Frequency Converters: Specifying the Module

A mixer translates frequency. A converter does everything that has to be true for the translation to be usable: it rejects the band that would otherwise arrive on top of the wanted one, it supplies an LO clean enough not to spoil the signal, and it hands the result to the next stage at a level and in a passband something downstream can accept. That is why converters sit in this components section as their own family rather than as an accessory to the mixer page.

The practical consequence is that most of a converter datasheet describes parts that are not the mixer.

Reading the datasheet

Parameter	What it describes	What it constrains	Where it comes from
Conversion gain	Net gain of the assembly	Level presented to the next stage	Mixer conversion gain or loss plus amplifier gain
Noise figure	Assembly noise	Sensitivity	Dominated by whatever is first in the chain
Image rejection	Suppression of the second input band	Which off-channel signals reach the IF	Preselector, image-reject mixer, or IF choice
Isolation and LO leakage	LO power escaping to other ports	IF stage saturation, radiated interference	Mixer balance plus filtering
Spurious, in dBc at stated offsets	Unwanted mixing products	Which frequency plans are usable	LO and RF harmonic combinations
P1dB and IP3	Large-signal behaviour	Upper end of dynamic range	Mixer topology and available LO drive
LO phase noise	Spectral purity of the LO	EVM, reciprocal mixing, close-in noise	The LO source and how it is locked
Gain flatness	Gain against frequency	Equalisation and multi-carrier balance	Filters and amplifier response

Use model-specific RF/LO/IF bands, drive, temperature and reference planes. No mixer-class range is an acceptance limit.

What is actually in the box

The satellite world names the assemblies plainly, which makes it a good place to see the pattern. A block upconverter is described as an outdoor unit that upconverts an L-band IF, at the IF band specified for that unit, to the transmit frequency, and its published contents are a mixer, an LO synthesizer locked to a reference, an image-reject filter and a power amplifier. The receive-side counterpart, the low-noise block, is the same idea with an LNA in place of the PA.

Four blocks, four different specifications, one part number. A general-purpose lab converter, an EW receiver front end and a point-to-point radio module differ in which of the four dominates, not in the list.

The frequency relationship itself is arithmetic: for the upconverter above, the output is the LO plus the IF. Which sideband you keep, and which one you have to remove, is the subject of upconverter vs downconverter and the frequency-plan identities in RF, IF and LO explained.

The mixer sets a floor, not the result

A passive diode mixer has conversion loss; an active mixer can provide conversion gain. Marki Microwave’s public Mixer Basics Primer, sections II and IV.A discusses diode mixers driven by a strong LO relative to the RF signal. Its loss ranges do not specify one topology, frequency and drive condition, so they are not retained here as selection limits. ADI’s MT-080, Active Mixers, pages 10–11 describes active alternatives.

Example, assumed inputs: a module has 20 dB net conversion gain and uses a passive mixer with positive conversion loss. The surrounding gain offsets that loss; the mixer still has its own compression limit. This example does not apply to every active mixer or module. Check the assembly’s gain distribution and noise figure at its stated ports.

Port isolation describes leakage between specified ports. Marki’s section IV.B distinguishes LO-to-RF, LO-to-IF and RF-to-IF leakage; section IV.C ties diode-mixer compression to LO drive. Compare each at the candidate’s actual RF/LO/IF frequencies, recommended drive and terminations rather than using a class-wide range.

Takeaway: Record the mixer architecture and test conditions before interpreting module gain or headroom.

A pinout change on the mixer itself is a separate check from the module numbers above. The port-swap sheet says which exchanges a bare passive diode mixer can be measured in, and where an amplified converter leaves that table.

Spurious products are a plan, not a fault

A diode mixer produces output at combinations of LO and RF harmonics. Marki’s public primer, sections II–III explains how balance suppresses particular product families; suppression levels still depend on the real circuit and drive.

What matters for specification is that every remaining product has an address. Its frequency is a fixed combination of your LO and RF frequencies, so it moves when the frequency plan moves. A spur that lands inside the IF passband at one LO setting may be harmless at another, and a single-path IF filter cannot separate it if it overlaps the wanted channel. Topology, balance and drive can reduce its level; converter datasheets therefore need conditions and offsets, not only one number.

When intermodulation falls inside the wanted IF channel, ordinary IF filtering cannot separate it from the signal. Assess tone spacing and levels in the dynamic-range budget; see the primer, section IV.G.

