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Hi all,
just for your records, attached is a VNWA plot of the NanoVNA RX input impedance.

73, Norbert, DG1KPN

yes, my NanoVNA CH1 also has VSWR=1.3 at 900 MHz. You can measure it by connect CH0 to CH1 with a good quality and short cable.

Also, according to my CH0 output voltage measurement, the output impedance of CH0 is about 39 Ohm. But it may be not very precise, because I measured it with 100 MHz bandwidth oscilloscope.

Hi qrp.dd
Incorrect, that is not a measurement of the CH1 input impedance. The cable between Ch0 and Ch1 create impedance transformation. You need to calibrate at the end of the test cable with a female kit. If done at the end of the test cable with the female female adaptor fitted and then use the male calibration kit you are getting closer, but still as you have removed the female female adaptor you measure inside th NanoVNA, as the measurement plane now is different than the calibration plane, and shifted inwards with a distance equal to the delay/offset equal to the delay of the female female adaptor.
Kind regards
Kurt

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Fra: [email protected] <[email protected]> P? vegne af qrp.ddc@...
Sendt: 22. september 2019 09:12
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Emne: Re: [nanovna-users] RX-Port Input Impedance

yes, my NanoVNA CH1 also has VSWR=1.3 at 900 MHz. You can measure it by connect CH0 to CH1 with a good quality and short cable.

On Sun, Sep 22, 2019 at 11:57 PM, Kurt Poulsen wrote:

Incorrect, that is not a measurement of the CH1 input impedance. The cable
between Ch0 and Ch1 create impedance transformation.

Actually, you're right my previous screenshot was taken with no calibration for cable end plane. :) Previously I already performed this measurement with calibration for cable end and without and didn't found significant difference for VSWR measurement. So I simply repeated measurement with no full calibration for my previous screenshot.

But how did you found that this is not cable end calibration? :)

Here is measurement with full calibration at the end of cable, you can check it on TDR screenshot. As you can see VSWR is almost the same. I'm using good quality 10 cm RG405 cable, so it doesn't affect VSWR measurement much even with no calibration on cable end.

I also tested CH1 with another type VNA and it shows the same result.

I measured the CH1 RL with a professional VNA and got this values:
100MHz 36dB
300MHz 28dB
500MHz 23dB
700MHz 20dB
900MHz 17dB

Measuring UHF filters would benefit from having a better termination impedance.

I tried to understand the CH1 input attenuator but i don't find any explanation to that topology. Using the SA612 input as single-ended, connecting IN_B to ground through a capacitor as indicated in the datasheet and simplifying the attenuator to a PI or just L arrangement could lead to a better RL with same loss. But i'm sure i'm missing something, as VNWA3 uses the same circuit although with different resistor values. It's input match is much better BTW. Maybe SA612 noise?

Carlos

To get a better CH1 input impedance I recommend to use a 10dB attenuator possibly with a 10dB amplifier when above 300MHz

It seems that CH0 output also has impedance about 40 Ohm. So, it's better to use two 10 dB attenuators. One on the CH0 and the second on CH1.

Can you show the measurement? preferably the smith chart of another VNA doing the CH0 measurement.

Hi qrp.ddc
The way you wrote "by connecting Ch0 with Ch1" it was obvious the you missed the end of cable calibration ?
Always remember a test cable is part of the instrument as you are forced to calibrate at the end of the cable. Doing thu calibration with two test cable the phase for S11 and S21 are in sync at the S11 calibration plane (at the end of the thru adaptor) and if you then measure a DUT with male SMA on input and female SMA on output the you measure accurate and phase sync (0 degree) is maintained but if removing the female female adaptor for measuring a DUT then phase relation ship between S11 and S21 is lost and you measure inside the DUT by the distance/delay of the thru adaptor. The NanoVNA or any of the PC application cannot offset the delay of the thru adaptor during calibration.
Kind regard
Kurt

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Fra: [email protected] <[email protected]> P? vegne af qrp.ddc@...
Sendt: 23. september 2019 02:25
Til: [email protected]
Emne: Re: [nanovna-users] RX-Port Input Impedance

[Edited Message Follows]

On Sun, Sep 22, 2019 at 11:57 PM, Kurt Poulsen wrote:

Incorrect, that is not a measurement of the CH1 input impedance. The
cable between Ch0 and Ch1 create impedance transformation.

Actually, you're right my previous screenshot was taken with no calibration for cable end plane. :) Previously I already performed this measurement with calibration for cable end and without and didn't found significant difference for VSWR measurement. So I simply repeated measurement with no full calibration for my previous screenshot.

But how did you found that this is not cable end calibration? :)

Here is measurement with full calibration at the end of cable, you can check it on TDR screenshot. As you can see VSWR is almost the same. I'm using good quality 10 cm RG405 cable, so it doesn't affect VSWR measurement much even with no calibration on cable end.

I also tested CH1 with another type VNA and it shows the same result.

On Mon, Sep 23, 2019 at 08:04 PM, <erik@...> wrote:

Can you show the measurement? preferably the smith chart of another VNA doing
the CH0 measurement.

I'm talking about OUTPUT impedance of CH0 (don't confuse with INPUT impedance). I didn't hear that it is possible to measure output impendance with another VNA :)

You're needs to measure amplitude on different loads and then calculate source impedance:

Z = (R1 - R1*(U1/U2)) / (U1/U2 - R1/R2)

My measurements for CH0 in attachment. R1 = 1 MOhm, R2 = 50 Ohm. Of course this is just a draft measurement for overage amplitude. I'm too lazy to perform measurement for specific frequencies. :)