On Wed, Aug 26, 2020 at 07:49 AM, Jean-Roger Roy wrote:
Erik, Herb
From my return of experience with different spectrum analyzers :
For old style / vintage equipment such a HP8558B, the reference level rotating button is mechanically coupled with the input attenuator (see picture). So the attenuator value follows the set reference level. However you can pull one part of the button for disengage this coupling function (as for manual setting on the tiny SA). The purpose is to have always the most suited RF signal at mixer level, regarding choosen reference level.
For most recent equipments, let's say 1990 and beyond, they increase their attenuation value automatically when the reference level is set to higher values. I have worked with HP8591 or FSA family fron Rohde et Schwarz, and it is always the same behavior. Please note that there is no "power level discover" at first sweep, like on the TinySA. You must pay attention at the default level settings before doing any measurements ! Practically most of (professionnal) users rely on RefLevel regarding RF input level. For professional equipment, it is sometimes hard to find what is the best tradeoff for the value of input attenuator (IIP3, noise figure, etc.). When required the user can decide to set (carefully !) the attenuator at 0 dB, in order to get the best sensitivity.
Now as I am retired, I own a Siglent spectrum analyzer which follows the same rules. Just as an example, here is the behavior for the (auto) attenuator about my Siglent analyzer :
Manual RefLevel set at -10 dBm, Auto Attenuator fixed at 10 dB
Manual RefLevel set at 0 dBm, Auto Attenuator fixed at 20 dB
Manual RefLevel set at +10 dBm, Auto Attenuator fixed at 30 dB
For information there are also intermediate values for attenuator such as 15 dB or 25 dB, when the Reflevel is set at -5 dBm or +5 dBm.
I understand your reasoning, as I just check what is the behavior of the tinySA according to RF input level. You have an automatic setting attenuation, that does not exist on all spectrum analyzers. The Siglent has something similar called "Auto Tune", but it doesn't work at first startup (sweep) of the equipment. RefLevel remains always at 0 dBm, so you need to pay attention at the level settings before starting any measurement.?
I understand perfectly the strategy in order to protect the first stages of the tinySA. My feeling is that there is nothing to change at this side, but if possible introducing the RefLevel driving when working manually. After all, if the user decides to set manually attenuator value or RefLevel, it means normally that he understands what it does. Any of these actions are equally dangerous (not RFLevel for the moment) ! Considering tinySA specifications (Wiki page), and its behavior, could we imagine roughly something like this (input level never exceeds +10 dBm, or +20 dBm for a very short duration) :
Manual RefLevel set at +30 dBm, Auto Attenuator fixed at 30 dB
Manual RefLevel set at +20 dBm, Auto Attenuator fixed at 20 dB
Manual RefLevel set at +10 dBm, Auto Attenuator fixed at 10 dB
Manual RefLevel set at 0 dBm, Auto Attenuator at 0 dB
It could be better for "educationnal purpose" (attenuation value increases with higher RefLevel) to have this kind of behavior, but it could be challenging considering the existing AGC strategy, so don't worry or waste time on this topic. Any of view of other users could be welcome.
Jean-Roger,
? ?Excellent response.? I think you hit it on the head that graduating from the tinySA to a professional level scope will be easier if the user is already familiar with attenuation value increasing with higher RefLevel.? But as you also reasoned, with the existing tinySA AGC strategy it may not be worth the risk of breaking the current well working interface.
? Its only users like ourselves that have used commercial grade equipment that question the automatic coupling between reflevel and attenuation.? Students, enthusiasts and hobbyists won't miss it, and it may be a good thing for them to learn to make the proper settings adjustments? manually.
- Herb
