CN102522955A - Mixer - Google Patents

Abstract

The invention discloses a mixer, which belongs to the technical field of mixers. The mixer comprises a transconductance stage, a switching stage and a load stage, wherein transconductance stage adopts a DMGTR (Differential Multiple Gate Transistor) structure consisting of a fully differential pair transconductance and a pseudo-differential pair transconductance; and the switching stage consists of two foldable Gilbert units, one Gilbert unit serves as an in-phase channel of signals, the other one serves as a quadrature channel of signals, the two Gilbert units are both connected with the output terminal of the transconductance stage, and the output terminal of each Gilbert unit is connected with the load stage consisting of two resistors. According to the invention, the linearity of the mixer is greatly improved, and higher gains can be obtained at the same time.

Description

A kind of frequency mixer

Technical field

The present invention relates to the frequency mixer in a kind of radio-frequency front-end integrated circuit, relate in particular to a kind of collapsible orthogonal mixer, belong to the frequency mixer technical field based on the DMGTR structure.

Background technology

Traditional frequency mixer generally adopts two balanced structures of gilbert (Gilbert) unit; As shown in Figure 1; Though this mixer architecture have each interport isolation high, to advantages such as supply voltage and ambient noise susceptibility are little; But the Gilbert unit is because piling up and the pressure drop of load of offset, mutual conductance pipe and switching tube is difficult to the saturation region that assurance all crystals pipe all is operated in hope under low supply voltage.In addition, be to improve the conversion gain of circuit, need to increase the bias current of transconductance stage, and the increase of bias current will cause the pressure drop in the load to increase, thus compressed switch to the voltage space of transconductance stage, reduced the linearity of circuit.

The linearity is the most important technical indicator of frequency mixer, and the method that generally improves the linearity is to increase transistorized overdrive voltage, but under the constant situation of electric current, can reduce transistorized mutual conductance like this, thereby cause gain to descend, and noise increases.

Frequency mixer as shown in Figure 2 is even adopted DMGTR (Differential Multiple Gate Transistors, difference multiple-gate transistor) structure; The linearity can improve greatly; But it is still with transistor stack, owing to make the transistor that piles up all be biased in the saturation region (pseudo-differential to except) of hope, so the value of RL can not be excessive; Otherwise resistance drop is crossed conference and is caused voltage space not satisfy the requirement of bias transistor in the saturation region; And the gain of frequency mixer is directly proportional with RL, so this mixer architecture can not obtain very high gain, it is limited to the effect of late-class circuit Noise Suppression.

Summary of the invention

The present invention is directed to the deficiency that existing frequency mixer technology exists, and propose a kind of collapsible orthogonal mixer based on the DMGTR structure.

This frequency mixer comprises transconductance stage, switching stage and load stage, and said transconductance stage adopts the DMGTR structure; Said switching stage is to be made up of folding two Gilbert cells, the signal homophase path of first Gilbert cell, and the signal quadrature channel of second Gilbert cell, two Gilbert cells all connect the output of transconductance stage; Said load stage comprises two sub-load stage, and the output of each Gilbert cell connects a sub-load stage.

Said DMGTR structure is made up of mutual conductance mutual conductance and pseudo-differential fully differential.

Said sub-load stage is to be made up of two resistance.

Technique effect:

1, transconductance stage adopts the DMGTR structure, has improved the linearity of frequency mixer greatly.

2, switching stage adopts accordion structure, makes frequency mixer can obtain higher conversion gain, has improved late-class circuit Noise Suppression effect.

3, folding switch is pair relatively independent with input pipe, has reduced the requirement of voltage margin.

Description of drawings

Fig. 1 is the circuit structure diagram of conventional mixer.

Fig. 2 is the non-collapsible mixer structure chart of DMGTR structure.

Fig. 3 is the FDT circuit structure diagram.

Fig. 4 is the PDT circuit structure diagram.

Fig. 5 is the circuit structure diagram of frequency mixer of the present invention.

Fig. 6 is the artificial circuit schematic diagram of frequency mixer of the present invention.

Fig. 7 is the simulation result figure of mixer linearity degree index of the present invention.

Fig. 8 is the simulation result figure of frequency mixer conversion gain of the present invention.

