1AD Radios Of The 2005 Contest Entrants
The set consists of a one-MPF 102 JFET beat frequency oscillator (BFO) loosely coupled to a modified MRL 39 single-tuned crystal set. The BFO is at left. You can see the MPF 102 JFET centered just above the back edge of the mahogany base. The UTC LS-12 matching transformer and RCA 'Big Cans' sound powered phones are not shown. Also absent are parallel tuned wave traps, mainly because they weren't used. The BFO is coupled to the MRL 39 by wrapping a few turns of a clip lead from its output around the antenna lead-in. The basic principle of operation is exalted carrier reception. More about that later.
Thanks to Steve McDonald, VE7SL, for reminding me of possibilities of this idea for the One-Active Device (1-AD) Contest. I used it for low frequency (LF) listening a year ago -- and proceeded immediately to forget about it. The resurrected idea has also proved eminently successful for LF listening this season.
The original BFO a year ago was a small EICO 330 solid state signal generator. Because of the 1-AD Contest limit of one active device, I planned to try a simple, straightforward, one-JFET Hartley oscillator as "proof-of-concept" and then to build from there. As it turns out, there is little need to go beyond this most basic set. The coil and tuning capacitor should be high Q to keep the oscillations sharp as possible. At one time I'd thought about more "sophisticated" FETs like the J310 and U310 but decided against these devices because of the harmonics issue described below. Their high frequency response might be detrimental.
Exalted Carrier Reception. Signal from a local oscillator (the BFO) is tuned to the carrier of the desired signal. This raises the voltage of the signal to the region of much more efficient detection. Remember that crystal sets are notoriously inefficient on weak signals. Exalted carrier --where the wanted signal is amplified -- is the active counterpart to passive wave trapping -- where the un-wanted signal is attenuated. It is amplification of sorts, but it is quite different from regeneration or reflexing -- the set is continuously in oscillation. In the case of an isolated weak DX signal, exalted carrier raises the signal to the level of intelligibility (readability). When the weak DX signal is 'buried' by a strong QRM signal, successful extraction of the weak DX signal becomes a game of changing the relative amplitudes of the DX and QRM signals.
There must be more high-powered explanations around,r> but that's the gist of it.
Exalted carrier reception is not new. Early on, hams discovered that weak AM signals could be greatly enhanced by tuning them in using the single sideband (SSB) mode. Here again a local oscillator in the receiver is tuned to zero beat with the AM carrier. These rigs had the further advantage that you could select the sideband (lower, upper) you want to listen to. Many of the older top-of-the-line communications receivers, Hammarlund, National, etc., include it in their bag of features. Exalted carrier reception differs from the more sophisticated 'synchronous detection.' In synchronous detection the carrier is replaced by an oscillator in the receiver, locked to the carrier frequency and phase. While synchronous detection reduces fading, its main benefit is increased fidelity, making it particularly useful for listening to music programming. I'm not sure synchronous detection could be had with only one active device.
An indicator of the sensitivity of this set is the ring in the phones when either BFO or MRL 39 tuning capacitor is lightly rapped, causing the plates to vibrate. This is classic frequency modulation about the zero beat. Equally significant is greatly enhanced selectivity -- so much so I didn't use a wave trap. Sure, there is splatter on the baddest actors, but that's the station, not the set.
The strongest (but non-splattering) local stations were noted to 'pervade' the spectrum several tens of kilohertz to either side of their center frequencies. This is because the MRL 39 set, good as it is, has only so much selectivity. You can tell when an off-channel local is present because its audio quality is not affected by re-adjusting the BFO. As described below, on-channel audio quality is greatly affected by tweaking the BFO exactly to its frequency. Next year, I would consider using a wave trap to attenuate the QRM from off-channel locals.
The BFO is very frequency-stable in the short term in spite of there being no supply voltage regulation provisions. (Voltage regulation was planned for Phase II, but I never got that far.) 'Warm-up time' is 10 to 15 seconds. I can tune in a station, turn off the BFO for an hour or so, then turn the set back on to hear a high-pitched whistle descend exactly to zero beat in a matter of seconds. One thing working for frequency stability is the low current draw, about 0.3 mA.
Now, you don't get all this without paying a price. Here are some of the downers:
The simple BFO does have harmonic content. The BFO is going to exalt any strong signals at 2x, 3x, 4x, etc. the frequency you're tuned to. For me, this means local stations at 1080, 1500 and 1540 make listening to 540, 750 and 770 kHz difficult. 2.5 MHz WWVH ties up 1250 kHz -- I also hear them on 625 kHz. A far greater threat is all the short wave (SW) going on, especially in the evening. Fortunately, my location is in the SW boonies. Other localities may find the SW situation impossible for this type set.
The spotter radio (a Sangean pocket digital) is near-useless -- dead as a doornail. Front-end overload by the BFO signal causes AGC clamping in the Sangean. When I turn the BFO off, the Sangean comes back to life. But I can't listen to both receivers simultaneously to verify identical program content. It's not that difficult without the spotter. One can tune to a known station and 'count channels' to either side by noting the ascending/descending pitch squeal to the next zero beat.
The BFO, although verniered, requires some deft handling to get exact zero beat. At the top end, this is virtually impossible -- so I tune as close as I can and then zero-beat by judiciously positioning my left hand near the BFO -- a hand capacitance vernier. Because BFO tuning is so touchy, I went with the MRL 39 single-dial set instead of the two-dial Lyonodyne-17. Even when the carrier is zero-beat the audio is degraded because the modulation sidebands are off-frequency. Speech sounds like SSB 'duck talk,' and music can get trashed beyond recognition. But hey, we're talking DX here, not hi-fi, easy music listening from the Lazy Boy.
I discovered that frequency stability does suffer over the long term as the battery supply decays. On the last night of the 1-AD Contest, I switched out of desperation to a new battery. (The old one had been in continuous evening use for almost four months.) With the new battery, BFO calibration points were shifted by a small but noticeable half-a-percent or so.
Fair Radio 500 pF capacitor to replace the 180 pF var. cap. plus 100 pF padder. May need to reduce the inductance of the coil a little to spread the band out.
Improved vernier dial -- something larger, perhaps even a double vernier drive.
Variable level injection -- through an actual var. cap. instead of the gimmick.
Harmonic suppression -- low-pass filter, or tuned parallel tank as a notch filter. At the same time I don't want to shoot myself in the foot by attenuating high-end BCB BFO output.
In spite of all this -- or perhaps because of all this -- it sure has been a load of fun!
-Mike Tuggle- 03/28/05