11 June, 2017

Simple interference fix for the Chinese Pixie

The Chinese Pixie transceiver operating at 7023 kHz has become very popular. It often costs less than 5 USD on Ebay. Like most Pixies it is susceptible to broadcast breakthrough and intermodulation. Much of this is caused by the keying circuit of the audio amplifier, the LM386. The cure is to move the muting diode from the power supply pin (no. 6) to the bypass pin (no. 7). I have described this in another blog post with title: "Using pin 7 of the LM386 to reduce BCI and add side tone to Pixie 2".

Here are two pictures that show how this can be done for the Chinese Pixie. One needs an additional resistor in the range 10 - 51 ohms. If you can fit it, then use the large 51 ohms resistor that come with some of the kits (I think it is meant for a dummy load). I have used 10 ohms in the picture. It replaces the old R3 of 1 k. The diode D3 is not mounted in the holes provided, and instead it is mounted under the PCB with the minus (denoted by the ring) connected to where D3's minus was, and the plus side connected to pin 7 of the LM386.

R3 is indicated by the lower left arrow, and the old
placement of D3 is shown with the upper arrow

23 February, 2017

Comparing two antennas with WSPR

Ultimate 3S with 5-band relay module in front,
variable LM2596 power supply (with voltmeter) for
the power amplifier behind left,
a variable LM2596 supply set for 5 Volts for the Ultimate 3S
in the middle, and the antenna switch to the right in the back.
WSPR - The system for Weak Signal Propagation Reporter makes it easy to compare antennas if your transmitter can instantly switch antennas. The system shown here can send on antenna 1 for almost two minutes and then switch immediately to antenna 2 for the next transmission.

The Ultimate 3S already has software that supports that and application note 3 from QRPLabs (Controlling additional relays using the Ultimate3S “Aux”) describes how. I built mine following that note and the experience from EA1CDV.

The circuit is controlled from pin D7 and consists of a transistor, a relay, a resistor and an electrolytic capacitor. In addition I have two LEDs that indicate which antenna which is in use. In the first picture the green LED in the back right under the BNC antenna connector shows that antenna 1 is connected.

27 November, 2016

Even better low-pass filters for transmitters

The last issues of QEX have featured two interesting articles by Gary Cobb, G3TMG. He outlines the advantage of using Zolotarev designs for the harmonic suppression filters of transmitters, giving even better suppression of the second harmonic than the more common Chebyshev or quasi-elliptic filters.

Chebyshev low-pass filter from the GQRP data sheet (issue 1)
My interest in this was triggered by the test of the Ultimate 3 QRSS/WSPR kit from QRP Labs in the Nov 2016 QST. The review was positive overall, but it was remarked that the harmonic suppression does not meet FCC requirements (-43 dBc or better). I am not sure whether this is due to PCB layout issues, or if better filters can alleviate it, but I note that the design uses the simplest filter of the ones I have listed here.

The evolution of filters for use for harmonic suppression follows at least these three steps:

06 November, 2016

Yet another Arduino clock

Does the world need more Arduino clocks? Maybe not.

But I needed another Arduino project as I had made a K3NG morse keyer. I love this keyer because it is unique in supporting a display where you can see what you send. But I wasn't using the morse keyer all the time, so I wanted the hardware to serve two purposes. That's the excuse for also making a clock.

Its main features are:
The hardware for the K3NG keyer includes a speed pot and a memory bank selector (to the right) as well as four push buttons on top for selecting memories. The pot now controls the intensity of the display, but the bank selector switch is not used. Of the four push buttons, only button 1 is used. With it one can toggle the clock through various displays as shown below.

