Vorläufig / Preliminary, work in progress
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In order to not extend my main wiki page nanoVNA not too much, I have split off the applications.
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:
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.
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.
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
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
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.
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 shold 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.
2018Cookbook.pdf, guru of chokes, Jim K9YC
RFI-Ham.pdf 2019 K9YC, A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing
Application Notes in groups.io/g/nanovna-users
Absolute_Beginner_Guide_NanoVNA_v1_5.pdf by Martin J.K.
Videos from W2AEW, Alan Wolke:
Alan Wolke W2AEW: 1:23:07 VNAs Explained and the NanoVNA last 20 min. discussion.
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-- RudolfReuter 2020-04-29 08:17:09