Saturday, October 06, 2007

Bandpass Controls for HF Digimodes

When working PSK31 or other "digimodes" on HF (high frequency) radio, we commonly use computer soundcards to analyze and transmit data. I like to use the Linux program fldigi for this, along with my Ten-Tec Orion transceiver.

The soundcard takes in the entire audio output from the Orion, typically 100 - 3,000 Hz. Fldigi (and similar software) allows you to point to a transmission of interest on a "waterfall" spectral display. This automatically sets the decoder to analyze data in a small band around the cursor using digital filtering.

This works well when the bands are relatively quiet, but when you have a band crowded with strong signals, the signal of interest can be strongly affected by "out of band" signals (but still in the 3 kHz audio region) that key the receiver's AGC (automatic gain control).

In this case, we need narrower IF (intermediate frequency) filtering than 3 kHz. Fortunately, a DSP (digital signal processing) rig like the Orion provides "infinitely" adjustable IF bandpass characteristics, so it is possible to "zoom in" on the signal of interest, largely rejecting any signals outside the small IF passband.

I have written a small Python/wxPython application "" that puts up a panel to allow Orion bandpass control from the same screen as fldigi. (Since fldigi communicates with the Orion via Hamlib at the same time as oFilter, there is a small potential for I/O conflict, but this is not a serious problem.)

Here are a few screenshots to show what is going on. Ultimately, it would be great to integrate the oFilter functions into fldigi or a similar program, using convenient mouse controls.

The PSK31 band at 14.070 MHz with many Europeans
coming in the 3 kHz default bandpass.

"Zooming in" the IF bandpass to 200 Hz,
more-or-less centered on the signal of interest.

Zooming in to a 100 Hz bandpass (minimum available).

The application.

Sunday, September 30, 2007

Report from DCC 2007

About 200 hams and friends gathered at the 26th TAPR/ARRL Digital Communications Conference for 2007 was held in Windsor Locks, CT, Sept. 28-30, 2007.

Many great papers and conversations. I presented my talk (PDF), "SourceForge, Hamlib, and Rigserve: Free Beer, Free Speech, and Rig Control", which is also printed in a somewhat different form (PDF) in the Proceedings.

A few cheap photos from my Treo 650 phone:

A typical session

Gerry Youngblood explains the Flex-Radio SDR-5000

Steve Bible, N7HPR, TAPR Vice President

Banquet speaker Bruce Perens, K6BP

Bruce had many fine insights into the state of the amateur radio world. For one thing, he noted that the average age at DCC is about 10 years less than at Dayton. "We are the future of amateur radio."

Tuesday, July 31, 2007

SourceForge, Hamlib, and Rigserve... DCC'07

My contribution for the TAPR/ARRL Digital Communications Conference, Sept., 2007 appears here, as a PDF.

DCC is always a good show for advanced Amateur Radio technology. This year, it will be in Windsor Locks, CT, not too far from me. Previous Conference Proceedings are available.

Saturday, June 16, 2007

SoftRock Antics

The "SoftRock" receiver kits have been around for a while. These are very inexpensive and basic quadrature sampler devices with a crystal controlled local oscillator. They make a complete receiver front-end for a Software Defined Radio. You can buy a version 6.2 SoftRock Lite kit from Tony, KB9YIG, for $10. I thought it was time to take the plunge.

For your $10, you get to spend hours putting together the tiny (1.5" square) printed circuit board, with a number of surface-mount components. After a little fussing, I got to the point of feeling some confidence with hand soldering the tiny components. The hardest part is organizing the right array of magnifiers to be able to see what is happening.

Here is how the board comes out:

I packaged it into a small aluminum box, and that was that!

To test it requires the following equipment: laptop computer, PowerSDR 1.8 software (from Flex Radio), a high-quality soundcard (I use the Edirol FA-66 Firewire box), and a good antenna (my 3-element SteppIR). With all that, we get the following view of the low end of 20 meters.

(Click on any image to see full resolution.)

Under normal band conditions, the SoftRock sounds just like my Orion transceiver. That is $10 well spent for self-education and fun. It calls out for a companion transmitter, so maybe the next step is to build a SoftRock Lite RxTx kit. Let's not get carried away - the RxTx kit costs $30.

Sunday, May 20, 2007

Hamvention Video Clips

A couple of short video panoramas from the Dayton Hamvention!

In the Hara Arena -- a small part of the exhibit space.

The fleamarket scene.

Saturday, May 19, 2007

Seen at Dayton Hamvention, 2007

Photos without comment from the Dayton, Ohio Hamvention, 2007. (Click for high-resolution versions.)