RF, IF and LO Explained

Every heterodyne radio has three frequencies, and most arguments about mixers, filters and oscillators are arguments about how those three are placed. This page names them and shows why the image is a planning problem. The rest of the signal chain section is what you do with the plan.

The three names

RF is the frequency at the antenna — the thing the system is trying to receive or transmit.

LO is the local oscillator: a tone the designer puts next to the RF so a mixer can translate it. In a receiver the LO is usually the part that tunes. Moving the LO is moving the radio.

IF is what comes out of that translation — low enough, or convenient enough, that the rest of the chain can filter, amplify or digitise it.

The identity for a downconverting mixer is:

|RF − LO| = IF

That is also the identity for downconversion versus upconversion. The same ports, the opposite direction.

None of the three names is a component family. An LO can be a YIG oscillator, a VCO or a DRO. An IF can be 70 MHz or 1.2 GHz. RF is just “wherever the antenna is”.

The image in a conventional nonzero-IF plan

For a conventional real-mixer plan with LO > IF > 0, two positive RF input frequencies differ from LO by IF: one above LO and one below it. One is wanted; the other is the image. The IQ exception below concerns how those responses are separated.

The spacing is forced:

|RF − image| = 2 × IF

On high-side injection (LO above RF) the image sits above the LO. On low-side injection it sits below. Both put the image 2 × IF from the wanted signal. After mixing into one real-IF path, the two responses occupy the same IF. An ordinary IF filter cannot separate them. A preselector can attenuate the image before mixing; a quadrature architecture provides a separate cancellation route.

That is why ordinary single-path IF filtering is not an image-rejection plan.

The IQ image-reject exception

Mini-Circuits’ public I&Q mixer explanation, figures 4–7 shows two mixers producing quadrature IF outputs. The wanted and image signals have different relative phase relationships in those paths. An IF quadrature hybrid combines them so the wanted response goes to one output and the image to the other, where it is terminated. The retained two-path information enables cancellation; amplitude and phase imbalance limit practical rejection.

Takeaway: Name the RF preselector or the complete IQ cancellation path as the image-rejection owner.

A worked placement

Example, assumed inputs: RF = 10 GHz, IF = 1 GHz, high-side injection in a conventional difference-frequency receiver.

• LO = RF + IF = 11 GHz
• Image = LO + IF = 12 GHz
• Image − RF = 2 GHz = 2 × IF

The preselector must pass the wanted band near 10 GHz and meet the project’s rejection target at 12 GHz. If, as additional illustrative assumptions, a YIG filter has a 30–50 MHz 3 dB width or a cavity has 5% width at 10 GHz (500 MHz), those widths alone establish no rejection guarantee. Check filter order, measured skirt, blocker level and required attenuation. In an alternative assumed plan with IF = 100 MHz, image spacing is 200 MHz; reassess the same requirements rather than declaring either filter sufficient.

Low-side injection on the same RF and IF puts the LO at 9 GHz and the image at 8 GHz. The filter problem is identical in width and on the other side of the RF. Choose the side that puts the image somewhere easier: off the antenna band, away from a known jammer, or onto a slope you already have.

The IF is a trade, not a quality

IF choice	What it helps	What it costs
Higher IF	Image farther from RF; easier preselect	Harder IF filter, ADC or second conversion
Lower IF	Easier digitiser and channel filter	Image closer; sharper RF filter required
High-side LO	Image above the RF	LO must reach RF + IF; possible harmonic issues
Low-side LO	Image below the RF; sometimes a cheaper LO band	Possible in-band image if the RF span is wide
Zero IF / direct conversion	No analog IF; no classical image at 2 × IF	I/Q imbalance, DC offset, a different image (the opposite sideband)

Qualitative planning table. A dual-conversion radio is this table applied twice, with a first IF chosen to make the first image easy and a second IF chosen to make the channel filter easy.

Wideband microwave receivers often pick a first IF high enough that a tracking YIG preselector or a simple fixed filter can kill the image, then convert again to a second IF the ADC likes. That is two LO plans, two images, and two places to spend isolation. It is also why a single “the LO” conversation is usually incomplete.

The LO is a phase-noise port

The mixer multiplies the RF by whatever the LO actually is — including the LO’s phase noise. A strong unwanted signal offset by Δf from the wanted RF mixes with the LO noise at that same Δf and lands in the IF as if it were on-channel. That is reciprocal mixing. No IF filter removes it.

Two hardware responses, and they do different jobs:

• A preselector attenuates the interferer before the mixer, which attenuates the reciprocal-mixing product with it, at the cost of insertion loss.
• A quieter LO reduces the noise density at Δf. A tracking YIG preselector cannot fix a noisy oscillator.

Which offset Δf matters is a system question: adjacent-channel, image, or a known blocker. Quote phase noise there, not at the datasheet’s favourite marker.

Common planning mistakes

• Assigning image rejection to an ordinary single-path IF filter. Overlapping responses require RF preselection or a specified IQ cancellation architecture.
• Picking the LO side after the preselector is bought. High-side and low-side put the reject band on opposite sides.
• Ignoring the second conversion. Dual-conversion radios have two images. The pretty first-IF plan can dump a second image onto something you care about.
• Specifying the LO as a frequency only. The LO is also a noise source at every offset an interferer can occupy.
• Forgetting that the preselector and a YTO both settle. A tracking pair has one time budget, not two.

Where this plan turns into a component choice: YIG oscillator vs VCO vs DRO for the LO, YIG filter vs cavity for the preselector, and radar RF components for the same chain under a waveform constraint.

Keep the plan reviewable

Fill one record per conversion and repeat it for credible blocker or spur cases. Include a second conversion separately. Calculate frequency locations first, then assign the rejection to actual hardware or an IQ cancellation path.

Field	Plan entry	Evidence / acceptance result
Wanted RF and channel width	Fill: range, bandwidth and wanted level	Fill: gain and usable passband
LO / IF / injection side	Fill: ranges and selected product	Fill: supported bands and calculated image
Image / harmonic / reference spur	Fill: frequency range and expected level	Fill: rejection at relevant frequencies
Largest in-band / out-of-band blocker	Fill: transmitter, level and offset	Fill: overload and reciprocal-mixing results
Rejection owner / reference plane	Fill: preselector, IQ path or other block and connector	Fill: installed order and rejection evidence
Environment and readiness	Fill: temperature, LO/filter command times and tolerances	Fill: later readiness and test conditions

Compare upconverter and downconverter placement where the direction changes. For acceptance, inject desired, image and credible spur inputs at representative tuning points; record false IF responses and blocker-induced noise rise with the installed filters. Read the noise requirement with the phase-noise guide.

Takeaway: Keep frequency, level, rejection owner and measured result together so another engineer can challenge the plan.