TheRadioBoard

[XtalHWJ]

Has anyone experimented with the "zero threshold" receiver featured in QST a few years ago? This uses a special MOSFET as a synchronous rectifier/detector.

I just built the AM Broadcast Band version and it does work better than a 1N34 germanium diode detector, but I can only get one local station using the ferrite rod antenna. If I attach 20 ft. of wire, I can get 5 or 6 stations within about 75 miles.

If anyone has had better luck, I'd like to get some tips.

Rich
http://www.arrl.org/qst/2007/01/culter.pdf
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Homebrew Radio ex-Silicon Valley

[Bob Weaver]

Looks like Gus built one:
http://theradioboard.com/rb/viewtopic.php?t=492
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The Bunker

[battradio]

I have built a couple , and found diode sets to be more sensitive and selective than the zero bias fets . If you add small voltage they work quite nicely ,but its now a powered set .
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Mark

[XtalHWJ]

I read Gus' thread and his experience parallels mine. I do think it's better than a 1N34, but without an external antenna, the results are not amazing.

Also used an outboard LM386 audio amp, because my ears are failing. Will play a bit more with the matching xfrmr. That Bogen gives a lot of options and my new home-made ITT 'phones may require tweaking the taps.

Just finished etching the board for the shortwave version; will report on that later this week, I hope.

I built a similar detector using an MPF102 JFET; it works, but not as well as the "zero Vth" FET.

Rich
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Homebrew Radio ex-Silicon Valley

[Broesel]

I built a loop antenna crystal set with a zero-voltage MOSFET. This is a fine crystal radio having good sensitivity & selectivity and good sound quality. I used it in the BHAM sprint contest in 2009 and was surprised to get far more than 100 stations.

Brösel

[XtalHWJ]

I have since built another breadboard with the larger 16-pin version of the zero Vth MOSFET.

It works even better than the original. Now I have 4 FETs in parallel. Definitely better than a single 1N34.

Have also tried a 2N2222 as a rectifier. I drive the base with a 1.5 V battery to reduce the Vce (on) voltage. At least as good as a 1N34.

Dave Schmarder says these detectors are "legal" as they equivalent to bias cells used with traditional detectors.

So, I will keep experimenting and learning.

Rich
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Homebrew Radio ex-Silicon Valley

[gusnaz]

I believe I'll dig out that set and play with it and see if I can get some DX with my up graded antenna.

Gus
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Old age is 'when you still have
something on the ball, but you are just too tired to bounce it.'

[Austin Hellier]

Guys,
I've also used a 2N5485 FET as a detector, and even an (unpowered) MK484 IC! Yep, and they both worked, but not as well as the ALD FET IC.
You have to tune just off the centre Fo for best results.

Austin Hellier