Measurement setup of noise diode source

Hi,

I have already designed a noise diode source. I tried to measure ENR of noise source using the spectrum analyzer (FSV, Rohde & Schwarz). But I couldn't see any change in the display during measure the output power of noise source. Could please help me to explain how can I measure the noise source using the spectrum analyzer?

The display will vary with the persistence of the display it must be a storage device, but changing the persistence of the display will cause the amplitude to change. The bandwidth selected and the sweep time also affect the results. Have you tried to use a rms voltmeter on the signal output of the spec. analyser with the sweep switched off?, i.e. as a fixed bandwidth receiver?
Frank

You mean, you see no output voltage of the source? What's the diode specification?

Yes I have tried but I couldn't see any change when the source is connected or disconnected !!!

---------- Post added at 18:35 ---------- Previous post was at 18:34 ----------

chuckey said:

The display will vary with the persistence of the display it must be a storage device, but changing the persistence of the display will cause the amplitude to change. The bandwidth selected and the sweep time also affect the results. Have you tried to use a rms voltmeter on the signal output of the spec. analyser with the sweep switched off?, i.e. as a fixed bandwidth receiver?
Frank

Click to expand...

---------- Post added at 18:39 ---------- Previous post was at 18:35 ----------

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FvM said:

You mean, you see no output voltage of the source? What's the diode specification?

Click to expand...

I mean ..... I couldn't see any change in the power level when the noise source is connected or disconnected from spectrum analyzer. I'm using NC304 diode from NoiseCom for x-band application.

Well, at higher frequencies the noise power might well be
below the instrument / ambient noise floor. That might
require you to make a low noise, high gain amplifier that
sits close-in to the diode and then characterize that with
(say) a shorted input, and then look at diode noise on an
additive basis. You usually see specialized equipment for
noise measurements of bare transistors / diodes. Back
in the day we had a shielded box in which everything
ran off dry cells including a low noise op amp buffer.

You might also be fighting the 50-ohm input impedance
of the spectrum analyzer and other things. Best to
test it like you intend to apply it, and use external
amplifiers to match to the instrument input.

I don't see NC304 listed under the present NoiseCom products. Other NC300 family diodes are achieving about 30 dB noise figure, should be 10 dB above FSV internal noise. For the calculation, the thermal noise of a 50 ohm resistor is -174 dBm @ 1Hz bandwidth.

FvM said:

I don't see NC304 listed under the present NoiseCom products. Other NC300 family diodes are achieving about 30 dB noise figure, should be 10 dB above FSV internal noise. For the calculation, the thermal noise of a 50 ohm resistor is -174 dBm @ 1Hz bandwidth.

Click to expand...

Sorry the corrected part number is NC305.....I have checked that the limited internal attenuation of FSV is 10 dB. When the noise source is connected, the output power is around -75 dBm and noise is 139 dBm/Hz at 11 GHz and when the noise source is disconnected, the output power level is around -75.5 dBm and noise is also 139 dBm/Hz, i.e. there is not big different. I know as written in data sheet of noise diode that the ENR is between 29 to 34 dB. I need to know, how can I measure ENR of noise source using FSV analyser? is it required a LNA for ENR measurements?

In any case, the internal attenuator must be disabled for the measurement.

FvM said:

In any case, the internal attenuator must be disabled for the measurement.

Click to expand...

There is no option to disable the internal attenuation in FSV analyzer!!!... it says that the minimum internal attenuation is 10 dB!!. Do you have use FSV analyser?

I have used it at times. Every SA can be set to zero attenuation, also FSV, as I see from manual. See e.g.

2.5.2.1 Measurement Example – Measuring the Intrinsic Noise of the analyzer at 1 GHz

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See page 108:

Note: Values under 10 dB can only be entered via the numeric keypad or via remote control command in order to protect the input mixer against overload.

Click to expand...

Actually, you have to take care when powering the noise diode.

FvM said:

I have used it at times. Every SA can be set to zero attenuation, also FSV, as I see from manual. See e.g.


See page 108:

Actually, you have to take care when powering the noise diode.

Click to expand...

Thanks for helping....