YIG Filters: Selection and Specification

A mistimed preselector can invalidate a receiver reading. Match the filter’s response and settled state to the wanted RF, LO timing and blocker positions before accepting data. This component guide covers magnetically tuned preselection; the YIG material guide explains the resonator.

Key specifications

Parameter	What it means	What it constrains
Tuning range	Span over which the magnet can place the band	Whether one filter covers the receiver, or you need several
Bandwidth	Passband width at 3 dB relative to minimum loss, or notch width at a specified rejection level	Channel width versus interferer rejection at the stated RF frequency
Stage count and coupling	Resonators and coupling geometry in the complete design	Check bandwidth, skirt rejection, ripple and loss together
Insertion loss	Passband loss at the tuned centre	System noise figure, if the filter sits before the LNA — or after it, the recovery you still need
Off-resonance isolation	Rejection away from the tuned band	Image rejection and blocking
Limiting level	Input level at a defined compression criterion	Nonlinear response under large signals; check damage rating separately
Settling time	Time to sit inside a frequency tolerance after a step	Ready time from actual command times, both tolerances and passband alignment
Passband VSWR / ripple	Match and flatness across the 3 dB width	Whether the LNA or mixer sees a stable load as you tune

Meanings are general to the device class. For values, work from the specific part’s datasheet — stage count, band and grade move every number.

Where it sits in the chain

In a receiver the filter is usually a tracking preselector: it sits after the antenna (and often after a limiter and LNA) and before the mixer, and it moves with the local oscillator. Its job is to pass the wanted RF and reject everything that would mix to the same IF — first of all the image, 2 × IF away. After the mixer it is too late; the image and the wanted signal are the same frequency.

In test equipment the same part is a tunable bandpass or band-reject in front of a detector or a spectrum analyser. The specification language does not change. The time budget does: record when the LO and filter are commanded and when each reaches its tolerance. Parallel commands overlap settling; serial commands include the delay before the second command.

How it works

The resonator is the same ferrimagnetic sphere used in a YIG oscillator. The difference is the coupling.

In a bandpass filter the input and output loops are set at 90° so that energy cannot cross except through the resonating spheres. Closer loop-to-sphere spacing widens the 3 dB bandwidth and raises insertion loss. Additional resonators and adjusted coupling can widen bandwidth and shape rejection; evaluate loss and skirts from the complete response, as described in the manufacturer’s YTF note, pages 1–3.

In a band-reject filter a single loop per sphere lies along the transmission path. Off resonance the loop is just line; on resonance it couples to the sphere and reflects. Rejection width depends on resonator count and adjusted coupling; check the complete response. Micro Lambda’s YTF note, page 2, describes 15 to 70 MHz at 40 dB rejection for its standard band-reject filters; check the selected model.

Both types follow coil current, not voltage. The driver is half the specification, for the same reason it is on a YTO.

For the figures we can attribute, the manufacturer’s published description gives standard bandpass 3 dB bandwidths of 15 to 40 MHz at 2 GHz and 20 to 50 MHz at 18 GHz, and standard band-reject parts with a 40 dB rejection bandwidth of 15 to 70 MHz.

The readable YTF technology note was reviewed on October 8, 2026. Pages 1–3 describe coupling, bandwidth and selectivity; page 4 defines tuning response against a specified frequency tolerance and sweep. These definitions support a response-based comparison, not guaranteed loss or rejection from stage count alone.

Bandwidth is not quite constant

Micro Lambda’s YTF note, page 2, describes approximately constant absolute bandwidth with an expansion of about 20% per octave. Its example is 30 MHz at 2 GHz becoming 50 MHz at 18 GHz; this is a manufacturer example, not a guarantee for every filter. Specify the width at the frequencies you will use, not only at the bottom of the band.

The manufacturer’s note gives 0 to +10 dBm as a limiting example, not a universal range. Its page 2 definition uses 1 dB compression of the transfer characteristic. Confirm the model’s limiting level and separate damage rating.

Against the alternatives

Criterion	YIG-tuned filter	Cavity filter	Switched filter bank
Tuning	Electrical, octave or more	Mechanical, or none	Discrete bands only
Bandwidth vs frequency	Roughly constant absolute width	Percentage of a fixed centre	Whatever each cavity was cut for
Insertion loss	Read the complete coupled response	Check loss at the required bandwidth	Include each filter and switch path
Selectivity	Stage count and coupling shape rejection	Response depends on design and bandwidth	Verify rejection in every selected band
Settling	Measure step and tolerance with the driver	No tuning for a fixed filter; include switching if used	Switch and downstream recovery time
Power handling	Limited by sphere saturation	High; radar and SATCOM use it	High per path
Size and power	Magnet dominates; coil current continuous	Machined metal; no coil	Several cavities plus switches
Typical use	Tracking preselector, analyser	Fixed channel, high power	A short list of bands

The pattern matches the oscillator comparison: YIG buys a moving, constant-width band and pays in loss, speed and size. When the band does not move, a cavity is the better filter. When you need both low loss and several discrete bands, a switched bank is the honest alternative — not a “faster YIG”.

Application matrix

Application	What dominates the choice	Where a YIG filter struggles
Spectrum / signal analyser preselector	Multi-octave tune at a constant absolute width	Sweep rate; insertion loss ahead of the converter
EW / SIGINT receiver	Tracking the LO across the threat band	Hop speed; limiting on large signals
Spectrum monitoring front end	Coverage plus a tracking pair with the LO	Combined settling of filter and oscillator
Radar front end	Image and blocker rejection that can move	Pulse-to-pulse agility; power handling
ATE / bench filtering	A single tunable bandpass or notch	Cost and coil power versus a drawer of cavities
SATCOM channel filter	Almost never YIG	Loss, size and a band that does not move

Qualitative. Every row is a judgement about which parameter binds first, not a vendor recommendation.

