15 April 2014

Not much gong on (on 10 metres)

As usual, Spring decreases my will to build circuits (that's why there were no updates for a while). But I feel the urge to radiate some RF too. So I threw out of the balcony a 1/4 wave radiator for 24/28 MHz and last Monday I could work W1AW/1 in PA with 5 W SSB. After that ... just two continental QSOs in PSK31 throughout the week. A WSPR session brought in some North America reports, one night.

Sadly, the culprit is not a deaf or noisy antenna, but closed (and empty) bands. Will keep monitoring 10/12 m, then maybe I will lengthen the radiator wire to 15 m.

24 February 2014

AD9850 board and temperature

My AD9850 DDS chip and the 125 MHz canned oscillator run pretty warm to the touch. No info were found online, so here you are my values in room temperature (23°C): both stabilise between 45 and 48 °C.

I let it run for few hours and it didn't break nor catch fire.

Chip temperature is not output-frequency dependant, but output-frequency depends on the oscillator temperature. Touch the canned oscillator and it will change of few Hz. I am wondering if I should add a small heatsink to the oscillator. Is it worth?!



18 February 2014

Arduino controlled AD9850 DDS signal generator

Been blog-silent for a while, but my AD9850 DDS is now complete. The Arduino (Nano) code evolved day-by-day while assembling the circuit. I re-assigned some pins to simplify wire routing and implemented an easily extensible menu. My code is based on AD7C work and it is available on request.

I have added a scan function with user settable lower/upper frequency limits.

I have added a menu driven with a resistive potentiometer, which requires only one button for confirming the action and theoretically can host dozens of functions. While it is not an innovative idea, it is very comfortable to use as long as there are 5-to-10 total choices.

I am now using a TWI/I2C interface to the display, which uses 4 instead of 8 total wires.

DC is provided with a step-down regulator. I will add DC filtering capacitors near the Arduino and DDS boards.


That's two knobs and one display that now require a suitable housing.

31 January 2014

Playing with AD9850 board

While trying to understand all the pins of the AD9850 board (marked "SR-HC08"), I could not find the square wave output. A good sign was that QOUT2 was at +5 V and QOUT1 at 0 V. A simple adjustment of the onboard trimmer (duty-cycle control) activated the AD9850 internal comparator and there appeared the square wave.

Therefore output pins in my board are:
  • ZOUT2: sine wave through an LPF
  • ZOUT1: direct sine wave
  • QOUT2: square wave
  • QOUT1: square wave (180° shifted)
 If you don't need the square wave output disable it with a fully (C)CW rotation of the trimmer.

Musings about going Bluetooth

I occasionally receive a question about transporting bidirectional audio over Bluetooth and not only CAT controls. The answer is "technically possible", but there are two possible scenarios.

Voice

This is the case for handsfree operation, be it in on the move or just around the house. Using the VOX this is possible, but on the RTX side we need a Bluetooth "master" device. In order to establish a Bluetooth link there must be a master initiating the connection towards a slave. Since all in-ear handsfree earphones are "slaves", there is need for a master device connected to the transceiver: I have not found yet a master bidirectional audio Bluetooth module (maybe the one mentioned in an earlier post?!).
The discontinued Jabra A210 seems to be the only device with such characteristics, and it must be modified in order to work with a transceiver.

Digimodes

Since digimodes are generated in the computer/tablet/smartphone, we already have a master Bluetooth device. So theoretically any handsfree BT earphone would work. Besides difficulties in modifying such tiny electronics, I have been told that the audio encoding and compression operated by BT audio devices introduces unacceptable noise and phase distortion. The former affects the ability to copy weak signals at our antenna, the latter makes it impossible to work phase-based digital modes (PSK31, for example, while RTTY would probably work).



29 January 2014

So DDS I

The discussion and web-xcitement around the MINIMA transceiver designed by the BITX author added yet another project to my to-do list. Before buying an Si570 for MINIMA, I still had to play with an AD9850 DDS board I recently bought.

Wiring it up to an Arduino Nano clone was a matter of half-an-hour. A working Arduino sketch was easy to find thanks to the work done by AD7C: in an hour my DDS signal generator was up and running.

The code is simple and gets the job done. I did a few easy changes:
  • corrected the "MHz" label (was "Mhz")
  • delayed the store timing of the latest tuned frequency (2s, now 20s, not to stress the uC EEPROM)
  • added a visual feedback whether the latest frequency has been stored or not
Even to my non-calibrated frequency counter, the DDS frequency was off by ~ +50 Hz. This was corrected with a simple math that I will describe in the future.

To my surprise the whole circuit draws about 165 mA: that's a lot of current! That's 0.8 W or power consumption. Even if the AD9850 DDS offers a huge frequency agility in an HF RX/TX, that's at the expense of current consumption. I will not give away my QRP frequency XTALs just yet!



22 January 2014

First 23 cm QSO

The first 23 cm monthly activity contest date came too soon. It was my first opportunity to test on the air a 20W transverter and 23 element beam. The antenna went up easily and I used the TV coax already installed plus two BNC-N adapters: losses everywhere!

The 23 cm antenna on the balcony, beaming 60°
Apart being away for a long time from an SSB contest, I immediately felt restricted to the 3000 Hz bandwidth of the FT817, while for the last 12+ months I was able to see 2 MHz of RF spectrum.

The RTLSDR does offer an advantage. Even though I have probably not missed a QSO, I was thrown back to the good old days of search and pounce with the VFO.

I was able to speak with all the people I have heard last year. All signals were very strong, with the ODX being S9+60. Curious band!

Next month I will try to arrange more room in the shack/lab desk so that I can switch the antenna to the SDR when done with QSOs: at last I will be able to compare the two RX solutions.