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#34
March 21st 07, 04:19 PM
posted to rec.radio.amateur.homebrew,sci.electronics.basics,sci.electronics.design
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VCXO frequency isn't high enough
"W3JDR" wrote in message news:lDuKh.7998$vV3.3900@trndny09... Just to set the record straight.... Someone suggested that you need to 'neutralize' the parallel resonance so the series resonance can be tuned toward it. This is completely wrong! The series resonance is, for practical purposes, invariant. The motional parameters (L and C) of the series resonance are such high reactances (small capacitance; high inductance) that external components have only a tiny influence on the series resonance. The series resonant frequency is the lower of the two crystal 'resonances'. The parallel resonance is above it. When you make a VCXO with any substantial tuneability, you're probably operating the crystal at its parallel resonance. This leads to the common observation that you can 'pull' a crystal up in frequency more than you you can pull it down. You can only pull the parallel resonance to approach the series resonant frequency, but you can't pass it because the crystal is effectively a short-circuit at that frequency. Also for the record, the crystal's quartz only has one fundamental and significant natural resonance - the series resonance. The so-called 'parallel resonance' is actually a controlled spurious resonance caused by the holder capacitance. At frequencies above series resonanve, the crystal's RLC equivalent looks inductive, and at some frequency the holder capacitance will resonate that net inductance. Joe W3JDR "Ian Jackson" wrote in message ... In message , Anthony Fremont writes Ian Jackson wrote: As I said, sorry for the ramble. Ian. OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You should set your clock to way in the future and repost that message so it sticks around for a while. ;-) So to make a long story short I need to put an inductor of roughly 400uH across the crystal to cancel the 5pF of C2. Wow that's a ton of inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do it. I'll let you know how that works out. I found another crystal, unfortunately it's identical and possibly from the same batch. I haven't tried it yet, but I'm not expecting any miracles. I'm tired of burning my fingers unsoldering parts, so I'm goint to tinker on the breadboard with another 602 set up just for the oscillator testing with capacitor changes. I will apply the new coil to the soldered up version though. The receiver hears, as we just had a storm earlier and I could hear lightning crashes in the distance. In my narrow tuning range, I can hear what is likely the carrier of a broadcaster too, or maybe my TV. Later tonight when the band opens up some more, I should hear something from W1AW hopefully. Thanks again As you say, at around 3.5MHz, you will need a fairly large inductor to resonate with 5pF. An alternative might be to make a bridge circuit, where you actually use another (5pF) capacitor to balance out the unwanted 5pF. I used to use an extremely simple balancing circuit to make accurate measurements of the resonant frequencies and ESRs (equivalent series resistance) of VHF crystals, and it should be possible to use something similar in an oscillator. However, maybe someone out there can advise on a tried-and-tested circuit which will definitely work. [There's no point in re-inventing the wheel!] Ian. -- |
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