Embodiment

Be described further in the face of the present invention down.

The circuit structure of frequency mixer of the present invention is as shown in Figure 5, comprises transconductance stage, switching stage and load stage.Said transconductance stage adopts forms DMGTR structure to mutual conductance (FDT:Fully Differential Transconductance) and pseudo-differential to mutual conductance (PDT:Pseudo Differential Transconductance) by fully differential; The FDT circuit structure is as shown in Figure 3; The PDT circuit structure is as shown in Figure 4, and in Fig. 5, M1 and M2 form FDT; M13 is equivalent to current source, and M3 and M4 form PDT; Said switching stage is to be made up of folding two Gilbert cells; In Fig. 5; The signal homophase path of first Gilbert cell that constitutes by M5～M8; By the signal quadrature channel of second Gilbert cell that M9～M12 constitutes, two Gilbert cells all connect the output of transconductance stage; Said load stage comprises two sub-load stage, and in Fig. 5, the output of first Gilbert cell connects the first sub-load stage that is made up of resistance R 3, R4, and the output of second Gilbert cell connects the second sub-load stage that is made up of resistance R 5, R6.

In Fig. 5; FDT in the transconductance stage and PDT meet into radiofrequency signal RF+ and RF-as two input pipes; First Gilbert cell in the switching stage (homophase path) meets into homophase local oscillation signal ILO_P and ILO_N, and second Gilbert cell (quadrature channel) meets into orthogonal local oscillation signal QLO_P and QLO_N, and load stage is as the output of frequency mixer; First sub-load stage output homophase (I road) intermediate-freuqncy signal IIF_P and the IIF_N, second sub-load stage output orthogonal (Q road) intermediate-freuqncy signal QIF_P and the QIF_N.

The present invention adopts SMIC 0.18 μ m CMOS technology to realize, is applied in DRM (Digital Radio Mondiale, the global digital broadcasting) receiver chip.The principles simulation circuit of frequency mixer of the present invention is as shown in Figure 6, and rf inputs PORT0 converts the double-end signal input mixer to through transformer; In-phase signal input PORT2 and orthogonal signalling input PORT3 all convert the double-end signal input mixer to through transformer; The output of frequency mixer is ac-coupled to late-class circuit through the electric capacity of 220uF, replaces late-class circuit with PORT1 here, and its internal resistance is set to the input impedance of late-class circuit, makes simulation result truer; VDD is a supply voltage, and VDMGTR is the right gate bias voltage of pseudo-differential, and remaining bias voltage has all been accomplished chip internal.

The simulation result of mixer linearity degree index IIP3 of the present invention and conversion gain G is respectively like Fig. 7 and shown in Figure 8, radio frequency signal frequency 20MHz, local frequency 55MHz, IF-FRE 35MHz.

Introduce action principle of the present invention below.

The DMGTR structure that this frequency mixer adopts is to utilize input transistors mutual conductance g _mSecond dervative g _m" and IIP ₃Relation improve the linearity.The drain current of common source configuration metal-oxide-semiconductor can use Taylor (Taylor) series expansion to represent:

$i_{\mathrm{DS}}=I_{\mathrm{dc}}+g_m{v}_{\mathrm{gs}}+\frac{{g_m}^{\&prime;}}{2!}{v_{\mathrm{gs}}}^2+\frac{{g_m}^{\&prime;\&prime;}}{3!}{v_{\mathrm{<figure-callout\; id="3"\; label="gs"\; filenames="HDA0000128006810000011.png"\; state="\{\{state\}\}">gs</figure-callout>}}}^3+L---\left(1\right)$

In the formula: I _DcThe DC component of expression drain current, v _GsThe voltage in expression transistor gate source, g _m ⁽ⁿ⁾Expression mutual conductance g _mThe n order derivative.

The IIP that then representes with voltage form ₃Can be expressed as:

${\mathrm{<figure-callout\; id="3"\; label="IIP"\; filenames="HDA0000128006810000011.png"\; state="\{\{state\}\}">IIP</figure-callout>}}_3=\sqrt{\frac{4}{3}\frac{g_m}{{g_m}^{\&prime;\&prime;}}}---\left(2\right)$

The linearity of visible crystals pipe and g _m" be inversely proportional to, if g _m" ≈ 0, non-linear can being left in the basket fully that then mutual conductance driving stage is introduced, and whole frequency mixer non-linear will be only by switch to producing.

