30 December, 2007

AT Sprint (the original)

Photo: LA8EKA
The AT Sprint is a very nice limited edition surface mount CW transceiver designed by KD1JV Steve Weber. The printed circuit board measures 85 by 54 mm. It was quite a challenge, but a fun one, to build! 

It was designed to be a rig of a size fit for camping. It covers the 40, 30, and 20 meter bands with plug-in modules. The whole transceiver fits in an Altoids tin and outputs about 4 W. The rig in the tin weighs about 85 grams. It uses a DDS - Direct Digital Synthesis - chip for the VFO, it has a superheterodyne receiver with a four-crystal filter, audio AGC, audio filter, and a very efficient class E output stage. It also has a built-in memory keyer and a morse code frequency annunciator. It can take any battery voltage between 5.5 and 13.8 Volts.

Unfortunately, after QSOs on all three bands, my AT Sprint suddenly died, and it turned out that both the DDS and the micro controller were malfunctioning. It is hard to tell exactly why, but one theory was that there might be a problem with different supply voltages for the DDS, which subsequently led to the voltage modification described on Monty - N5ESE's AT Sprint modification page. Of course, now it is up and running again with new chips.

I am using it from my 1. floor balcony with a battery and using a rain gutter as the antenna and am having fun making contacts all over Europe with it. In my experience, this setup gives contacts according to the predictions of the Australian Space Weather Agency for Oslo (Why I have to go halfway around the world to get these, I don't know). 
  •  Sidetone pot, photo: LA8EKA
    Alternative user's guide in table format that fits the Altoids lid
  • Addition of 5 k pot for sidetone level adjustment. The pin on the power connector has to be grounded on the underside of the PCB for this to work.
  • Clipping diodes, photo: LA8EKA
    Modification of the input clipping diodes for the RF-saturated airwaves of Europe. Take a close look  at the photo and you will see the 2x3 diodes in the 40 m module and 2x2 diodes in the 30 m module.
  • RF gain, photo: LA8EKA
    Addition of an RF gain control. Especially for the 40 meter band, it is nice to be able to turn down the RF gain when there is a loud background 'rumble' and you can hear the AGC pumping its level up and down with the keying of the desired signal. It has to be done per band module. I have used a 470 ohm trim pot with the center tap to L4F and the top connection to the low impedance side of T1 (secondary), and the last pin grounded.
  • On strong signals, the audio sounds a bit 'hard', something which I found is caused by distortion in the audio AGC circuit. As suggested by Steve, I removed this by decreasing audio gain by changing R5/R6 (originally 2.2k). First I tried 10k, but audio gain was too little, so I ended up with 4.7k. 
I have worked with KD1JV and upgraded the software of the original AT Sprint, so now mine has:
  • Support for 80 meters
  • Direct frequency entry
  • RIT LED indicator using port B5. This port is already used for DDS SDATA, but it can also be used to power a small LED RIT indicator by controlling its state between each setting of the DDS
  • It sounds out audible tics every 1 kHz (E - one dot), 5 kHz (I - two dots), and 10 kHz (S - three dots)
I wrote a two page article about the AT Sprint in "Amatørradio", September 2003, the monthly journal of the Norwegian Association for Amateur Radio.

Addendum: This web page describes the original AT-Sprint (spring 2003). It has come in several versions since then, versions 2, 3, 3b, 4, and 4b. They are so different that none of the mods on this page apply to later versions.