On 8/22/20 12:36 PM, Jerry Gaffke via groups.io wrote:
Jim,
Yes, there are sources of more or less random noise.
Filtering them out would improve performance.
But RBW is not a spec you typically see when a VNA is described.
Yup - measurement bandwidth, and buried way down inside some menu on most VNAs.
But useful to know if you're using the VNA as a radar in "zero span" mode.
I assume all these VNA's are using ADC's to digitize audio from a mixer
of the outgoing RF signal and a local RF signal that is perhaps 4khz away,
creating 4khz audio, to be digitized by an ADC. Should be able to reject
anything near that 4khz audio signal in the DSP firmware on the ARM.
It's actually 5 kHz away. (see attached plot of the sin/cos table in dsp.c)
The dsp code grabs 48 samples (at 48 kHz sample rate), so 1 millisecond - that gives you about 1 kHz bandwidth
Would need the two RF signals tightly locked to a common reference oscillator
to get stable 4khz audio at 6ghz.
That's what the Silabs part in the NanoVNA does.
However, there are other ways to implement a VNA.
I believe at least some of these new VNA's described by OwO
use the fundamental from the signal source throughout the range.
The original nanoVNA does use harmonics of the Si5351 to work above 300mhz.
The primary issue is that the fundamental is stronger than the signal of interest,
so the ADC needs more dynamic range to get the same margins.
Undesired harmonics (and the fundamental if over 300mhz) do not result in 4khz audio,
and are thus easily ignored.
So I doubt harmonics due to relatively small non-linearities in an amp would have much effect.
Perhaps - I've not thought about it a lot, but intermods are always weird. You have to think about things like 2x2 and 3x2 kinds of combinations.
Jerry, KE7ER
On Sat, Aug 22, 2020 at 11:28 AM, Jim Lux wrote:
On 8/22/20 7:27 AM, Jerry Gaffke via groups.io wrote:
This being a VNA (not a spectrum analyzer),
the only frequency present in the network is assumed to be that of the
signal injected by the VNA.
Not for active systems (i.e. if you're measuring an amplifier or active filter
or mixer).
So I doubt "resolution bandwidth" applies.
It sort of does, but not in the SA sense - there's a measurement bandwidth
which essentially sets the SNR of the measurements of mag and phase in the 3
receivers.
Harmonics of the injected signal due to non-linearities in the network
should not be an issue.
One hopes. However, an amplifier might introduce noise, some of which would be
in the VNA's receiver bandwidth. And, since higher frequency ranges in the
NanoVNA depend on using harmonics of the oscillators, one might get caught by
something like that.
