RF mixing - differential-ended to single-ended - Page 1
Forum section: electronics / beginners

author: DanioIO
author_rank: Contributor
timestamp: April 20, 2014, 09:13:36 pm
content: Hi,Im trying to design a simple spectrum analyzer using AD8343 RF mixer (http://www.analog.com/static/imported-files/data_sheets/AD8343.pdf). My problem is that this mixer works with differential signals and i have only single-ended signal (VCO and input RF signal). How can i use this device without a transformator like in Figure 69 in the datasheet?Is it ok to use a differential amplifier on the Output to convert the signal to single-ended?

author: Rudane
author_rank: Regular Contributor
timestamp: April 20, 2014, 09:17:10 pm
content: I can't remember for sure, so someone back me up if I'm right, but you can capacitively couple one input to ground and drive the other pin with your single ended signal. You will suffer a 3 dB loss of input signal I believe, so you'll have to ensure you have enough power.Edit: I'm trying to find the component I did this with, but I'm having terrible luck. It may be that the device I used just had this capability.Edit: The chip was an analog devices 4007. Check out the applications section where they drive the reference input single ended instead of differential (ADF4007).

author: DanioIO
author_rank: Contributor
timestamp: April 20, 2014, 10:39:31 pm
content: Thank you i will dig in ADF4007 datasheet.I found a simple circuit (http://www.linear.com/product/LT5560) can this work for my problem? Do I have to use a precise passives for this application?

author: Rudane
author_rank: Regular Contributor
timestamp: April 21, 2014, 09:25:03 pm
content: Well, if it meets the specs of your design it will work. Do you have any concerns that it won't?

author: KJDS
author_rank: Super Contributor
timestamp: April 22, 2014, 07:31:13 am
content: Quote from: DanioIO on April 20, 2014, 10:39:31 pmFigure 6 of the datasheet is a good place to start looking for passive component information.

author: rfbroadband
author_rank: Unknown
timestamp: May 08, 2014, 04:54:08 am
content: Quote from: DanioIO on April 20, 2014, 09:13:36 pmwhy do you want to replace the transformer? It sounds like in your application you need to sweep the LO of the mixer over a wide frequency range, so a narrow band circuit is not going to help you.The LT5560 passive circuit is a narrow band circuit centered around 900 MHz. It is a LC and CL circuit, each providing -90 and +90 deg phase shift, so differential you get 180 deg, but keep in mind this is narrow band only.fc = 1/(2*pi*sqrt(L*C))...use L= 6.8nH and C= 4.7pF and you will get 904MHz.For broadband operation use a 1:1 balun. You could use ETC 1-1-13 (Macom) 100MHz to 3GHz or so...

author: madshaman
author_rank: Frequent Contributor
timestamp: May 08, 2014, 02:03:03 pm
content: What are the general tradeoffs of using a transformer for this purpose as opposed to an active circuit? I'm curious.

author: rfbroadband
author_rank: Unknown
timestamp: May 08, 2014, 02:54:39 pm
content: Quote from: madshaman on May 08, 2014, 02:03:03 pm1) balun is passive, no power consumption2) a really broadband balun (3GHz) is still cheap, a broadband active circuit will cost more (you need find a really good broadband amp_)3) a balun does not introduce any distortion......4) Selecting the wrong active LO driver circuit can cause a variety of problems in the mixer (noise, conversion gain, LO leakage, distortion etc). So using a passive balun it's a lot easier to get the performance out of the device that is specified in the datasheet datasheet unless you worked with mixers before and are familiar with all the effects the LO signal can have on your desired output signal.

author: madshaman
author_rank: Frequent Contributor
timestamp: May 08, 2014, 07:56:16 pm
content: @rfbroadbandThanks!