The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> MIXER DESIGN IN Agilent ADS... Some questions...
https://designers-guide.org/forum/YaBB.pl?num=1139546773

Message started by fab_jor on Feb 9^th, 2006, 8:54pm

Title: MIXER DESIGN IN Agilent ADS... Some questions...
Post by fab_jor on Feb 9^th, 2006, 8:54pm

Firstly, sorry for my poor english...

I am having some troubles in my project. I am designig a double balanced mixer CMOS which uses a PMOS as a active load ( current source, exactly ). The RF and LO ports are perfectly matched, but the IF port no. While I got an excellent IIP3 point ( +14.2 dBm ) and reasonable Noise Figure ( NF = 14.4 dB ), my circuit´s conversion gain is low ( CG = -7.7 dB ). I use a current of, approximately 12 [mA] and the transistors´dimensions are : W_RF=200 [um] and L_RF=L_min ( 0.25 [um] ), W_LO=300 [um] and L_LO=L_min and, the active loads, W_AL=650 [um] and L_AL=0.35 [um]. I got these values to estipulate DC voltages on especific nodes. For example, the DC voltage at IF node is 2.4 [V] ( VDD = 3.3 [V] and TSMC 0.25 [um] process ). Please, can you help me ? I need a better CG... I am sending the schematic attached.

Thank you very much.

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by sonny on Feb 25^th, 2006, 6:40am

Hi.

An output DC voltage of this mixer varies depends on threshhold voltage of NMOS and PMOS. you can easily find out by corner simulation (fast / slow or slow/fast) . I recommend you to use CMFB(common mode feed back) circuit at the output node to stable the output DC voltage.
;)

Regareds,
Sonny

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by fab_jor on Feb 25^th, 2006, 1:16pm

Thank you for your answer, sonny.

What kind of CMFB ? I dont wanna to use resistors in my design. This is a project constraint. And what about the Conversion Gain ( CG ) ? How can I make it better ?

One more time,

Thank you very much.

Fabricio

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by fab_jor on Feb 25^th, 2006, 1:18pm

And what's "corner simulation" ?

Regards

Fabricio

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by sonny on Feb 28^th, 2006, 6:27am

fab_jor wrote on Feb 25^th, 2006, 1:16pm:

About CMFB , detect mixer output DC voltage by LPF, compare the voltage and specific voltage by comparator, control the PMOS current using comparator output to set the mixer output DC voltage to specific one. I guess low PMOS output impedance make conversion gain low, so I recommend longer PMOS L ( about 1um or 2um).

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by sonny on Feb 28^th, 2006, 6:30am

fab_jor wrote on Feb 25^th, 2006, 1:18pm:

And what's "corner simulation" ?

Regards

Fabricio

I mean simulation using corner model (maximum Vth, minimum Vth ) which is provided by foundary

Regards,
Sonny

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by fab_jor on Mar 1^st, 2006, 11:39am

Thank you for your reply, Sonny.

But I cant understand about the CMFB's circuit. What kind of circuit I must use ? Can you send me a topology/schematic ? I already tried another active load ( a current mirror and I got a better conversion gain ), but never better than 0 [dB] and the output impedance was not equals between the outputs. The IIP3 is great ( about +13 [dBm] ) and I utilize a 12.5 [mA] as the bias current. I think that, if I lower that current, I will get a better conversion gain. What do you think ? I dont understand because my circuit's conversion gain is too low, if I am using active loads ? Do you know why ?

Best regards.

Fabricio

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by fab_jor on Mar 1^st, 2006, 11:40am

LPF = Low pass filter ???????

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by RFenthusiast on Mar 1^st, 2006, 12:06pm

Hi,

Did you try increasing the current? I think increasing the current in the gain stage will help improve the CG as it will improve the gm of the transistor. May be you should try with 20mA.

Also you could use just one degeneration element between the sources of the transistors forming the gain stage (place it across the two sources), and use two current sink transistors at the sources instead of one common one. This will reduce the voltage drop across the resistors.

Regards,

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by fab_jor on Mar 1^st, 2006, 1:58pm

Hi RFenthusiast .

I will try...

Regards

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by fab_jor on Mar 3^rd, 2006, 2:11pm

Hi sonny and RFenthusiast

RFenthusiast:

Changing the current ( 3 [mA] and 20 [mA] ) dont make the CG better... I got a good IIP3...

sonny:

The PMOS's output resistance ( Rds or 1/Gds ) is higher than 10k. The RF stage's Gm is about 55 m.

So, I dont know what is wrong with my design. I simply cant get a good CG !!!

Any sugestions ?

Regards.

Title: Re: MIXER DESIGN IN Agilent ADS... Some questions.
Post by RFenthusiast on Mar 6^th, 2006, 3:55am

Hi fab_jor,

I have noticed that if the matching is not right it will not show you the right CG, so one suggestion would be to get rid of the balun and measure the differential voltage at the output, just to check........I dont know if this will help but it might.

But before this, try increasing your output load resistance to like 1 MOhm and measure the CG. Also you can just observe the output and input voltage waveform in time domain and see if there is gain.

regards.