A monitoring or EW front end may pair the filter with a YIG-tuned oscillator. Measure their ready times and tracking error together using the budget below.

Selection criteria

Buying and lifecycle

• Specify the response at required offsets and bandwidth. Extra sphere count alone is not an acceptance criterion.
• Ask at which frequency the 3 dB width was measured. The ~20% per octave creep makes a single number incomplete.
• Ask what driver the accuracy figure was taken with. Current noise on the coil is frequency modulation of the passband.
• Check limiting against the largest signal the antenna can present, including after the LNA if the filter sits there.
• Plan calibration with the LO. A tracking pair that is calibrated apart will not track in the box.

Obsolescence on a low-volume magnetic part is the same problem as on a YTO; the general discipline is in sourcing and quality.

Testing and integration notes

• Measure isolation at the image frequency of the actual IF plan, not only at a convenient offset on the bench.
• Verify settling with the same step size the LO will take. A full-band figure does not describe every small correction.
• Sweep insertion loss and VSWR across the tune; a filter that is pretty at centre and ugly at the band edge is a common surprise.
• Confirm the limiting level with the real source, using a defined input level and compression criterion.
• Check magnetic clearance before the enclosure is fixed. The control variable is a field.

Why spectrum analyzers use a YIG preselector

Keysight’s Spectrum Analysis Basics, chapter on preselection describes a tracking YIG-tuned filter ahead of the microwave input mixer. Selecting the wanted RF region before mixing suppresses image and other unwanted conversion responses that an IF filter cannot distinguish afterward. That filtering is not the analyzer’s resolution bandwidth: the preselector protects the conversion process, while later filtering determines which nearby IF components are resolved.

For a receiver using the same idea, check three things: the wanted signal stays within the tracked passband, the filter has settled before measurement, and its loss is included in the receiver sensitivity budget. Keysight also discusses preselector centering and post-tuning drift; a peak response obtained before centering is not automatically a valid amplitude comparison. Material background belongs in what YIG is, and the wider technology trade-off in YIG versus cavity filters.

Set a tracking budget, not two independent specifications

The common YIG-filter application is a receiver whose LO and preselector move together. It is tempting to specify oscillator tuning accuracy on one sheet and filter tuning accuracy on another, then expect the pair to work after installation. The meaningful quantity is their relative error. The filter has to remain centred on the wanted RF while its reject band stays useful at the image or blocker location. Two individually accurate current tables can still drift apart after temperature, cable resistance or a different approach direction are introduced.

Begin by deciding what the filter must pass. Include the wanted signal bandwidth, modulation shoulders, frequency error, calibration error and any required margin. Then state the rejection required at the calculated image and known blocker offsets. This creates an allowed tracking error across the band. A wide passband is more forgiving of tracking but sacrifices rejection; a narrow passband protects the mixer but makes error and settling visible. The right budget comes from the receiver plan, not from a favourite stage count.

Use this fill-in timing record with the actual driver, temperature and tuning direction. The ready rule below is an editorial budgeting method based on the manufacturer’s frequency-tolerance definition and Keysight’s preselection checks, not a measured device specification.

Budget item	Requirement / allowed error	Command and evidence to record	Acceptance / ready criterion
LO path	Frequency step: ; allowed frequency error: ±	Command time t_LO: ___; measured time into and staying within tolerance: s_LO ___; lock/reference state: ___	Meets frequency and required lock/noise limits
Filter path	RF center: ; allowed center error: ±	Command time t_filter: ___; measured settling interval s_filter: ___; driver/temperature/direction: ___	Wanted RF remains in the permitted passband
Tracking	Wanted bandwidth/margin: ; allowed relative error: ±	Measured center-to-wanted-RF error: ___; image/blocker offsets: ___	Passband alignment and rejection both meet the receiver plan
Receiver ready	Latest acceptable ready time: ___	Earliest candidate = max(t_LO + s_LO, t_filter + s_filter); include other recovery: ___	Both paths remain in tolerance and alignment passes before accepting data

For parallel commands at the same start time, the longer measured settling interval controls the earliest candidate. For serial commands, enter the actual later command time; add delays only when the sequence imposes them. A lock indication alone does not prove filter alignment. Test representative steps and reversals as discussed in how YIG oscillators work.

Noise and loss remain separate constraints. A quiet LO does not compensate for a preselector that lets a strong blocker reach the mixer, while a narrow filter does not improve a noisy LO at an in-band offset. Conversely, every dB of filter loss ahead of the first gain stage costs sensitivity. Test the complete chain at the intended RF points with desired and blocker signals, then compare the result with the phase-noise requirement rather than assigning all degradation to one part.

For maintainable systems, store the filter calibration, driver revision, temperature reference and acceptance sweep with the serial number. A replacement filter may match the catalogue range yet require its own table; traceable test evidence is what turns an interchangeable-looking component into a reliable tracking pair.

Related guides

• What is YIG — the material and why a sphere tunes at all
• YIG filter vs cavity filter — the decision this pillar assumes
• RF, IF and LO explained — why the image is 2 × IF away
• YIG oscillators — the usual tracking partner
• Understanding phase noise — reciprocal mixing is why the preselector exists
• Radar RF components — how the same parts are constrained in a radar