All,

I crossed some of articles recommending to use a fixed attenuator between the antenna amplifier and Port1 of NanoVNA for measuring S11 parameter of amplifier input ( or input complex impedance ) to prevent a damage of antenna amplifier when
VNA is in Auto mode providing transmit power about 0 dBm.
Looking at the block diagram of nanoVNA H4 the measurement of any impedance is done using Whinston bridge that gets calibrated with 50 ohms load. I guess using a fixed attenuator would be unappropriate for this arrangement of design.
Calibration using short, open and load, in my opinion, would create an error  in measurement when fixed attenuator with value is greater  than 3 dB is used.
A better approach  is to lower the transmit port level from 0 dBm to -10.4 dBm by changing the current from 8 mA to 2 mA of VNA driver.
Also using Whinstone bridge the measurement of S11 paramater is done indirectly through calculation process of complex S11 knowing the real and imaginary parts of impedance.
I am interested to hear other opinions as well whether use or not use a fixed attenuator.
Regards,

Raphael

On 12/12/22 9:40 AM, Raphael Wasserman wrote:
> All,
>
> I crossed some of articles recommending to use a fixed attenuator
> between the antenna amplifier and Port1 of NanoVNA for measuring S11
> parameter of amplifier input ( or input complex impedance ) to prevent a
> damage of antenna amplifier when
> VNA is in Auto mode providing transmit power about 0 dBm.



Yes, if you put a pad on CH0 (Port 1), the calibration still works
(calibrate at the "output" of the attenuator), but the dynamic range
will be reduced. That may not be an issue for you.



> Looking at the block diagram of nanoVNA H4 the measurement of any
> impedance is done using Whinston bridge that gets calibrated with 50
> ohms load. I guess using a fixed attenuator would be unappropriate for
> this arrangement of design.


I don't know that the bridge has any impact on the measurement with or
without a pad. I suppose that reducing the drive current (output power)
would allow full receiver dynamic range. If you put a pad in, you get a
hit either way. So a 10dB pad would reduce the dynamic range by 20 dB
(10dB lower power to the UUT, 10dB reduction in the reflected power, but
the noise floor stays the same).

If you've got circulators, you might be able to do a 2 port measurement,
attenuating the output power, but getting the reflected power at full
strength to CH1 (Port 2), and then use S21 measurements.

Some sort of clever rat-race type hybrid might let you separate outgoing
and incoming signals.

> Calibration using short, open and load, in my opinion, would create an
> error  in measurement when fixed attenuator with value is greater  than
> 3 dB is used.
> A better approach  is to lower the transmit port level from 0 dBm to
> -10.4 dBm by changing the current from 8 mA to 2 mA of VNA driver.
> Also using Whinstone bridge the measurement of S11 paramater is done
> indirectly through calculation process of complex S11 knowing the real
> and imaginary parts of impedance.
> I am interested to hear other opinions as well whether use or not use a
> fixed attenuator.
> Regards,
>
>  Raphael
>

Well, I was partially incorrect - Wheatstone bridge as a resistive bridge is intended for directional measurements as well in a wider frequency range vs. directional couplers.
Only unanswered left whether fixed attenuators larger than 3 dB can be used not causing measurment error.

On 12/12/22 5:14 PM, Raphael Wasserman wrote:
> Well, I was partially incorrect - Wheatstone bridge as a resistive
> bridge is intended for directional measurements as well in a wider
> frequency range vs. directional couplers.
> Only unanswered left whether fixed attenuators larger than 3 dB can be
> used not causing measurment error.



You can put a larger attenuator without trouble. The dynamic range will
decrease as 2x attenuation, so 30dB probably isn't a good idea. Do the
calibration with the attenuator installed.

How can you calibrate the output of the attenuator ?

When a short or open is applied on output of attenuator the bridge will see
some resistance in both events but the VNA will not see either short or open
conditions.

I tried both with attenuator 12 dB and lowering the drive current getting two
different results while measuring the load impedance.

With attenuator Rl was 73 ohm without attenuator with lowered current drive
about Rl 115 ohm.

