Sliders - the word from Silicon Labs
Before mailing sliders off to the antipodes, I thought some experimenters might want to re-read Scott Willingham's (of Silicon Labs) comments on how the Tecsun DSP radios work. I've highlighted the main point.
"As Roy has pointed out, the loopstick (or air-loop) inductance is resonated with
an on-chip varactor, which tunes in small discrete steps. This tuning is done
each time the frequency is changed. Roy is absolutely correct that it is vital
to repeatedly re-tune the radio as one evaluates different antenna changes. The
chip's tuning method actively adapts to the inductance, bringing the tank to
resonance regardless of the change. In other words, the varactor value is not
set simply by a lookup table corresponding to frequency. The chip measures and
peaks the resonance upon each tuning event."
Here's a link to the full post, from December 26, 2009: http://groups.yahoo.com/group/ultralightdx/message/6928
I would take a SiLabs engineer seriously. Some of our comrades must have missed that post, because this one just appeared:
"When Tecsun and Kchibo switched to loopstick coils without movable forms (as in the D96L, PL-310 and PL-380 models), it was no longer possible to use this sliding-coil method to find the optimal coil position providing maximum AM sensitivity. Rather than accept all these DSP models as already having the maximum level of AM sensitivity thoughtfully provided by the Chinese factories (a ludicrous thought, for someone who has performed as many AM alignments as me), I decided to set up an actual dynamic testing method that would perform the exact same function as the sliding-coil tests in the PL-300WT/ G8 models, allowing the use of a sliding a coil to peak a weak AM audio signal under actual reception conditions.
"I turned to one of our ULR group innovations, the Slider coil, which provides a variable inductance by means of a special 40/44 Litz wire coil that can be easily shifted across a 7.5" Amidon ferrite bar. When transplanted into these newer DSP models in place of the stock loopstick, this Slider coil provides the exact same AM-sensitivity testing function as the stock sliding coils in the PL-300WT/ G8 models (that Steve and I aligned)."When this 81-turn Slider coil is shifted along the ferrite bar after being transplanted into a PL-310 or PL-380 model, it can always quickly determine the actual coil inductance providing the maximum AM sensitivity in the model, by showing a sharp audio peak whenever the coil is close to the optimal position during reception of a weak AM station. This diagnostic test immediately corrects for any circuitry changes (from previous models) in the Si4734 radio, or in the DSP chip itself, since it is being performed under actual DXing conditions, with the best weak-signal AM reception being the bottom line. As such, it essentially creates a test where the radio tells you exactly which loopstick coil inductance it wants, to provide the best possible AM weak-signal reception. "
No matter where you put the coil on the ferrite, as soon as you retune the radio, the radio readjusts the varactor to peak it. If you keep moving the coil you're chasing your tail. If this is not clear, re-read the highlighted portion of Scott's post. The tests cited just above were performed before the experimenters understood how the chip worked. It was an honest mistake. They happen. But now that we know, direct from SiLabs, could we please stop repeating it? And BTW, Roy Dyball has verified what I just said on his digital test bench connected to a G8, with a sliding inductor on a 7.5" -61 rod. Are Roy, the chip and the SiLabs engineer all wrong?