raed_microwave
Advanced Member level 4
- Joined
- Jul 26, 2008
- Messages
- 107
- Helped
- 0
- Reputation
- 0
- Reaction score
- 0
- Trophy points
- 1,296
- Location
- Kassel, Germany, Germany
- Activity points
- 2,066
What I'm doing is....I want to test the square-law detector whether it is working in the square or linear region.....what power level should I consider? the power level read by power meter or by spectrum analyzer? I have designed a voltage doubler circuit using HSMS-2852 diode from Avago and the datasheet said that the max. input power of diode is -20 dBm but this case is for narrow band signal. How can I evaluate power level of IF with the case of wideband noise signal ( up to 1 GHz)?The power meter evaluates the heat generated by absorbing a wideband signal or noise spectrum. A spectrum analyzer analyzes the narrow sections of your spectrum while it can scan it over the wide band. The RBW is the width of a narrow window which scans over a wider bandwidth.
In spectrum analyzers you have three bandwidths: the first is the frequency coverage over which the narrow-band receiver is scanning. The second is the bandwidth of this narrow-band receiver. The third is the integration bandwidth of the video filter after detector.
Usually you cannot evaluate a real power of a wideband signal or noise by spectrum analyzer. You can measure a CW power and analyze signal-spectrum components by their full and relative powers. But the power meter indicates the full spectrum power.
If you can trust your spectrum analyzer that it is calibrated to correctly read a CW signal power, you can calibrate your detector with it.
Detector response is typically square-law from TSS, ~-35 dBm, up to ~ -20 dBm, to a CW signal or a wideband one.
The problem with a wideband signal or noise in a detector is that the detector may be mismatched at some frequencies.
The best way to calibrate a detector response to a wideband signal is to use a RF power meter.
Sorry I didn't get your wondering.......The output DC voltage of detector is 1.2 volt for -3 dBm of input powerI wonder if the -3dBm shown by your power sensor are (partially) caused by a DC voltage.
The difference, 30 dB, between the power meter and spectrum analyzer readings, is due to the sensed noise bandwidth: your estimated noise bandwidth is ~1 GHz and the power of -3 dBm is read on the power meter; On spectrum analyzer, the window is ~1 MHz wide, so it reads -30 dBm.
The correct power to the detector is -3 dBm which is too much for a square-law detector.
At -20 dBm, a good detector can have K ~ 2500 mV/mW, therefore ~25 mV detected voltage. Over that, a square-law detector usually behaves "linearly" but some fine detectors can be damaged with -3 dBm!
noise bandwidth is ~1 GHz and the power of -3 dBm is read on the power meter
Thermal noise is -174dBm/Hz, or -84dBm at 1GHz bandwidth. If the -3dBm is only from wideband noise, the noise would be 81dB above thermal noise. Isn't that a little bit too much?
edit: I might have misunderstood the question. Did you say that your input test signal is a wideband noise source indeed?
To relate the spectrum analyzer to the power meter measurement, we need to know the selected SA detector characteristic. Recent spectrum analyzers have power averaging and channel power measurement capabilities, that achieve results near to a power meter. The difference between measurements of about 2 dB isn't really large, however.
Both measurements suggest operation of the diode detector outside square law range. Also the slope in your measurement isn't true square law rather than transition mode, as far as I see. If I understand right, the -20 dBm square law limit is for CW signals. It should be lower for noise due to the crest factor.
Another question, that has been already addressed by jiripolivka is, if you actually achieve broadband matching and respective frequency independent power measurement in your detector circuit. To verify the frequency response, you would preferably use a signal generator.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?