Spectrum Analyzer questions

[videobruce]

For anyone that has had experiance with SA's, especially newer nodels, here are a few questions;

1. If I understand it correctly, the top of the dislay is the "reference level", correct? IOW's, if you set the level to say, 0dbmV (I use dbmV, not dbm), any peak that touches that top line would measure 0dbmV?

2. Is the sensistivity and the noise floor the same thing? Say the specs for the noise floor is -100dbm (a common spec), would that interpert into a sensistivity of -100dbm or is this different?

3. Where/how would a mismatch, say 50 to 75 ohms show up on a trace? Example, looking at the loss and trap depth through a series of filters that are 75 ohm using a tracking generator that outputs a 50 ohm signal into a 50 ohm input of the SA?

4. By increasing the reference level, does that attenuate the incoming signal as a attenuator would do?

[prcguy]

I'm a little sleepy this morning but will try to answer this. If a CW (no modulation) signal just reaches the top of the display (on most models) you can consider that signal at the same level as the reference level. If the signal is modulated, especially wide digital signals like 8VSB digital TV or DVB satellite, you have to take the resolution BW of the analyzer into account.

The resolution BW on the analyzer is the IF filter and a CW signal will appear at the same level through all the filters where a wide signal will appear to reduce in level when a filter narrower than the signal is chosen. A simple way to get the correct level on a wide signal is to measure the BW of the signal at about 6dB down and I like to use a 1MHz resolution BW for this. Then take the log of the BW of the signal and multiply by 10 to get a correction factor for the wide signal. For example, a 24MHz wide signal would have a 13.8 dB correction that you would add to the measured signal (in 1MHz RBW) to get the actual level.

The leads into sensitivity and noise floor. The analyzer will have a specified sensitivity which will be measured with the narrowest resolution BW filter. One purpose of the RBW filter is to limit noise into the analyzer and the noise floor will change at a predictable rate, cut the filter BW in half and the noise floor will drop 3dB, cut the filter BW by 10 and the noise floor will drop 10dB until you reach the limits of the instrument. The trade off is the sweep speed will slow down considerably at narrow RBW settings, so your measurement parameters are dictated by what you want to measure, how much span you need and how much speed you need.

I should also mention that the narrower RBW filter settings allow you to see more details in the signal or to discern multiple close spaced signals. For the purpose of this discussion, a CW signal has no BW, its just an infinitesimally small slice of one frequency. Yet when you look at a CW signal on a spectrum analyzer it has BW and it gets wider with larger RBW settings. This is the analyzer tracing its internal RBW filter. If you are trying to see several narrow signals together like three 12.5KHz spaced UHF public service transmitters, a 1MHZ or larger RBW setting will show a single lump on the analyzer. As you narrow the RBW filter you will start to see the individual signals, especially with RBW settings like 1KHz (narrower than the transmitters) and the noise floor will be pushed down at the same time, exposing more detail and possibly more signals buried in the noise.

Some analyzers automatically add attenuation when changing the reference level. The RF section only has a certain dynamic range and changing attenuation is the way for the instrument to measure a very wide range of signals.

A 50/75ohm mismatch will incur a small loss, around .18dB if everything else is matched, but if there are other problems in the system like antenna to coax mismatch, the loss will go up. Some devices will show a frequency change if not terminated properly like sweeping a 75ohm filter with a 50ohm instrument.

The spectrum analyzer is your best friend in the RF business and the more you know how to drive it, the more places it will take you.
prcguy

Originally Posted by videobruce

For anyone that has had experiance with SA's, especially newer nodels, here are a few questions;

1. If I understand it correctly, the top of the dislay is the "reference level", correct? IOW's, if you set the level to say, 0dbmV (I use dbmV, not dbm), any peak that touches that top line would measure 0dbmV?

2. Is the sensistivity and the noise floor the same thing? Say the specs for the noise floor is -100dbm (a common spec), would that interpert into a sensistivity of -100dbm or is this different?

3. Where/how would a mismatch, say 50 to 75 ohms show up on a trace? Example, looking at the loss and trap depth through a series of filters that are 75 ohm using a tracking generator that outputs a 50 ohm signal into a 50 ohm input of the SA?

4. By increasing the reference level, does that attenuate the incoming signal as a attenuator would do?

[videobruce]

One reason for all of this is I was trying to compare readings from a Signal Level Meter to a Spectrum Analyzer (Instek GSP-827). While I understand they are two different devices with different functions, one would think analog signal levels would/should be somewhat the same. I do understand digital signals have to be averaged, while analog signals do not.