Regards,

Raphael



> On Dec 13, 2022, at 12:15 AM, Raphael Wasserman via groups.io
<wassermanr46=gmail.com@groups.io> wrote:
>
>

> Well, I was partially incorrect - Wheatstone bridge as a resistive bridge
is intended for directional measurements as well in a wider frequency range
vs. directional couplers.
> Only unanswered left whether fixed attenuators larger than 3 dB can be used
not causing measurment error.

_._,_._,_

* * *

On 12/13/22 7:26 AM, Raphael Wasserman wrote:
> How can you calibrate the output of the attenuator ?
> When a short or open is applied on output of attenuator the bridge will
> see some resistance in both events but the VNA will not see either short
> or open conditions.
> I tried both with attenuator 12 dB and lowering the drive current
> getting two different results while measuring the load impedance.
> With attenuator Rl was 73 ohm without attenuator with lowered current
> drive about Rl 115 ohm.
>  Regards,
> Raphael

Do your open, short, load calibration at the output of the attenuator.

It doesn't matter what the VNA actually sees, that goes into the
calibration.

Consider the bridge inside the VNA - it's not perfect, so even if you
put a short on the output, the power seen by the receiver is less than
the outgoing power, because of losses in the bridge.

No different when there's a 10 dB (or any value) pad.

Let's say the receiver sees -3 dBm with a short on Ch0 with no pad.
That becomes the reference value (amplitude and phase) for a perfect short.
Let's say it sees -3dBm (but phase flipped 180) - with an open. That's
the reference value for perfect open.
Let's say it sees -40 dBm with a perfect load (yeah, the bridge isn't
perfect) - that becomes the reference value for a perfect load.

Same thing with a pad:
Short - sees -23 dBm
Open - sees -23 dBm
Load - sees -40 dBm (because it's dominated by the bridge leakage)

Then those values go into the calibration math.

The overall problem with putting a pad on the port is that you lose
dynamic range. The noise floor in the example is, say, -60 dBm. So
instead of a -3 - (-60) = 57 dB SNR, it's seeing -23 -(-60) = 37 dB SNR.
Which has more measurement uncertainty.

Raphael,

This topic was recently discussed in the NanoVNA Users group.
https://groups.io/g/nanovna-users/topic/77450439#18145

Roger

Jim Lux,
Your explanation is difficult to understand. I draw diagram of Wheatstone bridge exactly how it is designed for this VNA. So, the bridge all legs have 50 ohm resistors as well the resistor from power driver is 50 ohms and resistor of cross arm where the measured receive power is measured by system. First I looked what happens when the ch0
sees either short or open condition and what voltage will be measured across the cross arm resistor and later did this when to ch0 port is connected a 10 dB fixed pad for 50 ohms termination, say designed with 3 resistors.
Again when the output of attenuator is not terminated with 50 ohm load and there is either short or open condition the bridge will see different resistances and therefore different voltages across cross resistor arm will be measured. I am not sure whether those values can be considered as calibration values for further
Measurement of complex impedance load ( s11).

Raphael

> On Dec 14, 2022, at 8:59 AM, Jim Lux <jimlux@earthlink.net> wrote:
>
> On 12/13/22 7:26 AM, Raphael Wasserman wrote:
>> How can you calibrate the output of the attenuator ?
>> When a short or open is applied on output of attenuator the bridge will see some resistance in both events but the VNA will not see either short or open conditions.
>> I tried both with attenuator 12 dB and lowering the drive current getting two different results while measuring the load impedance.
>> With attenuator Rl was 73 ohm without attenuator with lowered current drive about Rl 115 ohm.
>> Regards,
>> Raphael
>
> Do your open, short, load calibration at the output of the attenuator.
>
> It doesn't matter what the VNA actually sees, that goes into the calibration.
>
> Consider the bridge inside the VNA - it's not perfect, so even if you put a short on the output, the power seen by the receiver is less than the outgoing power, because of losses in the bridge.
>
> No different when there's a 10 dB (or any value) pad.
>
> Let's say the receiver sees -3 dBm with a short on Ch0 with no pad.
> That becomes the reference value (amplitude and phase) for a perfect short.
> Let's say it sees -3dBm (but phase flipped 180) - with an open. That's the reference value for perfect open.
> Let's say it sees -40 dBm with a perfect load (yeah, the bridge isn't perfect) - that becomes the reference value for a perfect load.
>
> Same thing with a pad:
> Short - sees -23 dBm
> Open - sees -23 dBm
> Load - sees -40 dBm (because it's dominated by the bridge leakage)
>
> Then those values go into the calibration math.
>
> The overall problem with putting a pad on the port is that you lose dynamic range. The noise floor in the example is, say, -60 dBm. So instead of a -3 - (-60) = 57 dB SNR, it's seeing -23 -(-60) = 37 dB SNR. Which has more measurement uncertainty.
>
>
>
>
>
>

