David,
Thanks so much for you input, proper coupling internal to the circuit is surely interfered with by the probe. Good point to consider. Due to this and a number of other reasons, I have decided to abandon using the NanoVNA as a signal generator. I have a number of more attractive options. Many thanks to all who commented on my post.
30 Billy
________________________________
From:
[email protected] <
[email protected]> on behalf of David McQuate via groups.io <mcquate@...>
Sent: Saturday, December 18, 2021 18:57
To:
[email protected] <
[email protected]>
Subject: Re: [nanovna-users] NanoVNA as a Signal Generator
Another reason to include a high value series resistor (high, meaning a
few thousand ohms) is to reduce the load placed on the circuit under
test by the signal injection probe. The impedance looking into either
of the ports of the VNA is about 50 ohms. Many circuits that you might
want to test will have a much higher impedance to ground. Connecting a
low-impedance probe would almost short-circuit the circuit being tested.
The injection probe's series impedance could be made high using either a
high value resistor, or a small value capacitor (high reactance).
A note on "sweeping": the VNA probably does not continuously sweep the
frequency, but most likely jumps the frequency from one value to the
next, so that, if the VNA is set to cover 3 to 4 MHz with 100 points,
the frequency will make 100 jumps (steps) of 10kHz.
Dave WA8YWQ
On 2021-12-18 16:17, W0LEV wrote:
ONE CAUTION: You are using a decoupling capacitor. If you are coupling to
anything like vacuum tube circuits with high voltages, the spike which
occurs when you initially connect the "test end" of the decoupling
capacitor to your circuit could produce enough voltage impulse to damage
the input of the VNA. That rapid spike will propagate RF energy from the
small arc right into the NANO. I'd Strongly recommend some reasonably high
series resistance between the "test end" of our decoupling capacitor and
the circuit under test. After all, the NANOs operate on 3.5 volts and
even small sparks or arcs potentially produce RF energy from DC to
light.......
Dave - W?LEV
On Sun, Dec 19, 2021 at 12:03 AM N5SE <bwmoore@...> wrote:
Coming along nicely here, but here are some questions that might need
answers or indicate a need for more research:
1. One of the roles i am hoping the nanoVNA will fill is that of signal
generator. I want to program the device for the desired sweep frequencies,
use a cable from ch0 to a female F coax adaptor to a home brew coupling
capacitor DC isolation probe. The probe will supply a grounding clip. I
will not have anything going to ch1. I will use the probe to inject the
swept frequencies from ch0 into live circuits either to trace via other
devices, or to evaluate the circuit. Will this damage the device? Should I
leave ch1 open or short it?
2. If I understand what I have read, calibration establishes the
scaling relationship between the input parameters and output results. If
not calibrated for each set of input parameters, then the output results
are unreliable. This is taking the BIG VIEW as a general statement. I am
sure some parameters may be changed without re-calibrating but I am too
inexperienced to know what they are.
3. If (2) is correct, then the only time you HAVE to re-calibrate is
when you are interested in the results.
4. Even if (2) is correct, best to get into the habit of calibration
every time I turn it on or change parameters; like socks before shoes.
I have read the Guide and watched the videos. I mainly want to validate my
interpretation of all I looked at. Also, I have not found much of anything
about a capacitor for DC isolation.
All comments appreciated,
Billy
< [cid:55e85da0-003e-40c5-814b-f75d440c91ee][cid:2f624a96-5108-43a9-89db-54f54c537d27]< ><>
--
*Dave - W?LEV*
*Just Let Darwin Work*
N5SE