I have a SLM that also have a SA as part of it's function. Though limited, by SA standards, the Analyzer screen in this SLM (Tek RFM151 Signal Scout) is fairly decent. One huge difference is the sensistivity of the two is great. The SLM clearly shows signals where the SA does not. Others commented that SA's aren't that great when it comes to this. At least that was the impression I received.
I do understand BW filters reduce sensistivity and sweep speed.

Some analyzers automatically add attenuation when changing the reference level.

How would I know this?

A 50/75ohm mismatch will incur a small loss, around .18dB if everything else is matched, but if there are other problems in the system like antenna to coax mismatch, the loss will go up. Some devices will show a frequency change if not terminated properly like sweeping a 75ohm filter with a 50ohm instrument.

That loss isn't worth talking about. How would/could a 'mismatch' change the frequency?
Would/could it do anything else regarding the pattern returned on the screen. Ringing from the filters for example?

[prcguy]

I wouldn't say a digital signal needs to be "averaged" but the level shown on a spectrum analyzer can be off by 20dB or more if you have a wide digital signal and a narrow RBW filter in the analyzer.

I'm not familiar with your analyzer or signal meter but my experience with spectrum analyzers and signal meters used in the satellite industry (Birdog, Superbuddy, etc) the spectrum analyzers can see things that the signal meters cannot. I usually use HP/Agilent spectrum analyzers and I have a new Rhode & Schwarz hand held model on order (oh boy, I can't wait!).

Run the reference level up and down on your spectrum while watching the attenuator, HP and others will start adding attenuation automatically when the ref level gets up there like -20dBm or higher.

If you have gross mismatches between your cables, antenna and equipment there will be additional losses in the setup that will vary in frequency depending on the lengths of the cables. The longer the cable the more places you will have dips and peaks in amplitude levels across the frequency range you are measuring. One tell tale sign is looking at the tops of a spectrum of digital carriers that are usually very flat on top, with a bad cable/equipment mismatch you will see the tops of the digital carriers tilted, some to the right and some to the left. You can sometimes draw a sinusoidal wave across the tops of the carriers on the analyzer screen and see what the period is and actually determine the length of the offending cable using the period and velocity factor of the cable.
prcguy

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Originally Posted by videobruce

One reason for all of this is I was trying to compare readings from a Signal Level Meter to a Spectrum Analyzer (Instek GSP-827). While I understand they are two different devices with different functions, one would think analog signal levels would/should be somewhat the same. I do understand digital signals have to be averaged, while analog signals do not.

I have a SLM that also have a SA as part of it's function. Though limited, by SA standards, the Analyzer screen in this SLM (Tek RFM151 Signal Scout) is fairly decent. One huge difference is the sensistivity of the two is great. The SLM clearly shows signals where the SA does not. Others commented that SA's aren't that great when it comes to this. At least that was the impression I received.
I do understand BW filters reduce sensistivity and sweep speed.How would I know this?That loss isn't worth talking about. How would/could a 'mismatch' change the frequency?
Would/could it do anything else regarding the pattern returned on the screen. Ringing from the filters for example?

[videobruce]

I wouldn't say a digital signal needs to be "averaged"

It does when it is a 8VSB or a 64QAM TV signal.

I have a new Rhode & Schwarz hand held model on order

Tell me it isn't this one;
http://www2.rohde-schwarz.com/en/pro...odel_.03).html

The longer the cable the more places you will have dips and peaks in amplitude levels across the frequency range you are measuring.

Will that get worse if you have this 50 to 75 ohm mismatch?

[prcguy]

Ok, I won't tell you its that one, its one that looks just like it and happens to have the same model #.
prcguy

Originally Posted by videobruce

It does when it is a 8VSB or a 64QAM TV signal.Tell me it isn't this one;
http://www2.rohde-schwarz.com/en/pro...odel_.03).htmlWill that get worse if you have this 50 to 75 ohm mismatch?

[videobruce]

Slut.

How about the getting worse part of the question?

[prcguy]

I am an equipment slut.

See this site which can help calculate additional loss from equipment and cable mismatches:
http://www.vk1od.net/tl/tllc.php
prcguy

Originally Posted by videobruce

Slut.

How about the getting worse part of the question?