Where the image goes

Every downconversion has a second input frequency that converts to the same IF. There are three ways to deal with it, and a converter datasheet is partly a statement of which one the vendor picked.

Filter it. A preselector ahead of the mixer removes the image band before it reaches the mixer. This works well when the IF is high, because the image is then far from the wanted signal. It gets expensive as the IF falls and the two move closer together. When the receiver has to tune, the filter has to tune with it, which is the case for YIG-tuned filters; when the band is fixed, a cavity is usually the better filter.

Cancel it. An image-reject mixer splits the signal into quadrature paths and combines them so the image lands at a port that can be terminated. The rejection then depends on how well matched those paths are: 10° of phase unbalance and 1 dB of amplitude unbalance, described as easily achieved, give about 20 dB of image rejection, and the same manufacturer reports SMIQ-65H-D+ results at a 200 MHz IF in figure 9: typically 20 dB across most of its band, better than 25 dB from 40 to 52 GHz. Useful, integrated, and not on its own a substitute for preselection when the interferer is strong.

Plan around it. Choosing an IF that puts the image where nothing transmits costs nothing in hardware and is the first thing to try. It is also the option that disappears once someone else has chosen the module’s IF for you.

The LO is the other half of the converter

The LO is the block that most often decides whether a converter is acceptable, and the one least visible in a block diagram. Its phase noise contributes to the converted signal; noise inside the wanted IF passband cannot be removed by ordinary IF filtering: for the mechanism and how to read L(f), see phase noise. Where the LO comes from is a component decision in its own right — a synthesizer when frequency accuracy and step size matter, a YIG-tuned oscillator when the range is an octave or more and close-in noise has to stay low.

Three LO properties reach the module specification directly: spectral purity, which becomes EVM or reciprocal mixing; step size and tuning range, which decide the frequency plans available to you; and settling time, which decides how fast the converter can be retuned. A radar or EW receiver that hops is buying the third one as much as the first.

Which parameter binds, by application

Application	Binding constraint	What that forces
Wideband receiver or EW front end	Image and out-of-band interference	Tracking preselection and a wide, quiet LO
Spectrum analyser front end	Spur-free dynamic range	High first IF and a frequency plan chosen around spur addresses
Radar receiver	Retune or hop time	LO settling, not conversion gain
Satellite uplink chain	Output power and frequency stability	Locked reference and specified gain flatness
Point-to-point radio	EVM of a modulated carrier	LO phase noise budget and IF flatness
Laboratory band translation	Level accuracy and repeatability	Known conversion gain over temperature, and headroom

One constraint per row is deliberate. When two of these bind at once the answer is usually a different frequency plan, not a better module.

The radar row is worked through in radar RF component selection, where the waveform, not the part list, decides which constraint binds first.

Integration and test

Filtering interacts with the mixer rather than sitting politely after it. A reflective filter returns out-of-band energy to the mixer port, which shows up as conversion-loss ripple and as spurs that were not in the plan; the standard mitigations are to place the filter close to the output, and to use a terminated diplexer or a small pad where the interaction persists. Inside a module this has already been done for you, well or badly, and it is one of the things you are paying for.

Three checks are worth doing on receipt, because they catch the mismatches a datasheet cannot:

• Measure conversion gain at the LO drive and input level you will actually use, not at the datasheet’s condition.
• Sweep the LO across your real plan and look for spurs in the IF passband, rather than trusting a single quoted offset.
• Check the IF port for LO leakage with the IF amplifier in circuit, since that is where isolation turns into compression.

When the module you want is discontinued — common for converters, which are often built in small volumes for one programme — the sourcing route and its verification steps are covered in obsolete RF component sourcing.

Compare modules under one frequency plan

Fill one record per candidate, then measure with the actual LO, IF filter and gain stages. Use the RF, IF and LO guide to calculate the image before comparing modules.

Input or check	Project requirement	Candidate evidence / result
RF / LO / IF bands and injection side	Fill: ranges and image location	Fill: supported bands and measured response
Wanted signal and strongest blocker	Fill: levels, frequencies and IF bandwidth	Fill: compression, noise rise and rejection
Gain, P1dB and IP3 conditions	Fill: input level, LO drive and tone spacing	Fill: datasheet condition and test result
Reference planes and installation	Fill: input/output connectors, cables and filter order	Fill: gain, leakage and spurs at those planes

Takeaway: Accept the installed converter under the same plan and measurement conditions used to compare it.