setting bin size, RBW, frequency span, and center frequency

Hi everyone,

I’m currently trying to interface the instrument SA124B with my computer through the API provided by signalhound. I noticed that there’s no way for me to set exact bin size, frequency span, center frequency values that I want. Is this a general feature of the instrument and there’s no way to go around this problem?

If I understood correctly, the saBandwidthClamped status is returned by the saInitiate command when the supplied frequency span setting (or other settings) broke the limit set by the instrument.
Does that mean that the instrument automatically clamps the bandwidth to a certain value? If yes, is it possible to know what value does it clamp to?

As far as I have played with the instrument, the only way to change the bin size setting is through the RBW setting. Is this hard-coded in the instrument or there is a formula that governs the relationship between RBW and bin size?

Thanks :)!

Hi Sadak,

The bin size is a relationship between RBW and the window function used. For most of our processing, we use a variable flat-top window to achieve the desired RBW.

The saBandwidthClamped warning will get returned during saInitiate if the RBW had to be changed to fit within the limitations of the device.

We have a list of the RBW limitations in the user manual here, under section 2.3.1, many of them are related to center frequency and span. This manual is for the SA44B but applies to the SA124 as well.

https://signalhound.com/sigdownloads/SA44B/SA44B-User-Manual.pdf

If you have follow up questions, let me know.

Regards,
A.J.

Hi Andrew,

Thanks for the quick reply! Thanks for reminding that the window function also plays an important role in it. I’m quite new in this spectrum analyser stuff, my initial thought would be that the bin size is the same as rbw but apparently, it’s not.

It still makes sense to me that the bin size has to be smaller than the rbw in order to make sure that there will be at least one bin that captures the full amplitude of the signal (is this correct?). But it’s still not so clear to me what will be the exact relation between rbw and bin size in the case where a flat-top window is used. Could you please explain this part in more detail?

The motivation behind this question is that I would ideally like to provide the SA124B with a frequency array that I generate myself (thus a particular bin size of my preference). Since I don’t see any way to change the bin_size directly from the provided API, I hope to be able to influence this bin_size setting through the rbw setting.

Speaking of rbw limitation, the limits I found in the SA44B-User-Manual.pdf (your link) is different from the one in https://signalhound.com/sigdownloads/SA44B/SA-API-Manual.pdf
In Page 39 Appendix: RBW and VBW, the first point of the list basically says that only 0.1Hz <= rbw <= 100KHz AND rbw=250KHz are allowed.
This is not consistent with one listed on the SA44B user manual where rbw from 0.1Hz TO 250 KHz as long as the frequency span and start frequency requirement is satisfied.

Thanks!

Sadak,

There is unfortunately no way to directly pre-calculate beforehand the bin size because we create the window function at the time of saInitiate(). Other analyzers might use fixed windows and zero-padding which would allow you to pre-calculate the bin size beforehand. Depending on the RBW the window might affect the bin size between a factor of 3-6. (ie The bin size is roughly between 3 and 6 times smaller than the RBW), but again, there is no way to fully pre-calculate this value. This is why we provide the bin size with the start frequency, so you can calculate the frequency of any given bin and also target the frequency bin you are interested in with a small amount of arithmetic.

That said… if you require a sweep with a specific bin size spacing, you could convert the sweep the API returns by re-sampling the sweep at the frequencies you want. A very simple algorithm for this would be to simply select the closest bin to the frequency you desire and use this amplitude for your new bin. You would not want to ‘resample’ by a large factor, ideally you are ‘resampling’ by some small fraction to line up the bins on some frequency that is convenient for you. The flattop window would ensure that your peaks would remain. Certain measurements such as channel power might be affected but probably by negligible amounts.

Regards,
A.J.

Dear Andrew,

Thanks for the answer! It’s a bit unfortunate that I will not be able to precalculate the bin size, but the SA124B is still a great spectrum analyser!

A follow-up and probably related question regarding the start frequency and center frequency. For a sufficiently large RBW, I find it difficult to get the exact center frequency that I set through saConfigCenterSpan. Although as far as I know, it’s usually within a bin size distance away from the one I set through saConfigCenterSpan. Is this always the case that I will never be able to set it to the exact center frequency that I want?

Sadak,
Yes, the resolution bandwidth, or RBW, is essentially the resolution of your frequency readout. For exact frequencies we have two tools: the frequency difference meter, and the modulation analyzer in zero span.

If you are looking for precise numeric calculations on an exact frequency, I would encourage you to use our API to acquire I/Q data, which can be processed to provide whatever measurements you desire.

Dear Justin,

Thanks for the indication. I will look into the stuff that you mentioned and probably think of implementing the I/Q data acquisition as well!

[Author]

Posts