Update to Power Meter (AD8307), EMF detector, loop tuner

My power meter has a direct input to an AD8307 Log Amp module. This gives a range of -80 to +20dBm sensitivity, with reliable measurements from -70 to +10dBm. 

AD8307 characteristics

To use it at higher powers attenuators can be used. These can be made with an high wattage input resistor of 50R, to act as the dummy load, a series resistor into the 50R input impedance of the AD8307 module in a resistor divider. The maximum input to the AD8307 is +10dBm or 10mW for linearity (it can tolerate 20dBm, 100mW).

The aim is for 10W in to give a reading of 10mW on the meter, this needs a series resistor of 2200+100R. I have checked the readings by comparison with the power output indication of my ELAD FDM-DUO SDR.

This is the screen layout, the small signal showing is from a short antenna connected to the input picking up local RF noise... The bar has a range of 100dB

Power meter display, Bar is -80 to +20dBm

The new Arduino code for the power meter called SIMPLE_WATT is here.

LOOP TUNING

A new use for the now sensitive power meter is to tune my AL-705 loop. I connect a short antenna to the power meter and place the unit close to the loop. On transmit (either by my FDM-DUO 5-8W, or my new U3S WSPR TX 250-200mW) it deflects and allows me to tune the loop using my new remote motorised capacitor controller (see below)

Great stuff.

So how is the WSPR/PSK going?

 I have started on a more longterm project to look for myself into my propagation and reach. This is somewhat limited as I cannot mount outside aerials and I use only an indoor AL-705 loop.

These are the long term graphs I have right now for spots on WSPR, from my U3S 200mW 30m station and from my ELAD FDM-DUO 5W FT8 station

WSPR 200mW

WSPR

PSK FT8 5W

Thanks to Paul Marks down in VK land for the development of an excellent Mac app "WSPR Watch" for these graphs. More to come as I leave the WSPR on for a week or so (occupying the loop so no FT8..)

SWR meter, motorised loop antenna tuning, QRP-LABS U3S for WSPR

 I have some new projects in the pipeline. 

LOOP TUNER

A longer term idea is to fit a stepper motor to the loop tuning capacitor to remotely step up/down and eventually auto tune the AL-705 loop.

This is the state of the development with stepper and Arduino. Button drive it up/down and pot adjusts speed. Code is here.

Some progress has been made, my son has kindly designed a mount for the loop tuning capacitor in its box and the stepper motor. This he has 3D printed. We have a few problems with it. One is that the capacitor on the Alpha Antenna AL-705 tuner is NOT in the centre, but about 1mm offset to one side, meaning the stepper does not line up with the capacitor shaft! Thank you Alpha... will have to re-design and re-print...

Another issue is to find a flexible coupler from the stepper shaft to the capacitor shaft, the capacitor seems to have a strange diameter, may be American "Imperial" not metric? It measures just <8mm, but is loose on the 6-8 coupler I bought on eBay... my son has an engineerng company and he says he can drill out a 5-6 coupling to fit.

The other issue is whether the stepper has enough torque to drive the capacitor which is quite stiff. This can be check only when I can get the coupler problem solved.

I have tested it. Here are some photos

Stepper to loop connector, book is there to align shafts!

SWR on NanoVNA

Arduino and controls, two direction and speed

The stepper driver

There is an expected problem, the stepper driver is a switching signal of 1A, this generate a S9+ interference on reception. So the stepper driver must be switched on only to tune the loop, not all the time. This is implemented in the latest ANT_TUNER Arduino sketch, here

Mount 3D printing

Tuner and Arduino mounted

What it will look like, now to wire it up

There is wiring to do and a control box to build. And a software problem to solve - to inhibit the driver while not moving, so to stop the switching RF interference, hopefully.

Jungle wired

I have mounted all the electronics and wired it up, bit of a rats nest, but... And I have built a controller box, pot for speed control and a `left and `right buttons for tuning.

Control box

IT WORKS superbly, fast is just right to scan across the bands, and slow is just right to nudge across a band, while watching the NanoVNA resonance frequency and SWR.

I feel clever.

