开云体育

I've screenshotted the Number 8 and 10 of the new version (serial LC and L):
The capacitive parts at the bottom are good, but the top side isn't it. The measurement with the professional equipment are similar.

Aha! There is so "matching" if there is a network involving reactive components,
regardless of the designer's intent for the operating frequency.

Also, if the cable between the DUT's output and the VNA is problem-free, it doesn't
matter which end of that cable gets the attenuator.

Anyway, I, too, suspect that the amplifier is oscillating. A check with a spectrum
analyzer or a wide range tunable receiver should settle that question pretty
quickly.

The wiki at /g/nanovna-users/wiki/jog_switch has a link to OEM multi-directional switch

This is why I reflashed my Acer chromebook with GalliumOS. Since it's a Linux distro, it isn't dependent on talking to the Internet to function. I still have a newer ASUS Chromebook that does everything I want it to (I don't use it for radio), so I didn't reflash that one. The Acer is an older 11" model, but it runs GalliumOS (which is optimized for Chromebooks) just great.

73,
Gwen, NG3P

HI Chris,
The Smith Chart is a plot of the reflection coefficient, gamma. Gamma is a ratio as follows: The voltage across the transmission line due to the reflected wave divided by the voltage across the transmission line due to the incident wave or Vref/Vfwd. AND very important, the phase angle between these voltages at a particular point on the transmission line locates where the point is on the chart. Gamma is a number that can be from 0 to 1.0. The angle can be zero to 360 degrees but is plotted as an angle between 0 and +180 deg or -180 deg (360 degrees around the chart). Now, plotted on the Smith Chart are circles of resistance and reactance that correspond to each possible gamma and its phase angle. When an antenna is connected to the far end of the transmission line and its Z is not equal to the transmission line's characteristic impedance (the SWR is >1) and the energy that is not absorbed by the antenna and radiated, returns as the reflected wave. Going along the transmission line back to the source this reflected wave meets the forward wave with all of the possible phase angles between them. Therefore at each point the impedance is described by the intersection (coordinates) of the resistance and reactance circles. The point is the impedance is changing depending on where you are on the transmission line with respect to where the antenna is. I know this is a lot to try and explain in one paragraph:)
The Rig expert analyzer and others can mathematically rotate around the chart so you can see what the Z is at each point.

The attenuator should be placed at the output of the amplifier, not the
input of the VNA. Output attenuation is a technique used to assure an
amplifier remains stable throughout its gain/bandwidth product or gain
slope and stabilizes the match to the following stage. Of course, this
attenuator will reduce the measured value of S21 by the attenuation
amount. It's purpose is to stablize the amplifier. - maybe a bit like loss
matching.

Dave - W?LEV

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

S21 is likely peaked due to the self resonance of the inductor in the
drain/collector.

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

I got so curious that I had to test a 10dB attenuator as David Eckhardt suggested even it is late here. The gain went down 10dB which is good I think, nothing changed much. I noticed that S11 is almost normal but noisy below 200-300Mhz, so I quess this is some kind of feedback. BF199 is not made for 200MHz or more. And the attenuator is next to VNA and there is some 30cm of coaxial between that and the amplifier, so it is not so effective taming the circuit.

To
Dana Whitlow. I build this some time ago and now I noticed that there are two capacitors 22pF and 68pF from collector to ground and output is between them. I wish I could remember what coil inductance was.

Hans
Your suggestion would work if you know how long the transmission line is in wavelengths. How can you simply measure the impedance at the shack end and know if the impedance you measured is due to the antenna's impedance or the rotation around constant SWR circle? Another way to look at it is, you measure the Z at the rig end and plot this point on the Smith Chart. Now to find the antenna's Z, you rotate around the chart by the electrical length between the rig end and the antenna (towards the load). Another way to do it is to measure the Z at the rig end and use a program such as TLW (comes with the ARRL Ant Book) to calculate the antennas Z BUT again you need to know the electrical length of the transmission line (or if you know transmission line type and it is in TLW's library of cables, you can enter the physical length). 73, Pete, WB2UAQ

I evolved from Mickey Mouse "tails" to Donald Duck bill:

Looks *much* better! Now, for results?

Oh, I'm lately working at < 10 MHz and am satisfied to get with 10% of
expectations.
I wish nanoVNA worked at < 10 kHz..

Wow! I'd say that S11 is not the only strange thing here.
For example, why is S21 so peaked?

When you say "no matching" does this mean that the input
is applied directly to the base (or gate) and that the output
is taken directly from the collector (or drain), with no tuned
networks or anything like that?

Could you perhaps supply a schematic?

Dana

Looks *much* better! Now, for results?

Dave - W ?LEV

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

For flying wires and alligator clips to work, everything must be in the *exact
same position* as when the calibration was done

I evolved from Mickey Mouse "tails" to Donald Duck bill:

Thanks I will.

There are several SMA board edge connectors on Ebay. There are models for 1.6mm and 1mm thicknessess. And of course normal models for soldering into board.

it looks like oscillation. Connect the output of the amplifier to the VNA
through a 6 or 10 dB attenuator and have another look at s11.

Dave - W?LEV

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

This is probably not NanoVNA specific.
I made a quick and simple amplifier of leaded BF199 and some passive components. Collector is via a coil I happened to have. There is no matching. Input is calibrated with a 20dB attenuator.
Gain is 23dB but S11 looks strange in a Smith chart. Is this oscillation or just feedback from output.

On Wed, Jan 8, 2020 at 03:22 PM, Brian wrote:

" .. I am not yet convinced that series inductance of the capacitor leads explains why I measure 26pF more at 28MHz. .."
=========================================
Even with a perfect fixture the capacitive reactance of a capacitor will decrease as the frequency across its plates increases. Therefore, the measured value is going to change in inverse to the frequency and not remain the same. How accurately the NanoVNA, or any other VNA, can measure the change will depend on the test fixture you use and how accurately you calibrate out its effects.

- Herb

A RF measuring instrument works much better withouta built-in QRM source!

The power inverter, microcontroller, USB and display are already RF sources.
BlueTooth should energize only between sweeps.

Thanks all for the comments. I am not yet convinced that series inductance of the capacitor leads explains why I measure 26pF more at 28MHz. The 100pF was connected with minimal lead length so I wouldn't expect 60 to 80 nH stray inductance.
I agree that at low frequencies it measures the same as my LC meter.
I think Dave Eckhardt is correct and the answer to the original question is that the test fixture must be pretty rigid and stable even at 30MHz. At 300MHz you need something really good to accurately measure components. Ideally matched to 50 ohms all the way to the component.
Thanks Brian.