This seems like taking an arbitrary resistor from the bin and calibrating your ohmmeter to it. From that point you only know how other resistors compare.
This also reminds me of some of the time-nuts discussions where they debate whether a millihertz matters in a 10 MHz reference oscillator. Try bringing that into the real world where the last 5 or 6 places can¡¯t even be reliably set.
Can someone please tell me to what levels of return loss it matters in different applications? For example, setting an antenna tuner, designing a RF amp, low level reception?
Is an an expensive and challenging to maintain lab VNA going to have any practical advantage over these $50 units? And under what circumstances and applications?
Peter
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On Aug 5, 2019, at 10:12 AM, tuckvk3cca <tuckvk3cca@...> wrote:
Thank you Warren. This is no argument over a pin head. Do you believe any of your loads are 50dB or better or even 38dB or better. How do these figures degrade with Frequency?Sent from my Samsung Galaxy smartphone.
-------- Original message --------From: Warren Allgyer <allgyer@...> Date: 05/08/2019 13:46 (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] NanoVNA Under The Covers I am not sure if I am adding to or further confusing this discussion which has descended into, in my opinion, an argument over the number of angels on a pin head.First, when a three point calibration is done on any VNA, the result will be to null the return from the load used in the calibration. When the calibration is finished the display will be showing the noise floor of the instrument, not the return loss from the calibration load. The only inference you can draw about the return loss of the load in this is that it is below the noise floor of the instrument.As an illustration, I have attached a chart showing the results of three different 3 point calibrations. I used two random "50 ohm" terminations from my junk box and the calibration load supplied with the nanoVNA. After each of the three calibrations I recorded the indicated return loss for all three loads in succession.As you can see from the chart, the reading for any of the the three loads when used as the calibration load is the noise floor i.e. "<-70 dB". What can also be seen from the chart is the symmetry of the readings with different calibration loads. For example, when I calibrate with Load 1, Load 2 indicates -53.6 dB. When I calibrate with Load 2 then Load 1 indicates a similar -54.3 dB.The point is that, by definition, it is impossible to get an accurate measurement of the return loss of a calibration load on the instrument that was calibrated by that load. The instrument has been set by the calibration to assume that load is perfect and will return only its noise floor level.The nanoVNA cannot and should not be expected to match the performance of a $15,000 lab instrument. But it does a remarkable job for its cost. In my "lab" I do not care about the difference in return loss between -40.5 and -41.0 dB. What I care about is at what frequency the return loss is best and, at that frequency, whether or not the load is reactive or not. I also care about having an instrument that can show a filter profile and allow me to optimize it for the performance I want. I care about what the input to my linear amplifier looks like for return loss and impedance. For these things I find the performance of the nanoVNA to be more than adequate and the value to be tremendous!