WSPR

Second I want to TX on WSPR on an ongoing way, i.e permanently on. I have built a QCX+ CW transceiver which has, it claims WSPR programming, but Ihave been unable to get it to work. I think the problem is that the class-E PA is not running in class-E due to some inductances being wrong - I get only about 2W output on 40m*. This in turn generates quite a lot of heat in the PCB substrate which is used as the PA heatsink. This heat in turn is heating the Si5351 xtal and causing it to drift over the 100+sec TX period. I can see this drift quite easily on my base station RX (ELAD FDM-DUO). It is also not decoding with WSJT software as a consequence. Anyway I don't like the QCX+ software UI for Beacon use...

So I will build the QRP-Labs U3S WSPR TX which has a GPS calibrated frequency control or an oven controlled xtal oscillator circuit to stabilise the frequency. This kit was ordered back in Dec 2020, but QRP-Labs have had long delays in getting a new GPS module built and tested and so my order was completed only at the beginning of June...

* since found my power meter is reading about 6dB LOW! So this is probably 4-5W in reality...

QRP-LABS U3S built. 

Quite a tricky kit as very tiny boards and lots of link options for different operating uses. I have the GPS board, but cannot fit it as QRP-LABS do not seem to supply the nylon standoffs for this and I have had to order some from eBay. Also had to order new push buttons as one is faulty. I have bought a small 12V to 5V regulator board for the power supply. I think it is wrong that QRP provide a 1/4" jack plug for 5V input as this size is traditionally used for 12V and could cause an accident. Could have used a USB B socket? To start with I have not built the OCX temperature controlled synthesiser module and will try with the standard Si5351 module. Here's the result on 30m when feeding 200 mW to my loop antenna:

I struggled with the menu system. But it is ingenious and when I had it sussed out it was easily done. Initially when I programmed in the frequency of 10,140,100 the U3S output was 10,140, 316 which is outside of the WSPR band (Only +/-100Hz wide). But after I left the system running for 24 hours and self GPS calibrating after each transmission, the frequency had been reduced to 10,140,206 which is closer to the band centre, probably as close as it goes... We shall see if it moves further. Very impressed with self calibrate feature.

By the way I calibrated mu ELAD FDM-DUO at 10MHz by reprogramming a small Ublox GPS module to output 10MHz in place of the usual 1pps. See down this blog...

U3S transmitting WSPR on tuned frequency 10,140, 200Hz

from location IO92 at 23dBM (200MW)

The output spectrum has tons of harmonic side lobes (phase noise?) not a single neat carrier. This is typical of the Si5351 and I have seen it in many other projects I have built. Locally decoded this brings 3-5 decode lines, the main central frequency and 3-4 other side lobes, only 10-20dB down. Not much to be done about this, unless there is a better way to program the Si5351 to remove these side lobes... 

Here's the output power into 50R load, as seen on my home made power meter (described further down this blog as part of BARSICLE project)

I have tried it driving a small loop (Wonder loop) and the result are disapppointing, pity I was planning to use this a a permanent installation for propagation monitoring. But connecting it to my new AL-705 loop brings results.

By the way there is a new version, an update, of the excellent MAC and iPhone app "WSPR Watch" which you can use to see your spots and display some stats and trends.

Update to U3S. Have installed all 3 BS170 MOSFETs, power output unchanged, as need to use higher Vcc to get more. May try 12V line to PA..

Loopy

 It's arrived. Even if Waters and Stanton confused me... I placed the order for an "ICOM" AL-705 loop antenna, and this large box arrived from Alpha antennas USA. No paper work so it made me think a bit. I called W&S they confirmed it was mine. Whoopee.

Now built the antenna

And "borrowed" my wife's camera tripod, got an iPhone clip mounting screwed on top and wired it up. 

First trial I found the tuning very sharp, but I found a way to do it. Display the frequency you want on the SDR specturm display (I use an ELAD FDM-DUO TRX). You can then peak the noise. I found I could get an SWR of 2-4. So I put my MAT-125E auto ATU inline and this was able to tune it down to an SWR of 1.4 or better!