Provide the g of FDT and PDT below _m":

For frequency mixer, the IIP under the small-signal situation ₃More important compared to large-signal conditions; This is because the signal strength signal intensity that receives when receiver greatly the time; Often can adjust the gain of LNA through automatic gain control circuit, thereby the input signal power that can guarantee frequency mixer is lower, still can be used as small-signal and handle.Therefore, level off to zero the time g of FDT as differential mode input AC small-signal Δ v _m" can be expressed as:

${g_m}^{\&prime;\&prime;}{|}_{\mathrm{\&Delta;v}=0}=\frac{-3}{4\sqrt{I_{\mathrm{SS}}}}{\left({\mathrm{\&mu;}}_n{C}_{\mathrm{ox}}\frac{W}{L}\right)}^{\frac{3}{2}}<0---\left(3\right)$

It is thus clear that the g of FDT _m" forever less than zero, this also is to adopt FDT to be difficult to do high reason as traditional Gilbert mixer linearity degree of mutual conductance driving stage.

Discover that the PDT that is operated in sub-threshold region can make g _m" greater than zero, with the parallelly connected g that then can offset FDT of PDT with FDT _m", make g _m" level off to zero, thereby can obviously improve the linearity of frequency mixer.

When Δ v levels off to zero the time g of PDT _m" can be expressed as:

${g_m}^{\&prime;\&prime;}{|}_{\mathrm{\&Delta;v}=0}=\frac{{\mathrm{<figure-callout\; id="0"\; label="I"\; filenames="HDA0000128006810000051.png"\; state="\{\{state\}\}">I</figure-callout>}}_0}{4{\left(\mathrm{\&xi;}{V}_T\right)}^3}{e}^{\frac{v_b}{\mathrm{\&xi;}{V}_T}}>0---\left(4\right)$

Comparison expression (3) and formula (4) can be found out, as the g of FDT _m" with the g of PDT _m" linearity of frequency mixer is best when adding up to zero, so linearity optimal conditions is:

$\frac{3}{\sqrt{I_{\mathrm{SS}}}}{\left({\mathrm{\&mu;}}_n{C}_{\mathrm{ox}}\frac{W}{L}\right)}^{\frac{3}{2}}=\frac{I_0}{{\left(\mathrm{\&xi;}{V}_T\right)}^3}{e}^{\frac{v_b}{\mathrm{\&xi;}{V}_T}}---\left(5\right)$

In the following formula: I _SSWith W/L be the FDT design parameter, I ₀And v _bIt is the PDT design parameter.

In addition, the gain that the collapsible switching stage structure that this frequency mixer adopts can effectively improve frequency mixer, the conversion gain of frequency mixer can be expressed as:

$G_v=\frac{2}{\mathrm{\&pi;}}(g_{m1}+g_{m3})R_L---\left(6\right)$

In the formula: g _M1The mutual conductance of expression transistor M1, g _M3The mutual conductance of expression transistor M3.In Fig. 5, the R in the formula (6) _L=R3=R4=R5=R6, under the situation that the drive stage transistors mutual conductance is confirmed, can be through suitably increasing load resistance R _LObtain higher gain, this is difficult for realizing in the frequency mixer of conventional stack structure.

Claims (3)

1. a frequency mixer comprises transconductance stage, switching stage and load stage, it is characterized in that: said transconductance stage adopts the DMGTR structure; Said switching stage is to be made up of folding two Gilbert cells, the signal homophase path of first Gilbert cell, and the signal quadrature channel of second Gilbert cell, two Gilbert cells all connect the output of transconductance stage; Said load stage comprises two sub-load stage, and the output of each Gilbert cell connects a sub-load stage.

2. frequency mixer according to claim 1 is characterized in that: said DMGTR structure is made up of mutual conductance mutual conductance and pseudo-differential fully differential.

3. frequency mixer according to claim 1 is characterized in that: said sub-load stage is to be made up of two resistance.

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