N9923A的噪音过低问题

Joel Dunsmore博士在另一个关于VNA的噪声系数噪声系数的线程中说
*有三种或四种常见接收器类型，最差到最佳：* * 1）基于采样器：非常高的噪声系数 - 8753和8720使用采样器在VCO的200至300次谐波上运行* * 2）高次谐波混频器（或
低谐波采样器） - 许多毫米波系统使用它;
谐波操作高达10或12或更多* * 3）低谐波混频器 - PNA在26.5GHz以上的分析仪中使用三次谐波混频，PNA-L在13 GHz以上使用它。我认为。* * 4）基本混音 - 噪音最低
图。*当我问N9923A FieldFox使用什么时，他说“基本混合”。
我想要比较我拥有的两种完全不同的VNA的噪音水平：1）HP 8720D http://www.home.agilent.com/en/p ... wave-vector-network
-analyzer？cc = US＆amp; lc = eng至少10年，50 MHz-20 GHz，实验室仪器，它使用具有最高噪声系数的接收器类型。
2）Agilent FieldFox N9923A http://www.home.agilent.com/en/p ... rk-analyzer-4-6-ghz当前型号，2 MHz -
6 GHz，便携式仪器，使用噪声系数最低的接收器类型。
毋庸置疑，我期待N9923A的噪音更低。
但至少根据我使用的方法，情况并非如此。
也许我做错了什么。
两种仪器都使用了以下设置。
平均值= 1 IF带宽= 1 kHz跨度= 50 MHz至6 GHz每次扫描点数= 1601 I然后在任一机器的测试端口上没有任何内容测量S12。
由于噪声的本质，走线上的峰值偶尔会非常高。
收集到1601个点后，我调整了参考电平，使每次扫描噪声超过参考值一次或两次。
这是通过眼睛完成的，而不是从GP-IB，USB或LAN收集数据。
所以这接近噪声水平的最大值，而不是任何平均值。
只有偶尔的数据点显示噪音高于此值。
结果如下：* HP 8720D = -87 dB * Agilent N9923A = -75 dB调整参考电平，因此大约一半的点高于参考值，一半低于当前的噪声水平。
* HP 8720D = -100 dB * Agilent N9923A = - 90 dB因此，在我看来，旧型号的噪音水平比新型号低约10-12 dB * *。
N9923A上的噪声在50 MHz时比6 GHz低约5 dB，而在8720D上噪声是平坦的，频率为6 GHz - 这是我有一个N校准套件的最高频率。
（我本可以切换到3.5毫米并校准到9 GHz，但我没有这样做。无论如何，N9923A的最大频率是6 GHz）。
事实上N9923A更新，它几乎肯定有更高分辨率的屏幕，因此当扫描1601点时，在更高分辨率的屏幕上可能会出现偶尔的峰值，而不是旧的HP 8720D。
但这样的效果无法解释10-12 dB的差异。
有什么想法，我得到这些结果？
戴夫

以上来自于谷歌翻译

   以下为原文

  Dr. Joel Dunsmore said on another thread about VNA noise figure
Noise figure of a VNA

*There are three or four common receiver types, for worst to best:*

*1) Sampler based: very high noise figure - 8753 and 8720 use samplers operting on 200th to 300th hARMonic of the VCO*
*2) High harmonic mixer (or low harmonic samplers) - many mm-wave systems use this; operation on harmonics up to 10 or 12 or more*
*3) Low harmonic mixer - PNA uses third harmonic mixing in analyzers above 26.5GHz, PNA-L uses it above 13 GHz, I think.*
*4) Fundamental mixing - Lowest noise figure.*

when I asked what the N9923A FieldFox uses, he said "fundamental mixing".

I deceided to to compare noise levels of two quite diferent VNAs I own:

1) HP 8720D
http://www.home.agilent.com/en/p ... er?cc=US&lc=eng
At least10 years old, 50 MHz-20 GHz, lab instrument, which uses the type of receiver with the highest noise figure.

2) Agilent FieldFox N9923A
http://www.home.agilent.com/en/p ... rk-analyzer-4-6-ghz
Current model, 2 MHz - 6 GHz, portable instrument, which uses the type of receiver with the lowest noise figure.

Needless to say, I was expecting the noise to be lower on the N9923A. But at least according to the method I used, this was not the case. Perhaps I'm doing something wrong.

The following settings were used on both instruments.

Averaging = 1
IF bandwidth = 1 kHz
Span = 50 MHz to 6 GHz
Points per scan = 1601

I then measured S12 with nothing on the test ports of either machine.

Because of the very nature of noise, the peaks on the traces can occasionally be very high. With 1601 points collected, I adjusted the reference level so the noise exceeded the reference only once or twice per sweep. This was done by eye, rather than collecting data down a GP-IB, USB or LAN. So this is near the maximum of the noise level, rather than any sort of average. Only the occasional data point shows a noise higher than this.

The results were:
* HP 8720D = -87 dB
* Agilent N9923A = -75 dB

Adjusting the reference level so about half the points are above the reference and half below gives lower noise levels of course.

* HP 8720D = -100 dB
* Agilent N9923A = - 90 dB

So it appears to me the noise level on the older model is about 10-12 dB *less* than on the new model.

The noise on the N9923A appears to be about 5 dB lower at 50 MHz than 6 GHz, whereas on the 8720D the noise was flat with frequency to 6 GHz - the highest frequency at which I have an N calibration kit. (I could have switched to 3.5 mm and calibrated to 9 GHz, but I did not do so. In any case, the N9923A's maximum frequency is 6 GHz).

The fact the N9923A is newer, it almost certainly has a higher resolution screen, so when sweeping 1601 points, an occasional peak is perhaps more likely to be seen on the higher resolution screen than on the lower one of the old HP 8720D. But there is no way such an effect could explain a 10-12 dB difference.

Any thoughts why I'm getting these results?

Dave

回帖（1）

刘华湘

2019-7-15 08:44:07

我的实验室中没有872x进行任何比较测量，但您真正需要注意的是两种不同仪器的指定本底噪声。
确实，技术通常随着时间的推移而变得更好，但我们通常在仪器系列中提供许多不同的性价比选择。
N9923A是一款成本较低的手持设备，即使该仪器老了很多，它也不会像平板式高成本型号那样具有良好的规格。

以上来自于谷歌翻译

以下为原文

I don't have an 872x in my lab to do any comparison measurements, but what you realy need to look at are the specified noise floor of the two different instruments. It is true that technology typically gets better with time, but we usually offer many different price/performance options in an instrument family. The N9923A is lower cost handheld device and it wouldn't be very surprising for it to not have as good a specs as a benchtop higher cost model even if that instrument is a lot older.

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李青

N9923A的噪音过低问题

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刘华湘

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