Active Components

Conversion Gain (Mixer)

Q: What is the difference between conversion gain and conversion loss?

They describe the same IF-to-RF power ratio with opposite sign conventions. Conversion gain is used when the mixer adds net signal power, which happens in active mixers built from transistor transconductance stages, such as Gilbert cells; a typical active downconverter shows +8 to +15 dB. Conversion loss is the term used for passive mixers built from Schottky diodes or FET switches, where the output is always weaker than the input by 4 to 9 dB; this is equivalent to a conversion gain of -4 to -9 dB. A passive double-balanced diode mixer with 6.5 dB conversion loss has a conversion gain of -6.5 dB. Both are measured at a single RF, LO, and IF frequency set with the LO at its specified drive level.

Q: How does LO drive level affect mixer conversion gain?

Conversion gain rises with LO power until the mixing devices are fully switched, then flattens. For a passive diode mixer, conversion loss improves by roughly 0.5 to 1 dB as LO power increases toward the rated level, for example +7, +13, or +17 dBm for level-7, level-13, and level-17 cells, then stays nearly constant above that. Under-driving a +7 dBm cell by 3 to 4 dB can add 1 to 2 dB of conversion loss and degrade the third-order intercept. Active mixers are less sensitive once the switching pairs are saturated, but excessive LO drive raises LO-to-IF leakage and DC offset. Designers specify conversion gain at the nominal LO power and verify it holds across the LO power tolerance.

Q: How is conversion gain measured on a mixer?

Apply a single-tone RF signal at a known power, typically -20 to -30 dBm to stay well below the 1 dB compression point, drive the LO port at its rated power and frequency, then measure the resulting IF tone power on a spectrum analyzer. Conversion gain in dB equals the measured IF output power in dBm minus the RF input power in dBm. The measurement must account for cable and connector loss on both the RF input and IF output paths, and a bandpass or lowpass filter is used on the IF port to reject the LO feedthrough, the RF leakage, and the image and sum products so only the wanted IF tone is read. Sweeping RF frequency while holding the IF fixed maps conversion gain flatness across the band.

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Expressed in dB, this figure of merit is the ratio of the IF output power a mixer delivers to the RF input power applied to it, measured at a fixed set of RF, LO, and IF frequencies with the local oscillator driven to its rated level. Active topologies built around transistor transconductance stages, such as the Gilbert cell, produce a net positive value of roughly +5 to +20 dB, relaxing the gain demand on the following IF amplifier. Passive diode and FET-switch mixers instead show a negative value equal to their conversion loss, typically -4 to -9 dB. The sign and magnitude flow directly into the cascade gain and noise budget through the Friis equation, so an accurate conversion-gain number is essential for predicting receiver sensitivity.

Category: Active Components

Active mixer gain: +5 to +20 dB

Passive mixer gain: −4 to −9 dB

How Mixers Add or Subtract Signal Power

A mixer translates a signal from one frequency to another by multiplying the RF input with a strong local-oscillator tone, producing sum and difference products. How much of the input power survives into the wanted IF product is captured by the conversion gain. In a passive mixer the nonlinear element, a Schottky diode quad or a ring of FET switches, can only redistribute the applied power among the many mixing products, so the wanted sideband always emerges weaker than it went in. The theoretical floor for an ideal single-ended switching mixer is about 3.9 dB of loss, set by the power diverted into the image and higher-order products; real diode mixers land at 4 to 9 dB once junction resistance, balun loss, and LO drive limits are included.

An active mixer breaks that ceiling by adding energy from the DC supply. A Gilbert-cell core uses a transconductance input stage to convert the RF voltage into a current, then a switching quad commutates that current at the LO rate. Because the transistors provide gain, the IF current developed across the load resistance can exceed the equivalent input power, yielding positive conversion gain. This comes at a cost: active mixers draw bias current, generate more noise, and usually have a lower input third-order intercept than a comparable passive diode mixer, so the choice between them is a system trade rather than a clear win.

Two distinctions matter when quoting the number. Voltage conversion gain (the IF-to-RF voltage ratio) and power conversion gain coincide only when input and output impedances are equal, common in 50-ohm test setups but not on-chip. Single-sideband conversion gain, the usual specification, counts only one input sideband; double-sideband gain, relevant when both sidebands carry signal as in a direct-conversion receiver, is about 3 dB higher for the same hardware.

Governing Equations

Conversion gain (power ratio):
G_c (dB) = 10 log₁₀(P_IF / P_RF) = P_IF(dBm) − P_RF(dBm)

Relation to conversion loss:
G_c (dB) = −L_c (dB)

Ideal Gilbert-cell voltage gain:
A_v ≈ (2 / π) × g_m × R_L

Cascade noise (Friis), mixer as stage 2:
F_sys = F₁ + (F₂ − 1) / G₁

Where P_IF, P_RF = IF output and RF input power, L_c = conversion loss, g_m = input-stage transconductance, R_L = IF load resistance, the 2/π factor is the fundamental of an ideal square-wave LO switch. Example: a Gilbert cell with g_m = 30 mS and R_L = 300 Ω gives A_v ≈ 5.7, near +15 dB voltage gain.

Conversion Gain by Mixer Type

Mixer type	Conversion gain	LO drive	Input IP3 (typical)	Noise figure	Best use
Passive single-balanced diode	−5 to −7 dB	+7 to +13 dBm	+12 to +18 dBm	≈ loss + 0.5 dB	General downconversion
Passive double-balanced diode	−6 to −9 dB	+7 to +23 dBm	+15 to +30 dBm	≈ loss + 0.5 dB	High-dynamic-range receivers
Passive FET (resistive)	−6 to −8 dB	0 to +10 dBm	+25 to +40 dBm	≈ loss	Very low distortion
Active Gilbert cell	+8 to +18 dB	0 to +6 dBm	0 to +10 dBm	8 to 15 dB	Integrated transceivers
Active diode (with IF amp)	+10 to +20 dB	+7 to +13 dBm	+10 to +20 dBm	6 to 12 dB	Gain blocks, MMIC

Common Questions

Frequently Asked Questions

What is the difference between conversion gain and conversion loss?

They are the same IF-to-RF power ratio with opposite sign. Active mixers (Gilbert cells, transconductance stages) add net power, so a downconverter shows positive conversion gain of +8 to +15 dB. Passive diode and FET-switch mixers always output less than they receive, by 4 to 9 dB, so their conversion gain is negative and equal to the conversion loss. A diode ring with 6.5 dB conversion loss has −6.5 dB conversion gain.

How does LO drive level affect mixer conversion gain?

Gain rises with LO power until the mixing devices are fully switched, then flattens. A passive cell rated for +7, +13, or +17 dBm reaches its best conversion loss near that level; under-driving it 3 to 4 dB adds 1 to 2 dB of loss and lowers IP3. Active mixers are less sensitive once the switching pairs saturate, but too much LO raises LO-to-IF leakage and DC offset. Always specify conversion gain at the rated LO power.

How is conversion gain measured on a mixer?

Apply a single RF tone at −20 to −30 dBm (well below P1dB), drive the LO at its rated power, and read the IF tone on a spectrum analyzer. Conversion gain (dB) = IF output (dBm) − RF input (dBm). De-embed cable and connector loss on both paths, and filter the IF port to reject LO feedthrough, RF leakage, and the image and sum products so only the wanted IF tone is measured.

Related Terms

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