nanoVNA-Applications

Contents

nanoVNA Applications

$/!\$ Vorläufig / Preliminary, work in progress $/!\$

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nanoVNA Applications

In order to not extend my main wiki page nanoVNA not too much, I have split off the applications.

RF Demo Kit

The RF Demo Kit NWDZ Rev-01-10 is available via Ebay for about 15 EUR.

A documentation can be found at: http://deepelec.com/rf-demo-kit . I will just extend it a bit about details and practical usage.

The same text is also on groups.io/g/nanovna-users.

It contains on a 100 x 100 mm PCB with 18 Test fields. See the picture on the right.

Test fields, Diagram, Frequency span, Resonance

LPF-30 MHz S21 LogMag 10 MHz - 150 MHz
HPF-100 MHz S21 LogMag 50 MHz - 200 MHz
BPF-433 MHz S21 LogMag 400 MHz - 470 MHz
BSF-6.5 MHz Ceramic S21 LogMag 5.5 MHz - 7.5 MHz
33R SWR = 1.5 S11 SWR-Smith 50 KHz - 900 MHz
75R SWR = 1.5 S11 SWR-Smith 50 KHz - 900 MHz
Capacitor 115 pF S11 Smith 50 KHz - 300 MHz
Inductor 470 nH S11 Smith 50 KHz - 300 MHz
C--R 115 pF 50R S11 Smith 50 KHz - 30 MHz
C--L 18 pF 24 nH S11 Smith 50 KHz - 300 MHz, 240 MHz
C-- R || L,100pF,0.4nH,S11 Smith 50 KHz - 900 MHz, 800 MHz
R || C--L 50R S11 Smith 50 KHz - 900 MHz, 500 MHz
Short S11 Smith 50 KHz - 900 MHz
Open S11 Smith 50 KHz - 900 MHz
Load 50R S11 Smith 50 KHz - 900 MHz
Thru S11 LogMag 50 KHz - 900 MHz
Att -5 dB S21 LogMag 50 KHz - 900 MHz
Att -10 dB S11 LogMag 50 KHz - 900 MHz

For Test field 12 I made an ELSIE simulation figuring out the component values: f = 510 MHz, BW = 48 MHz, Notch = -45.5 dB, C = 1.0 pF, L = 100 nH Q = 170

The micro coax plug is named U.FL/IPX, 50 Ohm, about 2mm diameter. For more details see on Wikipedia.

$/!\$ Excerpt: "Female U.FL connectors are not designed with reconnection in mind, and they are only rated for a few reconnects approximately 30 mating cycles before replacement is needed. The female U.FL connectors are generally not sold separately, but rather as part of a pigtail with a high-quality 1.32 mm doubly shielded cable, which allows for a low-loss connection."

Because probably the female part of the U.FL plug (on the cable) wears out faster, you can buy cheap on Ebay: "UFL U.FL IPEX IPX to SMA male plug RG178 Coax Pigtail", usually 20 cm long.

$/!\$ The cable crimp in the little U.FL connector in not very solid. In order to avoid a cable and plug separation you should solder the coax cable shield crimp, see the picture on the right.

I made once this experience, and it was very difficult to repair the cable connection. In order to fit the center wire, you need to solder it with very little tin, and open the socket a little with a needle. And you need some patience, good light and a good magnifier glass.

$/!\$ To make a connection with the U.FL coax plug, use a 3 mm wide screw driver tp push down while holding with the other hand the plug centric, see the picture below. Use a 1 mm wide screw driver to lift off the U.FL female plug, while holding down the cable end of the U.FL connector.

Dr. David Kirkby recommended an Extraction Tool from company HIROSE to lift up the U.FL connector.

You have to pay about $18 + shipping, so more than the RF-Demo-Kit costs.

If you have some mechanical tools and a bench vice you could make your own Extraction Tool from an empty can, see the picture on the right.

After Calibration with the Test Fields Short, Open, Load and Thru (in my case in save3) I tested with Test Field 1 Low Pass Filter 30MHz. See the nanoVNA-Saver diagram below.

The blue Marker 2 was placed by the nanoVNA-Saver Analysis function (Low Pass -3 dB). The red marker 1 is the -40 dB point.

Test field 8, inductor

1. In Test field 8 the inductor looks like to have nominal 470 nH. If you accept a tolerance of +/- 10 % = 423 - 517 nH, the usable frequency range is measured to 3.4 - 126 MHz. Above 394 MHz the inductor becomes capacitive. See nanoVNA-Saver diagram: RF-Demo-Kit_8-470nH_Saver.png

2. A comparable measurement with the semi-professional FA-VA5 and VNWA software.

If you accept a tolerance of +/- 10 % = 423 - 517 nH, the usable frequency range is measured to 1 - 128 MHz.

