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The OLD FRIEND Transceiver™ is a single-board, 6 Watt Direct-Conversion Transceiver specifically designed by Dave Benson, K1SWL for CW operation on the 80m, 40m, 30m, or 20m bands. This project pays homage to our early days in QRP. For homebrewers, a functioning D-C rig was a real accomplishment in those days. The Old Friend ("OF") incorporates newer technology in the form of digitally-controlled tuning and an LCD readout and includes both Iambic Keyer and Straight Key functionality. The OF further features good audio quality by virtue of its ‘relaxed’ approach to audio filtering. |
SPECIFICATIONS
Frequency
coverage:
Transmit: • Rotary Encoder- Tune/CW Speed
Short press- CW speed adjust |
The ‘Old Friend’ ... a Long
and Winding Road
Dave Benson, K1SWL
Over the years, I’ve had the pleasure of designing and fielding a fair number of kit offerings. The kits and projects for my use alone have tended toward the ambitious at times. This is NOT that story; I’m going to veer in the opposite direction!
This all started with a Direct-Conversion ‘Challenge’ rig that Craig Johnson, AA0ZZ and I had developed. We coordinated on the packaging but pursued the circuit details independently. (My version of the rig appears on my QRZ.com page.) These were functionally updates to W7EL’s classic design. I made about a dozen contacts with mine before relegating it to a shelf.
The direct conversion rig may have been the starting point for many of us getting into ‘homebrewing’ in the late ‘60s and early ‘70s. As such, their relative simplicity was hard to beat. They suffered various ills in that era: notably, hum and shortwave broadcast interference. Heathkit even produced one, the HW-7. And yes- it suffered those problems. As such, the direct-conversion approach is an ‘Old Friend’ from that bygone era and worth a second look.
So… what is a Direct-Conversion rig and why should I try one?
The D-C rig has the simplest receiver architecture in our amateur radio hobby. This simplicity is illustrated in the figure below. A rudimentary bandpass filter passes signals from a swath of the spectrum. A low-gain MMIC amplifier provides a measure of gain. The following stage, a diode-ring mixer, converts the RF signal to baseband (audio). This is a fairly robust device that is much better able to withstand high inband signal levels than the ubiquitous SA612 mixer (no longer available). To support this, the necessary Local Oscillator runs very near the received signal. Its difference frequency provides an audio tone to the following amplifier stages.
Each audio amplifier stage has both gain and a measure of low-pass filtering. Collectively, they provide about 80 dB of power gain. This is a lot of gain! I partitioned the DC biasing for this block with two separate 5V regulators. They’re inexpensive and this approach avoids the need for ‘single-point ground’ layout gymnastics.
In operation, both ‘sidebands’ are received equally well while tuning across a signal. This is a fundamental limitation of basic D-C receivers. In practice, when you tune in on a signal from the high side you’ve got it right. As such, you wouldn’t choose this kind of rig for contesting, but it’s fine for day-to-day operation. (More on that later.) A specific benefit of this architecture is the opportunity for superior audio quality. With a gentle hand on the audio filtering characteristics, it’s a design you can listen to by the hour.
Here’s a ‘dirty little secret’ about single-signal reception: aggressive QRM-fightin’ can be extremely fatiguing to listen to. We’d all love a passband characteristic with constant gain and infinitely steep skirts beyond the passband. That approach yields copious ‘ringing’ behavior due to filter impulse/step response. The effect is like listening with a drinking glass to your ear. Suffice it to say you’ve probably heard it on inexpensive QRP rigs at some point. The treatment of this topic is a little beyond the scope of this article. It’s on my ‘To-do’ list.
Anyway, that ‘challenge rig’ came off the shelf when I needed a ham buddy’s help with a plumbing job. He wanted the radio and we struck a deal. Before we made the exchange, I put it on the air in the New England QSO party in May. That netted me 34 QSOs on 40M. I was hooked!
I whomped up a new printed circuit board, driven primarily by the need for a more robust BNC jack and added audio filtering. There were enough last-minute changes that I ran the board with cuts, jumpers and a dangling daughterboard in the June ARRL Field Day event … and I made 81 contacts with it on 40M. I was so tickled with this that it became my sole focus for that event.
Recall again, this is a direct conversion rig. In my defense, I can admit to working people through lots of QRM! More than 55 years of CW operation enhances that skill. By about 7pm that Saturday and after hours of operating, though, I’d had it in that FD event. Wall-to-wall bedlam! I took it up again early Sunday morning and it was like shooting fish in a barrel … virtually everyone I called came back to me. The high point was a back-to-back pair of contacts with ND and AR. I quit the event only after tuning around and not finding any new ones to work.
Elated by that success, I continued to move forward with the project. The original ‘Challenge rig’ has now mutated to incorporate that additional audio filtering and a more capable LCD.
Recent ‘Old Friend’ Activity
If you've had contact with me on the air this fall, there's a good chance it's been while I was using a proto OF rig!• 8/15 to 9/10 -- 43 contacts
• 9/11 -- The 20M version was my sole rig on a Parks On The Air (POTA) Activation. 25 contacts in 40 minutes including 5 in the high-speed CWT Sprint
• 9/13-to-present -- 25 contacts. Both periods of ongoing activity were 80% on 20M, the remainder on 40M.
