receiver the LO mixes with the incoming signal to produce the intermediate frequency. In a transmitter the LO or a related source mixes with the audio or the exciter signal and produces the carrier that goes to the amplifier. Because the same noise is present, the defect shows up in both directions, as described in the following sections.

Reciprocal Mixing in the Receiver

A mixer multiplies the incoming signals by the LO. If the LO has noise skirts, the multiplication moves them. A strong unwanted signal at 10 kHz from the wanted frequency mixes with the noise of the LO that is 10 kHz away from the main line, and the product falls into the intermediate frequency passband. The strong signal acts as if it were a source of broadband noise on top of the weak one. This is called reciprocal mixing. The noise it creates is proportional to the level of the strong signal, so a stronger neighbour makes the noise worse decibel for decibel.

A calculation shows the size of the problem. Assume a strong signal at -20 dBm, which is about S9 plus 53 dB, 10 kHz away from the wanted frequency. The receiver uses a bandwidth of 2.4 kHz, which is 34 dB above 1 Hz. If the phase noise is -130 dBc/Hz at 10 kHz, the noise produced is -20 – 130 + 34 = -116 dBm. The noise floor of a good receiver with a noise figure of 10 dB in the same bandwidth is -174 + 34 + 10 = -130 dBm. The reciprocal mixing noise is 14 dB above the floor, and a weak signal that would have been copied is lost. If the phase noise were -145 dBc/Hz, the noise would be -131 dBm, about the same as the receiver noise, and the problem would not be noticed. That is the reason why a lower number in the specification matters so much.

Dirty Transmitters

The same defect in the transmitter makes the noise a property of the signal that goes out. A transmitter with a phase noise of -130 dBc/Hz at 10 kHz, running 100 watts, produces noise at 10 kHz offset of 50 – 130 = -80 dBm per hertz at its output. In a 2.4 kHz bandwidth this is -80 + 34 = -46 dBm. If another station is close, so that the coupling loss between the antennas is 40 dB, that station receives the noise at about -86 dBm, which is roughly S7 on the meter. The two stations do not need to be on the same frequency; they only need to be near each other on the same band. At a contest site with several transmitters or in a neighbourhood with two active stations, the quality of the transmit synthesizer decides whether they can work together.

The effect should not be confused with the clicks and the splatter described in our guide to key clicks and splatter. Those come from the shaping of the envelope or the overdriven amplifier and they are cured in the keying or in the audio settings. Phase noise exists on a steady carrier, and there is nothing that the operator can adjust, other than reducing the power or choosing a different radio. A rule is that the noise falls with the power, decibel for decibel, so a transmitter that runs 10 dB lower in power makes 10 dB less noise.

Typical Values and What They Mean

The values are only a guide. Different radios give the best results at different offsets, and a number at 1 kHz may be much worse than at 100 kHz. If you compare radios, compare them at the offsets that matter for your use: close-in values of 1 to 5 kHz for crowded CW bands, and 10 to 20 kHz for the neighbours in a contest.

How It Differs From Blocking and Intermodulation

Phase noise is one of three limits of a receiver next to strong signals. The others are blocking, where a strong signal compresses the front end and desensitizes it, and intermodulation, where strong signals create false products. The first is explained in our article on receiver dynamic range, blocking and IMD. The three are different, and the cure for one is not always the cure for another. An attenuator in front of the receiver helps with blocking and intermodulation, because those effects fall faster than the wanted signal when the level is reduced, as our guide to the preamplifier and attenuator shows. It does not help with reciprocal mixing, because the noise made by the strong signal falls by the same amount as the wanted signal. The ratio of wanted signal to reciprocal noise does not change.

The same reasoning explains why a better IF filter after the mixer cannot help: the noise is already in the passband when it leaves the mixer. The cure is to reduce the strong signal before the mixer, or to use a cleaner oscillator. Direct sampling receivers and traditional superheterodynes have different phase noise behaviour because of the clock source and the architecture, as we compare in the article on direct sampling and superheterodyne receivers.

What You Can Do About It

• Reduce the strong signal at the antenna. A directional antenna, a receiving loop or a null toward the strong station reduces the unwanted signal and the noise it creates.
• Use a separate receive antenna. A second antenna with a different pattern can put the strong signal into a null that the main antenna does not have.
• Add band-pass filters for multi-station sites. They cut down the energy of the transmitter noise on the other bands, but they cannot remove noise on the same band.
• Lower your own power if a neighbour suffers. A reduction by 6 dB halves the field of noise and may be enough to restore peace.
• Choose equipment with published measurements. Independent laboratory tests give figures for phase noise, and a radio with a good value at 10 kHz is worth the price in a crowded environment.
• Increase the distance between stations. Every doubling of the distance adds about 6 dB of attenuation to the noise that reaches the other station.

Reading the Spectrum Display

A spectrum scope or waterfall can show phase noise directly. A very strong signal that is displayed with a narrow span shows a smooth shape, and a skirt of noise on both sides that falls off gradually. If the skirt is higher than the noise floor of the display at an offset of 5 or 10 kHz, you are looking at the transmitter noise of the signal, or at the limits of your own receiver, or both. To tell the difference, change the attenuator setting. If the skirt disappears with the attenuator on, the noise was due to the overload of the receiver. If the skirt stays at the same height above the floor, it belongs to the transmitted signal. The practice is useful, and the display of our guide to the panadapter explains how to set the span and the reference level for such a check.

The Bottom Line

Phase noise is the short-term random fluctuation of an oscillator, shown as noise skirts around the carrier and measured in dBc/Hz at a given offset. In a receiver it causes reciprocal mixing, which raises the noise floor next to a strong signal by an amount equal to the strong signal plus the phase noise plus the bandwidth in dB. In a transmitter it creates noise that nearby stations hear, in proportion to your power. An attenuator and a narrower filter do not cure reciprocal mixing; the answer is a cleaner oscillator, a lower signal level at the antenna, a directional or separate receive antenna, a lower power for a neighbour or more distance. Compare radios on their measured values at the offsets that matter to you.

Frequently Asked Questions

What is phase noise?

It is the short-term random variation in the phase of an oscillator signal. In the spectrum it appears as a skirt of noise on both sides of the carrier, measured in dBc/Hz.

What is reciprocal mixing?

It is the mixing of a strong unwanted signal with the noise sidebands of the receiver oscillator, which puts noise in the passband and raises the noise floor.

Does an attenuator reduce reciprocal mixing?

No. The attenuator lowers the wanted signal and the noise produced by the strong signal by the same amount, so the signal to noise ratio stays the same.

How can I tell whether the noise comes from my receiver or from the other station?

Switch on the attenuator. If the noise falls with it, the receiver was overloaded. If the noise stays at the same height above the floor, it was in the transmitted signal or the oscillator.

Why does a dirty transmitter matter more with high power?

The noise is proportional to the carrier power, so doubling the power raises the noise by 3 dB. At multi-operator contest sites this adds up quickly.

What phase noise value is good enough?

For normal operation about -130 dBc/Hz at 10 kHz is good. For contesting and for sites with several stations, -140 dBc/Hz or better is preferred.