Noise levels are much lower (S4-5) than the small 60cm loop (S8-9) I used before, and signals are up 2-3 S points.

I checked it out also with my NanoVNA, and oh boy is the tuning sharp, and the knob very sensitive.

I have now had a few weeks to use the loop. And this is what I have found

1. It does work from 40-10meters (Alpha antenna can supply a 2 extra turn loop to expand to 80m)

2. It is a bit floppy and wuld not stand up in any windy conditions

3. It is sensitive to the shape of the loop and position of the coupling loop

4. It is incredibly sharp tuning as the frequency goes up, and to get a low SWR you must be exactly on tune.

I have used my NanoVNA to look at the SWR, and also to tune the loop This is the SWR on 17m (18.100MHz for FT8)

As you can see the SWR is around 2.3 at the best point. The tuning was very very sharp and difficult to target exactly the right frequency. The problem is that touching the tuning knob affects the resonant frequency, so you have to tune a little bit low so that when not touching it the resonance goes up 5kHz or so. I am starting to think about a motor driven system which I can remotely control back and forwards. Very low RPM (3rpm?) motor or a stepper motor...?

Looking at the Smith chart shows

Here you can see the impedence is not pure resistance at 50R which we want. This is the measured characteristics

An impedance of 61R plus a reactance of -j65 (Capacitive).

I use the loop via my auto0tuner MAT-125e. This brings the SWR to my transceiver down to 1.22 when I add a little inductance.

I use  small antenna switch to 

change over from Operate to Tune

I have started to be impressed with this loop. I have it indoors and even so it is reasonably sensitive, especially on 30m and above. I have taken up tuning it with my NanoVNA to whatever frequency (+/-20kHz at best) that I want to use. Then connecting it via my MAT-125E auto ATU and optmising the SWR again at the freuency of transmission. Normally I am able to get the SWR from 1.2-1.8.

RF Combiners (or Splitters)

One of the tangled things for which there seems to nothing very clear on the web is about combiners. specifically a way to combine two RF signals with zero phase shift and equal gain on each channel.

These are typically used in an SDR to combine to two outputs of the RF mixers which have 0/90 phase RF and 0/90 AF inputs.  So here is what I have found, three solutions:

The single transformer solution, 50R in 25R out

Port A = 50R

Port B = 50R

Port S = 25R

Rint = A+B = 100R

Obviously this does not have both 50R input and output, the two ports on the right are 50R, but the combined one on the left is 25R.

The two transformer solution with matching 2:1 to give 50R inputs and output

J2  = J3 = 50R

L1 tap mat 0.7 to give 2;1 ratio, J1 = 50R, tap = 25R. 10:7 turns FT37-43

L2 Bifilar 10t+10t FT37-43

R = J2 + J3 = 100R

C for HF compensation.

A single transformer with an impedance ratio of 1.4:1, to give 50R inputs and output

R = 50||50 = 25R (51+51R parallel)

10t / 7t+7t bifilar FT37-43

Starting on a new SSB DIGITAL EXCITER

PHASING SSB_EXCITER

Note: all files and sketches on my M0IFA site

I have fiddled about before to use a Teensy 3.2 processor to create 0/90deg audio outputs, using Hilbert filters, and then mixing these with RF 0/90deg from a Si5351 synthesiser (inspired by ZL2CTM). I got fairly good results using a Teensy 3.2 processor. But I could not improve the filters which used just 100 taps in the filters. This caused the +/-45deg phase shifts to fall off at lower frequencies, which in turn lead to unwanted opposite sideband generation at lower audio frequencies.

The solution to this I think is to increase the number of Hilbert taps, to 200-400 say. I have now mapped out a new design:

1. A Teensy 4.0 will both generate the 0/90 deg audio, but also scan the rotary encoder and drive a small OLED display - showing frequency, tuning step and sideband selection LSB/USB.

2. Use commercial mixers, a pair of ADE-1+'s and a combiner (see blog post above). This will drive an output amplifier which will be an MMIC such as the MAR-6 or ERA-1.

