Re: Your NanoVNA version
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Dr. Dave
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I was not addressing theoretical simplicity. I was addressing the practical simplicity of “dynamic range” as it applies to this device. I HAVE done serious measurements of the NanoVNA. The dynamic range of this device is limited by the RF output levels of the Si5351 as it cycles through fundamental frequencies up to 300 MHz and enhanced third harmonics from 300 to 900 MHz. The “Through” calibration compensates for frequency dependent variations in the Si5351 output and establishes an arbitrary level of “0 dB” in each of its 101 bins. The quoted 70 dB dynamic range is based, as you have said, on the difference between this somewhat arbitrary level and the noise floor of the device. Whether or not there is a theoretical dynamic range greater than 70 dB available by virtue of receiver performance is really immaterial from a practical standpoint because there is no more level available from the Si5351 “transmitter”. So, with regard to NanoVNA, and for the purposes of this board, which is about NanoVNA, it is indeed as simple as the difference between the compensated level displayed as “0” and the noise floor. I completely understand the theory that there may be a different value available with the same receiver and a different exciter. For the purposes of this board that is an “angels on the head of a pin” argument. WA8TOD On Aug 22, 2019, at 7:06 PM, Dr. David Kirkby from Kirkby Microwave Ltd <drkirkby@...> wrote:
On Tue, 20 Aug 2019 at 22:21, Warren Allgyer <allgyer@...> wrote:
Warren, I am afraid to say it *is* more complicated than what you state. There are actually at least *two* dynamic ranges on a VNA. The lower limit of useable measurements is usually set by the noise floor of the VNA, although Internal spurs sometimes exceed the noise floor. That’s relatively simple concept, and is what I believe you are talking about. I The upper limit of usable measurements can be set by one of two very different characteristics of the VNA. 1) The *system dynamic range* is what you are referring to, with your “0” dB reference. This is relevant to measuring passive devices. 2) The *receiver dynamic range*. Is set by the maximum signal the receiver can tolerate before it goes into unacceptable levels of compression. For professional VNAs, “unacceptable” is usually considered the 0.1 dB compression point of the receiver, but for low cost VNAs, it would not to define “unacceptable” differently . This is relevant if the source power is amplified, by an amplifier added for that purpose, or by the DUT. I have not done any serious measurements on my NanoVNA. However, if after calibration the difference between the 0 dB reference and the noise floor is 70 dB, then the *system dynamic range* is 70 dB. That is what is usable with a passive device. If after calibration the VNA can measure an amplifier with up to 20 dB of gain, the the receiver dynamic range is 70+20=90 dB. So it is not as simple as you think! -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@... Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales. Company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom |
Warren Allgyer