Isidro Berniol 2020/09/06 14:28
If you use a attenuator in front of a RLB you should also use an amp at
the coupler port to have enough level for a good dynamic.
This is the reason why professional VNAs have often 2 SMA jumpers on the
input port and the coupler port of the RLB.
This makes it possible to work with higher or lower levels.
You can try also to modify your MiniVNA to use a externel RLB. Then you
are free to add attenuators and amplifiers like you wish.
A sidenote. Thr NanoVNA isn't suitable for amplifier measurement. First
because of this level issue, second because of the missing ability to do
power sweeps. So no automated way to record the 1dB compression point
over the frequency.
To do measurements an amps, mixers, multipliers, downconverters, etc.
you also need the ability to have frequency translated calibration,
nothing to be done that easy, because then you need an absolute power
calibration of port 1 via level and frequency.
I also don't have such nice tools, this is why I do that with my
sweeper, a RLB connected to my Wiltron 560 display unit and a
directional coupler at the output of the DUT going to a second detector
for S21 plus the coupling port going to my spectrum analyzer. So I can
do some simple 1dB compression point measurements and also most of the
times the 2nd and 3rd harmonic increases very fast when a wideband amp
in going into compression. Because of a nice linearity over level and
frequency of the detector heads and a nice power stability over
frequency also frequency translation devices can be measured. It is much
more timeconsuming then with a modern VNA, but it is hobby, so no need
to do it faster :-)
The major difference in using a SNA or a VNA is that the SNA has
wideband detectors, not tuned receivers like a VNA. Because of very high
quality made detectors they have a level linearity of +-1dB over a wide
frequency range. If you add then a sweeper accuracy of +-1dB and some
hopefully short cables you can meaesure frequency conversion devices to
+-3dB over a wide range and non frequency conversion devices to +- 0.5dB
in a +- 10dB range with an old but well calibrated SNA setup. Also the
SNA sweeps much faster. I have a sweep rate of 100ms for a full band
10MHz to 18GHz sweep. perfect to find faulty cables and other
intermittent faults.
The NanoVNA V2 is using different signal sources for the source and the
receiver tuning. so it should be possible to have a different tuning at
Port 1 and Port 2. So theoretically also frequency translation devices
could be measured, but this needs an absolute power calibration of Port
1 and a high power stability. Nothing that can be done without a
temperature compensation or a compensated leveling loop.
To add such features to the NanoVNA would mean for my opinion to
redesign the whole source to a stable leveled and calibrated design.
Nothing that can be done with this design.
I could imagine that a extension with a pin diode leveler and a temp.
compensated wideband detector could help to reach that goal.
Also the behavior with the pulsed output isn't suitable for amplifier
mesurement, because you like to measure also the power consumptin over
power and frequency, also the thermal behavior. For this a constant
input level is required.
Regards,
Isidro
DB1SBI
Am 06.09.2020 um 13:51 schrieb Siegfried Jackstien:
> hmmm what about using attenuator plus an external bridge ...
>
> adding attenuators keeps the amp happy (and keeps his input power in
> an area where its real input rl is fine) ... after the attenuator the
> bridge (so you can accurate measure on amp end of the attenuator) ...
> and the return port of the bridge in second port of vna ... you then
> measure your s11 value as s21 value in the device ... problem solved
>
> greetz sigi dg9bfc
>
> Am 06.09.2020 um 11:12 schrieb Bob M.:
>> I've got two 10dB SMA attenuators on the way; they should be here
>> Tuesday.
>>
>> In the meantime I connected my return loss bridge (RLB) to my
>> spectrum analyzer (SA) and discovered that a high signal level
>> feeding the preamp causes undesirable things to happen. With 0dBm
>> feeding the RLB, the Return Loss (RL) is horrible (7-8dB) however
>> when I reduced the drive to -30dBm, the RL increased to almost 20dB
>> and the tuning changed drastically as well. Now the maximum gain
>> point is very close to the best RL point. The same tuning change
>> happens when I use the SA for measuring the passband gain of the
>> preamp. Even an input level of -10dBm alters the gain tuning.
>>
>> My issue with having attenuators at Port1 is that there will no
>> longer be a true "open" nor a true "short" condition if those
>> calibration plugs are placed after the attenuator. They will reduce
>> the signal level going to the preamp and will improve the indicated
>> RL greatly, which will not give me the true RL of the preamp itself,
>> which apparently can not be done with the NanoVNA I have. While the
>> NanoVNA is great for dealing with bandpass filters and other passive
>> devices, it just doesn't seem to be the right tool for the job when
>> testing some preamps that can't deal with the -10dBm output level of
>> the NanoVNA I have.
>>
>>
>>
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