Above 375 MHz the inductor becomes capacitive.

See VNWA diagram on the right.

In my understanding both diagrams show comparable results.

Coax Cable Impedance

A useful application is to measure (estimate) the Impedance of a coax cable. I had one case of a 100 cm coax cable labeled RG58 50 Ohm which behaved strange, and now measured to about 75 Ohm.

$/!\$ Connect the coax cable to the nanoVNA Reference Plane and leave the end open. Best result is achieved by calibrating at the SMA plugs of the nanoVNA (Reference Plane, e.g. C1).

Jon Gord has explained how to measure it with the nanoVNA.

As an example use a 1 m long 50 Ohm coax cable RG58.

Turn on Smith Chart display if it is not already on, and set the Smith marker for R+jX mode. See the screenshot on the right.

Connect the unknown cable, open circuit at the far end.

The upper frequency can be estimated with a Lambda/4 calculation:

1 meter cable length ~ 300 MHz / 4 = 75 MHz
- Because of the Velocity Factor of the coax cable it is actually lower (e.g. RG58 0.66 * 75 = 49.5 MHz).
25 cm cable length ~ 1200 MHz / 4 = 300 MHz

Adjust Stop frequency until the arc on the Smith Chart goes from 3 o'clock (open) to 9 o'clock (short). This is the frequency at which the cable is 1/4 wavelength long.

In this example 49.05 MHz.

Set the marker to half the Stop frequency. At this frequency the cable is 1/8 wavelength long.

In this example 24.55 MHz.

Read -jX value from the marker. The X value is the characteristic impedance of the cable. (The impedance of an 1/8-wavelength open cable is -j times the characteristic Z of the cable.)

In this example 48.95 Ohm.

Because Python 3 program nanoVNA-saver is multi platform and has a lot of features I prefer that for documentation.

In program nanoVNA-saver set the following:

Upper frequency to the estimated value from the cable length.
Lower frequency later to a value which gives a good resolution for measuring the dip frequency (Lambda / 8).
Diagram Type upper right to S11 |Z|
scan Segments to about 10, in order to easy estimate the dip frequency.

Then do a scan and adjust the Markers as following:

Marker 1 to the S11 |Z| minimum frequency.
Marker 2 to the S11 |Z| minimum frequency / 2.

Then read the coax cable Impedance from Marker 2.

See the 2 sample diagrams below, measuring a 26 cm RG316 cable and a 100 cm RG58.

Crystal measurement

$/!\$ With the standard nanoVNA-V2 firmware (and DiSlord) it is not possible to make crystal measurements.

With the nanoVNA-V2 objisan firmware it works.

A good paper about crystal measurement can be found at Agilent, 5965-4972E with the title Crystal Resonator Measuring Functions. More basic details are found at wikipedia, English and wikipedia, German.

On page 9 there is a diagram of a crystal Pi-network test fixture. The crystal impedance is assumed with 12.5 Ohm. The mentioned values of the E192 resistors gives an attenuation of 2 x 14.78 dB. That looked a bit high value for me, considering the not so good amplitude dynamic range of the nanoVNA. Another drawback was the use of the E192 series for the resistors. Fortunately I found an On-Line Pi-network calculator with variable input and output impedance parameters.

The minimum attenuation achieved was 2 x 11.44 dB with the E24 resistors R1 = none, R2 = 43R, R3 = 15R.

Next step was to build a small test fixture. See below my test fixture for HC-6/U and smaller crystals. The perforated board size is 25 x 20 mm.

2021-02-01 Update

Another good hint comes from Tom DG8SAQ in his paper VNWA_HELP.pdf version 36.7.9 page 441 and following.

He recommends to measure crystals without 12.5 Ω matching. I have tried that with a nanoVNA-V2 with ojisan firmware in chapter LC.2C_Crystal_series_resonant_measurement with good results.

With a SDR-Kit Testboard clone, see above.

The ojisan firmware allow also semi-automatic to calculate crystal parameters. See the screen shot above.

Fs= 7.997345 MHz
Fp= 8.0130 MHz
Ls= 13.5 mH
Cs= 29.3 fF
Cp= 7.47 pF
Rs= 17.5 Ω
Q= 38791

First I will show below the measurement of a 1.8432 MHz HC-6/U crystal. The -3 dB bandwidth is 35 Hz.

Next is the measurement of a 71.5 MHz HC-18/U crystal. The -3 dB bandwidth is 1.127 KHz.

Last is a 6 pole 9.0 MHz crystal filter XF-9 with 50 Ohm impedance. The -3 dB bandwidth is 5.5 KHz, the -60 dB bandwidth is 8.8 KHz.