Local time, solar and lunar state
Line 1: Local day, date, time
Line 2: Sunrise, maximum solar elevation (actual solar angle during the day), sunset
Line 3: Civil dawn, local time at maximum solar elevation, civil dusk
Line 4: Lunar phase, arrow showing that it is rising, days since new moon

22 October, 2016

What it takes to make the AP510 APRS tracker useful

This small VHF APRS tracker can easily be improved with some simple measures:
  1. The 1 Watt of output power is often too little to reach the desired APRS digipeater reliably enough. It is much simpler to improve the antenna than to add an amplifier and it can be done as follows:
    • Use a longer telescopic antenna. In the picture I have used an antenna that can be extended from 16.5 cm to 45.2 cm. Depending on how you use the tracker, always extend the antenna as much as practically possible.
    • Add an external counterpoise or "tiger tail" of length a quarter of a wavelength. That's about half a meter. In the picture it is fastened on the antenna's BNC connector by means of an 8 mm ring terminal.
  2. Update the firmware, if you haven't done so already, to the version from 3 Nov 2015. I have written before about my experience with that firmware.
  3. Get rid of the pirated USB chip in the interface cable. I did that last year and now interfacing it to the PC and updating it is so much simpler.
These simple steps have made my AP510 tracker much more useful.

The post "What it takes to make the AP510 APRS tracker useful" first appeared on the LA3ZA Radio & Electronics Blog.

16 June, 2016

DIY Powerpole voltage and current meters

Powerpole voltage and current monitoring is quite nice to have. One can buy commercial meters, but due to the availability of nice and cheap modules, it is very easy to make them oneself.

To the right you'll see my combined voltage and current meter as well as my volt-meter on top of the power supply.

Both of the modules have been bought on Ebay:
  • Miniature 0-30 V DC LED 2 wire Digital voltmeter (371333527599) where the display is 22 by 10 mm. Cost slightly more than $1
  • 0-100 V, 0-10 A Dual Voltmeter Ammeter (262455987311) costing less than $3. The module size is 48 x 29 x 26 mm and the letters are 7 mm tall just like the miniature voltage display.
The wires to the voltmeter are connected directly to Powerpole connectors as shown in the second figure (upper right). Then the voltmeter itself is enclosed in transparent shring-wrap tubing of diameter approximately 20 mm like the one you also can buy on Ebay (252004328030).

The voltage-current meter is a little more complex to connect. First the volt meter has a power lead (4-30 V) and a measurement lead (0-100 V) which are connected together as I will only be using it for 12 Volts. The current measurement loop is between the negative, black, Powerpole connectors. The positive, red, Powerpole connectors are wired together.

I hope this can inspire others to make something similar. And if you do, then please let me know in the comments field!

11 April, 2016

Improved GPS reception with a ground plane

My poor-man's 10 MHz reference based on the Ublox Neo-7M GPS module didn't always receive GPS satellites. Since I rely on reception indoors, conditions were sometimes too marginal to lock the oscillator output to 10 MHz. Inspired by the QRPlabs GPS module of Hans Summers (G0UPL) with its large 6 x 6 cm PCB groundplane, I therefore decided to do something similar.

The first picture shows the unit with the 8.5 x 6.5 cm single-sided PCB ground plane attached with double-sided tape. It definitely helped make indoors reception in my shack much more reliable. In addition to the improved conditions for the patch antenna, it probably helps too that the antenna now is shielded from the digital circuitry of the GPS module, the 10 MHz pulse shaper, and the USB interface. I also added a small LED to the right so that I could see from the outside whether the GPS locks properly.

The second picture shows the interior prior to adding the ground plane.

This post is a continuation of these other posts about the 10 MHz reference:
  1. Just good enough 10 MHz reference (3 Oct 2015)
  2. Better with SMA (15 Oct 2015)
  3. Curing amnesia in the 10 MHz GPS reference (19 Nov 2015)

27 February, 2016

Teeth marks in the K3

DX-expeditions love their K3s. And I love my K3. But look closely at the MENU button and you will find the marks of someone who literally have put the K3 on their menu as well.

Neither has the BAND button escaped this. Judging from the size of the teeth marks it is perhaps not so hard to guess who did this.

This is our club station's K3 and off weekends the only inhabitants there are mice, who seem to have taken their fancy on the soft buttons of the K3. They let every other piece of equipment alone, such as the Yaesu FT-1000MP, so there is definitely something special about the K3. I would guess that this was not part of the original Elecraft design specifications for these buttons.
The remedy is shown here: A custom-designed acrylic cover that is fitted on the K3 whenever it is not in use.

This article originally appeared on the LA3ZA Radio & Electronics blog.