On 12/14/22 7:55 AM, Raphael Wasserman wrote:
> Jim Lux,
> Your explanation is difficult to understand. I draw diagram of Wheatstone bridge exactly how it is designed for this VNA. So, the bridge all legs have 50 ohm resistors as well the resistor from power driver is 50 ohms and resistor of cross arm where the measured receive power is measured by system. First I looked what happens when the ch0
> sees either short or open condition and what voltage will be measured across the cross arm resistor and later did this when to ch0 port is connected a 10 dB fixed pad for 50 ohms termination, say designed with 3 resistors.
> Again when the output of attenuator is not terminated with 50 ohm load and there is either short or open condition the bridge will see different resistances and therefore different voltages across cross resistor arm will be measured. I am not sure whether those values can be considered as calibration values for further
> Measurement of complex impedance load ( s11).
>

The whole point of the calibration process is that you solve a set of
simultaneous equations to get coefficients using 3 known impedances
(open, short, and load happen to be easy).
So whatever properties the bridge (and fixture, which could be an
attenuator) have, you have 3 measurements and 3 unknowns.

The bridge can be hideously bad, and it still works. The constraint is
this:
The uncertainty of the "calibrated" measurement (after applying the cal
factors) is determined by the SNR of the measurement, and the
uncertainty of the cal factors.

The uncertainty of the cal factors is determined by the uncertainty of
your 3 reference impedances and the SNR of those measurements and how
well the 3 standards "span" the range of measurements.

> Raphael
>> On Dec 14, 2022, at 8:59 AM, Jim Lux <jimlux@earthlink.net> wrote:
>>
>> On 12/13/22 7:26 AM, Raphael Wasserman wrote:
>>> How can you calibrate the output of the attenuator ?
>>> When a short or open is applied on output of attenuator the bridge will see some resistance in both events but the VNA will not see either short or open conditions.
>>> I tried both with attenuator 12 dB and lowering the drive current getting two different results while measuring the load impedance.
>>> With attenuator Rl was 73 ohm without attenuator with lowered current drive about Rl 115 ohm.
>>> Regards,
>>> Raphael
>>
>> Do your open, short, load calibration at the output of the attenuator.
>>
>> It doesn't matter what the VNA actually sees, that goes into the calibration.
>>
>> Consider the bridge inside the VNA - it's not perfect, so even if you put a short on the output, the power seen by the receiver is less than the outgoing power, because of losses in the bridge.
>>
>> No different when there's a 10 dB (or any value) pad.
>>
>> Let's say the receiver sees -3 dBm with a short on Ch0 with no pad.
>> That becomes the reference value (amplitude and phase) for a perfect short.
>> Let's say it sees -3dBm (but phase flipped 180) - with an open. That's the reference value for perfect open.
>> Let's say it sees -40 dBm with a perfect load (yeah, the bridge isn't perfect) - that becomes the reference value for a perfect load.
>>
>> Same thing with a pad:
>> Short - sees -23 dBm
>> Open - sees -23 dBm
>> Load - sees -40 dBm (because it's dominated by the bridge leakage)
>>
>> Then those values go into the calibration math.
>>
>> The overall problem with putting a pad on the port is that you lose dynamic range. The noise floor in the example is, say, -60 dBm. So instead of a -3 - (-60) = 57 dB SNR, it's seeing -23 -(-60) = 37 dB SNR. Which has more measurement uncertainty.
>>
>>
>>
>>
>>
>>
>
>
>
>
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