Re: sBitx V2 SN #141 Spurious/Harmonic Measurements
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I think you know this, but 1N4007’s are not PIN diodes… but we force them into that service and sometimes they work (just like forcing IRF510’s into RF service… something they were not technically designed for… but we do it).? They do not have exacting specifications for use as a PIN diode so your gut is probably right… wholesale replace them as they are cheap. ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ VP2EHZ ? Moderator: North American QRO Group at Groups.IO. Moderator: Amateur Radio Builders Group at Groups.IO. ? email:? bill@... ? ?
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From: [email protected] < [email protected]> On Behalf Of Gordon Gibby Sent: Friday, October 20, 2023 4:26 AM To: [email protected]Subject: Re: [BITX20] sBitx V2 SN #141 Spurious/Harmonic Measurements ? After pondering this over night, I'm starting to more and more suspect that it has to be some diode that is AFTER the filters, since the filters look very impressive.
Even if you had a 3rd Harmonic EQUAL to the fundamental, those filters would push it almost 15 dB below the specification.? ? It is unbelievable that my 15meter 3rd harmonic of -28dBc is the result of anything BEFORE the filter.? ?Just unbelievable that it could be the drive, or final or anything!? ?Because a 60dB filter would wipe it out!
Also, in this post?/g/BITX20/message/105519? ?I showed proof that with some filtering AFTER the last diodes (an external filter) the harmonics can be? easily reduced to nearly undetectable (with a given spectrum analyzer setting).? ?This again validates my measurement setup.? ?If I can reduce the harmonics to undetectable....they don't seem to be the result of some failure of the measurement system shielding or linearity or anything else.
One of the most likely candidates for a diode past the filter is the 1N4007 used as a "PIN" diode at the output side of each filter.? ?These were not marketed as PIN diodes meeting any given harmonic suppression specification.? ?It could be possible that the intrinisic layer is of differing composition in different manufacturing outputs, and this might explain why some bands do well and others not in my particular Serial # 141.? ? And perhaps there are resonances or something in the circuitry?? ?
I found that PIN diodes are well known for producing harmonics.? ?There are IEEE papers on it that i can't get to because of a paywall.
I found specifications for high quality PIN diodes that guarantee 80 dB reduction in harmonics.??
I found a discussion on PIN diodes and their possibilities of making harmonics:?? "The primary intermodulation generator in a PIN switch is the forward biased series PIN diode. Intermod is generated in the diode when the stored charge becomes close to being swept out (or depleted) from the “I”layer region. Hence, low intermod switches employ diodes with longer than minimum minority carrier lifetimes and are biased at relatively high forward currents to store a log of charge in the junction. The degree of linearity is controlled by the percentage of charge depleted from the junction by the RF cycle. Highly linear switches have small percentage of charge depletion. See reference 3 for a more complete discussion of Intermodulation Distortion Mechanisms. A secondary intermod generator is the non-linear capacity vs. voltage characteristic of the reversed biased PIN diode. This phenomenon is relatively easily controlled by selecting diodes with flat capacitance vs. voltage characteristics and biasing the device into that region of the curve."
Unfortunately I'm leaving town in about 8 hours and have two classes to teach in between, so not sure I can get ANYTHING more done experimentally.? ?But what I think is the next thing to do is this:? ?On my 15 meter filter, which appears to be the most troublesome, use a bit of wire or a short jumper to SHORT OUT the output PIN (1n4007) diode, while the other three remain reverse biased -- and see if that changes the output spectrum.? ?I will have to reverse the soldering that I did to make the measurements of the filters before this can be done, but it may give some insight as to whether it is possible manufacturing variations in the 1N4007's are affecting the spectral purity of various serial numbers of the product.? ? I think it is the OUTPUT diode that is the most crucial.? ?If I am correct, then possibly some sort of test rig can be built to preselect the ones to use, or batches of diodes might be found to be good or bad.
All of this could be completely wrong, of course.? ?It might be something else.? ?I may have made another big mistake somewhere.? ?But more and more, I think I am validating my measurement technique and honing in on where the harmonics are coming from.? ?It should be fixable, since I was able to so clearly clean it up with a simple 2-section butterworth bandpass filter built for Field Day.?
73,
Gordon KX4Z
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