Here's a pretty poor hand drawn circuit I will try out - this is in no way definitive, just to give you an idea of my thinking...

USB/LSB switching is in the Teensy audio chain.

The Si5351 two outputs CLK0 & CLK1 have a 0/90 deg phase shift, programmed in software and good across the HF bands, though initially I target to test them out at 7.1MHz. These feed through a couple of pi attenuators to set the drive and impedance to the mixers (target +7dBm). The audio inputs to the mixers are via a couple of diplexers. I am looking into combiner designs for the two outputs of the mixers - see next blog post, but will try out first a single transformer with windings of 7+7/10turns.

The Teensy connections are

 

Teensy 4.0 and Audio board

The Teensy is very flexible with the function of the pins and great care has to be taken to use the right ones. Several are dedicated to communication with the Digital Audio interface board. The Rotary encoder is on pins 2 CLK, 3 DT, 4 SW as usual. And the OLED shares the Audio board I2C serial bus in pins 17 SDA, 18 SCL. The Teensy will be supplied by a 5V regulated supply and the Encoder and OLED run from the 3V3 output. Pin 15 is brought out by the audio board as a "vol" pin.

The first effort software uses the Arduino IDE with the Teensyduino (on PRJC web site) add-on installed. And,

1. The Teensy Audio library, installed when you install the Arduino Teensyduino app above

2. The digital audio development web site, here to define the architecture of the DSP processing

3. Hilbert filter design software, here

3. My OLED header, Oled.h defines the graphic library used (u8g2lib) and a range of display init & functions

4. The actual code is SSB_EXCITER. This includes the Hilbert filter taps (HT_45_200.h).

Hilbert filter taps are generated by the filter design program, and the floating point values are cut and pasted to a spreadsheet, this is output as a CSV file to comma delimit the values. These are then pasted into the sketches HT_45_200m or p. The outputs on the serial monitor of these sketches is code that is put into a single "HT_45_200.h" text file for use in the SSB_EXCITER sketch. Bit comploicated but it works.

The DSP organisation is simple, mixer1 has a gain of +/-1 to switch sidebands.

And the Audio System design Tool outputs the headers and wiring as a header you can paste into the SSB_EXCITER.ino sketch.

I will update my progress as I get to grips with the hardware build. 

And progress is being made, here's the PCB on which I am building the SSB Exciter, lots of wiring and soldering to go... going to be a forest of wire, but hey ho...

Progress - decided not to mount transformer in the middle as gets in the way of wires, will find another spot for them

Close up of board, massive bus board needed! For SCL, SDA, GND, 3V3 & 5V

And it works, it tunes, it displays it selects sidebands & Si5351 outputs signals

Working board, so far

Output at 7.1MHz

And now it has been tested with audio MIC input and generates 0/90 audio outputs. (Note if you are following that new code is now on my web site here, as the first code had wrong DSP signal paths.

When running the Teensy with the SSB_EXCITER.ino sketch and an audio input to the MIC connection it gives these audio outputs with a phase shift of 0/90 deg for LSB or USB transmission, the BW is around 100Hz to 3kHz.

DISASTER: made a huge mistake when wiring board: connected PSU 5V to 3.3V rail. Blew up everything. So project stopped for now... and may never start again... expensive to buy another Teensy...

5W PA

 I have been working for some time to fathom the secrets of power amplifiers. I have been targeting 7MHz and an output of 5-10W. The output stage needs a load impedance according to this

This is the circuit I have come up with, sorry poor photo, click to enlarge,

5W 7MHz PA

Transistors 2N3904 - 2N5109 - IRF530

The gain is adjustable by the trimmer on the emitter of the first stage. With the supply at 13.8V an input of -2dBm gives a gain with outputs at each stage of +12dB, +20dB and +37dBm. The board copper is used as a heat sink and does not get excessively warm. An external 40m LPF is used, but could easily be included on the board - may need a little more space.

5W PA board, 

the emitter resistor is made of 3 x 3R0s as I don't have any 2W 1R resistors.

Power output, via a 40m LPF,  is an easy 5W @ 12V, with quite a bit more at 13.8V

Power output 50R