Re: limited dynamic range of the BITX40

Jerry Gaffke
 

I'd like to do a uBitx like rig without relays as they seem large and unreliable, use analog switches and PIN diodes instead, perhaps 1n4007's for the TR switch.   Lots of folks claiming success with the 1n4007 in that position, including "ElectronicAntenna Switching" by Wes Hayward in QEX, May 1995.  But worth looking at some of the rants here:
    http://forums.qrz.com/index.php?threads/looking-for-a-pin-diode.147454/page-2

Specifically this one:

"Since the 1N4007 has been made by probably fifty different companies over the years, and is offered in epoxy, glass, ceramic packages with square, round, and hexagonal die, and with die geometry from less than .040 square to larger than .090 round (and everything between), and may be fabricated by single diffusion, double diffusion, epitaxial, suprataxial or just about any other method a manufacturer chooses, nobody having a 1N4007 really has any idea what he has.The 1N4007 is also available as a die alone, without any package, in several geometries and diffusions.Since its Cj and trr are unspecified, they vary a lot from fabrication to fabrication. The really good 1N4007s made 30 years ago by Motorola in Scottsdale are long gone and now they're all made offshore, to absolutely no standards.This is why I wouldn't generalize about using a 1N4007 as a PIN substitute: Some might work great this way, others absolutely won't."

 
That guy strikes me as a bit cranky and opinionated, but he may have earned the right.  Apparently worked at Unitrode, and contributed to their "The PIN Diode Circuit Designer's Handbook".  I'd pick a few different 1n4007's from big name manufacturers on Mouser, stick with whatever brand works best.
As you push toward 30mhz the capacitance across a 1n4007 gets significant, whereas that $0.25 NXP part is good for several GHz and is designed for low distortion and a wide adjustment range as an attenuator.

If you look back to my post of a week or so ago, you will see that I didn't really know what a PIN diode was.  
So take my opinions with a large grain of salt.

Jerry, KE7ER
 


On Tue, Sep 5, 2017 at 12:54 am, Ashhar Farhan wrote:
yeah. you could even try a 1N4007, at audio frequencies, it will do just as well. These rectifiers have PIN structure too. They work very well as attenuators through the HF range.
 

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