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Measurements >> RF Measurements >> Receiver test setup and calibration
https://designers-guide.org/forum/YaBB.pl?num=1397057304

Message started by mixed_signal on Apr 9^th, 2014, 8:28am

Title: Receiver test setup and calibration
Post by mixed_signal on Apr 9^th, 2014, 8:28am

Hi,

I want to plan test setup of my receiver. My receiver needs RF+ , RF- , LO+, LO- which I am generating using offchip baluns and introducing inside chip using RF probes as in Fig. 1.

1. I want to calibrate balun/probe loss because I want to subtract it from my total RX noise figure. I have a plan as shown in Fig. 2 where I want to measure the S21, find half the loss and remove it from total NF. The pads are identical, close and shorted. Is that OK? Has anyone better suggestion?

2. I have a baseband buffer @ 30MHz . The input is obviously not matched to 50 ohm. Is it necessary to match the output to 50 ohm @ 30 MHz since it will drive 50 ohm instruments.

3. I want to calibrate the NF and gain of the buffer and hence I have a lonely version. Is it required to matched I/P and O/P at 30 MHz now?

Title: Re: Receiver test setup and calibration
Post by aaron_do on Apr 9^th, 2014, 10:53pm

Hi,

1) I think that method is quite OK. I think it is accurate as long as your design is more or less matched to 50-ohm. Outside of your design's bandwidth, I suspect the answer won't be right since there will be reflection.

http://www.designers-guide.org/Forum/YaBB.pl?num=1233236788/1#1

2) This answer is just an IMO...I think it needs to be able to drive 50-ohm, but doesn't need to be matched to 50-ohm. In the first place, one wavelength at 30 MHz is 10m (free space). Also, I think you would only have problems if your load is unmatched, but in your case, its the analyzer (50-ohm). So for example if your load was not 50-ohm, then there would be reflection at the load, and when the reflected wave reached your "unmatched" device, it would be re-reflected. I think the concern is that if there is too much bouncing around, the resulting distortion can cause ISI.

3) For the output, just do whatever you did in the full version. For the input, if your input impedance is high, then you will get an automatic 6dB gain (it will be slightly less). However, if you add a 50-ohm matching resistor, then it will affect your NF. If it were me, I would not add any matching. Just make sure you interpret the s-parameter data properly when you want to de-embed the gain.

A lot of IMOs in my answers. Maybe somebody can give their opinion.

hope it helps,
Aaron

Title: Re: Receiver test setup and calibration
Post by mixed_signal on Apr 10^th, 2014, 9:44am

Hi aaron_do,

Thank you very much! Highly appreciate your feedback.

Title: Re: Receiver test setup and calibration
Post by mixed_signal on Apr 10^th, 2014, 3:24pm

Hi aaron_do,

Part 3) Can I measure noise figure if input of the buffer is not matched to 50 ohm. I plan to use Y factor method using spectrum analyzer.

Title: Re: Receiver test setup and calibration
Post by aaron_do on Apr 10^th, 2014, 6:14pm

Hi,

unfortunately its been a while since I've done NF measurements. I don't see any reason why you need to match the input. A side note, I seem to recall situations where my device noise was very close to the instrument's noise floor and this resulted in inaccurate results. Perhaps you should consider this...

regards,
Aaron

EDIT: I've just looked at some documents, and it seems that mismatch may be important for Y-factor NF measurement. The document is called "Noise Figure Measurement Accuracy - The Y-Factor Method - Application Note" and it can be found here:

http://www.home.agilent.com/en/pd-331902-pn-E4402B/esa-e-series-option-219-noise-figure-measurement-personality?&cc=SG&lc=eng

If you want to double-check, you could do some noise simulations and try to calculate noise figure using the Y-factor method. See if there's any difference in the result when the device is matched or not...

Title: Re: Receiver test setup and calibration
Post by mixed_signal on Apr 11^th, 2014, 9:10am

Thank you very much aaron_do!

Title: Re: Receiver test setup and calibration
Post by LazyDesigner on Oct 30^th, 2014, 7:47pm

HI Aaro_do,
Whats your opinion on output impedance matching as asked in #2, if signal BW changed from 30MHz to 1GHz, do we only need to drive 50ohm or we should match it to 50ohm?