Amplifying two 100MHz signals wothout disturbing phase - Page 1
Forum section: electronics / beginners

author: mzeeshanarshad
author_rank: Newbie
timestamp: January 13, 2015, 05:04:38 am
content: Hello,I'm working on a task where I need to amplify two 100MHz analog signals (voltage and current) without disturbing their phase difference. The two sine wave voltage signals actually measure current and voltage in RF. One of the two signals is upto +/-7V while the other is upto +/-1V.How can I use high speed op amps to make both signals amplitude equal by using opamps and variable resistors without affecting the phase difference?Can I use two sets of high speed (>500MHz bandwidth) opamps for each signal and adjust the gain of both seperately to make the opamps amplitude equal?Please recommend me some opamps as well.Thank you.

author: c4757p
author_rank: Super Contributor
timestamp: January 13, 2015, 05:39:33 am
content: Evenreally fastop amps will have trouble. Phase offset will be dependent on lots of things, including the gain of the amplifier. Also:Quote from: mzeeshanarshad on January 13, 2015, 05:04:38 am7 Vpk at 100 MHz? That is also nontrivial - that's a4400 V/µsslew rate! Only a very select few op amps can achieve this in practice - one of the highest slew rate op amps I know of is theTHS3001, which does 6500 V/µsunder all the right conditions, but it'll have trouble getting even close to that under more "normal" conditions.QuoteHow do you plan to do this? You're not getting a trim pot in there - even a stray picofarad will ruin your phase offset.What if you attenuate instead? Cut the 7V down to match the 1V? A wideband attenuator would be a walk in the park compared to amplifying.Even better, can you think of a way to make your designnotdepend on their phase being unshifted?More info?Make sure this isn't an X-Y problem- what is the actual task? Also, are these 100 MHz signals, orup to100 MHz? Those are two entirely different problems to solve.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 13, 2015, 05:41:10 am
content: Note that matched amplifiers will have the same absolute phase shift. But yeah, this doesn't sound like something you should need to be doing, and there's probably a good reason -- that you're missing -- why this is the case.Tim

author: c4757p
author_rank: Super Contributor
timestamp: January 13, 2015, 05:44:42 am
content: Quote from: T3sl4co1l on January 13, 2015, 05:41:10 amEven if they have different gains? I suppose they wouldn't be matched then...

author: mzeeshanarshad
author_rank: Newbie
timestamp: January 13, 2015, 06:22:46 am
content: Thanks for the great replies.I just wanted to clear that the reason I need to do this is that there are two sensors that measure current and voltage. The amplitude of current sensor is +/- 1V (max), however the voltage sensor amplitude is +/-7V (max). We would like to read these signals in an ADC whose input range is +/-4V. The problem is we cannot change the sensor design to attenuate or amplify these outputs.Is there a way to attenuate the +/-7V signal using some variable resistor in the device to reduce its amplitude to within +/-4V without hurting the phase (by also treating the other signal through some kind of buffer)?* The reason we can't let the phase shift is because we need to measure the phase difference in the software after the ADC.IT MAY BE POSSIBLE TO CALIBRATE THE PHASE IN SOFTWARE IF THE EFFECT ON PHASE IS JUST A FIXED ERROR (CONSTANT PHASE ERROR).

author: vaualbus
author_rank: Frequent Contributor
timestamp: January 13, 2015, 01:13:29 pm
content: Why not use an anlog phase detector? Also why ou need 100Mhz is seem to me strange for a sensor to output such a high frequency?

author: katzohki
author_rank: Frequent Contributor
timestamp: January 13, 2015, 04:37:13 pm
content: You could add in a phase shifter to compensate or do it in software yeah. But are you sampling at 100MHz? Sure you can't sample less often or downconvert or something?

author: DmitryL
author_rank: Regular Contributor
timestamp: January 13, 2015, 05:02:42 pm
content: Quote from: mzeeshanarshad on January 13, 2015, 06:22:46 amA current sensor that produces +/- 7V at 700 Mhz ?sounds very strange.Quote from: mzeeshanarshad on January 13, 2015, 06:22:46 amWell.. there is a way. Use a cheap carbon variable resistor, it won't hurt the phaseIt doesn't matter what you'll use, you either seem not to understand what you want and what needs to be done or don't have enough knowledge for RF job.

author: vaualbus
author_rank: Frequent Contributor
timestamp: January 13, 2015, 06:02:58 pm
content: Quote from: DmitryL on January 13, 2015, 05:02:42 pmor do what you should when attenuate RF signal use PIN diodes. Ah Ah.Too me seem soo strange have a sensor that output 100Mhz.

author: dmills
author_rank: Super Contributor
timestamp: January 13, 2015, 06:51:12 pm
content: I have actually been there (at 50MHz but still) with the 'sensor' being a current and voltage transformer pair wound on a binocular ferrite in the output stage of a HF transmitter.I deliberately padded the transducer down to give a stable 50 ohm output with good return loss (Pads are nice like that), and ended up with a few mw at the adc when running 1KW into 50 ohms, and it was almost independent of frequency over 1.8 - 54MHz.Now the phase did tend to drift somewhat as you changed the frequency, but that is what a lookup table and a cordic core are for (There was lots of FPGA action anyway so this was trivial).My application was optimising the power amplifier supply voltage to match the drain voltage swing into the load, which needed to measure load Z over about a 2:1 range.If you are working narrowband at 100MHz, you can fine tune the phase by simply tweaking the relative cable lengths...Depending on exactly what you are doing, you may find some of ADs log amps interesting, they make lovely limiting amps for phase detectors.HTH.73 Dan.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 13, 2015, 07:06:20 pm
content: This site needs like and dislike buttons on postsor maybe +1V and -1V pushbuttons on each postUser dmills first post is extremely +1 Informative. And I really want to be able to search for Informative posts later@dmills, Welcome to the shop!