WSPR Audio Filter

In order to improve the sensitivity of my ham radio WSPR (digital mode) reception, I checked the incoming audio signal with an DSO, and found very much noise on the signal. The WSPR reception works under normal conditions up to 3000 km distance, but I thought it could be improved by an audio filter.

I looked first with the DSO to the audio output of my F850 transceiver, at the 20 m band (14.0956 MHz). See the screenshot below, left.

Then I inserted a 3 KHz Cauer Low Pass Filter in series, and measure after the filter. See the screenshot below, right.

In order to see what it does in the spectrum, I run a DSO FFT, see below:

Filter design

For the filter design I used the very good Windows program Elsie.

I wanted as few as possible inductors in the schematic. The filter type Cauer was selected because of the steep stop curve, with few parts. The following parameters must be defined for that filter:

ELSIE menu: Design
- Topology -> Capacitor-input lowpass
- Family -> Cauer
- Bandwidth: 3K
- Order: 5
- Input termination: 600 Ohm
- Stopband width: 6 KHz
- Stopband depth: 35 dB

See the schematic of the calculated design below on the left.

Because I wanted to build the circuit with E12 series parts, I tried to find good matching values for the components, see on the right.

For the measurement with the nanoVNA with 50 Ohm impedance I added a resistor of 510 Ohm on both sides with a SMA female connector, to match the 600 Ohm impedance of the filter.

See the ELSIE calculated plot of transmission and phase on the left. On the right with the edited component values.

Now comes the reality test with the nanoVNA, see below the S11 transmission and phase values from 1 to 20 KHz. Usually the nanoVNA Firmware starts from 50 KHz upwards, but with a different firmware this was lowered to 1 KHz. This is a compromise, but better than nothing.

With the new feature Bandwidth from DiSlord in firmware 1.0.50 the noise in the Audio range is much improved with a bandwidth of 100 Hz, see below.

Filter board

The inductors (chokes) are bought at Ebay, the capacitors are SMD 0805, all soldered on a Perf-board.

I have also measured the inductance of the used chokes with nanoVNA, in order to see if it is possible, see below.

WSPR reception

It looks like an reception improvement.

It has also to do with the sun light. The whole distance must have day light, see:

From WSPR Spot Database

Timestamp -- -- -- Call -- -- -- MHz -- SNR Drift Grid - Pwr Reporter RGrid -- km --- azimut
2020-04-28 07:20 VK3QN 14.097200 -19 0 --- QF22 5 -- DL5FA -- JO40cb 16336 306 Australia, Melbourne

See the Day and Night map, date adjustable

Then later in the early afternoon the window to the USA opens:

Timestamp -- -- -- Call -- -- -- MHz - SNR Drift Grid -- Pwr Reporter RGrid -- km - azimut
2020-04-28 14:42 KK1D 14.097158 -21 1 --- FN31vi 1 -- DL5FA -- JO40cb 6012 51 USA, New York

See the Day and Night map, date adjustable

RF Calculator

You can find a useful RF calculator in the Internet.

For example a calculator to match LC components, different resistance matching.

SMA Torque Wrench Review

It is recommended, to tighten a SMA connection with a specified torque, in order to have a reproducible connection. That is the more important, the higher the frequency is (> 1 GHz).

The torque is specified by Keithley with 0.56 Nm.

Company sdr-kits tells a value of 0.45 Nm. Unfortunately a price of about 94 EUR is more than a nanoVNA costs.

There is an offer at aliexpress, search for Mxita SMA torque wrench RF. The price is about 19 EUR including shipping. The question is, how good is that tool, at such a low price?

There is a statement of Owen Duffy Review of MXITA SMA-8, in summary The thing is unusable and unrepairable. He observed several mechanical problems.

In my case I did not had any of those mentioned problems.

A good paper to read is from company Rohde & Schwarz, connector handling.

So, let's come to a practical topic. How do I adjust the torque at this torque wrench.

In most households you have a kitchen scale with >= 2 kg scale. That is enough for to use.

So, which weight do we expect at 0.56 Nm?

At an arm length of 4 cm at the wrench, 1 kg = 9.806 N and a value of 0.56 Nm you can calculate: (1 kg = 9.806 N is valid only on the earth surface and varies locally about 1%)

0.56 Nm * 1000/40 mm = 14.3 N  -> 1430 g
or
0.45 Nm * 1000/40 mm = 11.3 N  -> 1130 g

wikipedia tells a torque spread of 0.3 to 0.6 N·m for brass

But how to measure?

You press the 40 mm long arm of the torque wrench down to the scale, short before it trips. It should be done in both directions, see the pictures below. The trip point can be adjusted with a 5 mm hexagon wrench. I estimate a tolerance of about 10%, considering the wide spread of the definition.

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