Same thing again, just like an antenna. However, when I increase the series capacitance, the frequency drops instead of going higher. This I don't really get.
Did this, it helps, but when capacitance is below about 10pF, it seems to get unstable and I still can't achieve the frequency I desire.
Can you suggest a value to start with?
That's what I'm thinking now, it just won't pull that far high. I can sure pull it low though. Maybe I should try a 4MHz crystal and pull it down to
3990, this is where the voice transmissions are anyway.
I know that I'm "violating the specs", but that's the intent here. Most of the circuits I've encountered tend to pull the xtal down in frequency (just add capacitance), but this case is odd in that W1AW transmits just above the colorburst frequency. I can pull it down several kcs pretty easily, I just can't get it above the specced value by more than about 500Hz. I think I just need to try a different crystal or maybe two in parallel. The parallel inductance idea is interesting as well, never done anything like that before.
As for stability, as long as it's not causing warbling audio, I'm not too concerned. :-)
Its only a pull of ~100ppm, this should be easily pullable for most fundamental xtals. ofc if the tolerenace all add up against you you might find it hard.
I would also try reduce the 100pf caps on the sa602 too.
100pf is higher than most crystals I use would like,
50pf or 20pf or if youve got some spare trimmers ... you can adjust the ratio too, say just reduce the one accross pin 6-7
The inductor will pull the crystal down in frequency, as others have suggested.
In fact it is extremely difficult to pull a crystal's series resonance up in frequency more than a few Hz. This is because the crystal's parallel resonance is just above its series resonance. If you put a capacitor in series with the crystal the series resonant frequency goes up...BUT...if you approach the parallel resonant frequency you can no longer get a low impedance resonance condition since the crystal's parallel resonance makes the crystal look like an open circuit, regardless of what you put in series with it.
If you want to understand this better, try to find a reference with a good discussion of the equivalent circuit of the quartz crystal resonator. the only one I know of at present is Kenneth K. Clarke and Donald T. Hess, Communication Circuits: Analysis and Design, Addison- Wesley Publishing Co., 1971. It may be a bit hard to find outside a good university library.
As Ian Jackson suggested a parallel inductor might work....this modifies the parallel resonance. Ian..is there a schematic available for that ? I'd be interested in what actually worked.
I figured that they were voltage dividers to set the amount of feedback, but I can certainly see how they could have an effect. Since I'm close to where I need to be, I will try a couple of 33pF caps to see what happens.
I understand now. I knew that adding capacitance tended to slow xtals down. I was unfamiliar with how an inductor would act in series. I knew that this circuit was trying to pull the crystal higher than spec so I, quite wrongly, assumed that the inductor would have the opposite effect of capacitance. And it seems that it does when used in parallel and that seems sensible to my feeble mind. But in series, it has the opposite effect. What still throws me is that raising series capacitance doesn't have the opposite effect of adding parallel capacitance.
Hmm....food for thought. :-)
Doodling with reactance formulas, it appears that 20uH (coincidence?) would offset 100pF of capacitance fairly well by having a an opposing reactance (well resistance at this point) of about 450 Ohms at 3581kHz, the same as
100pF. I'll try putting the coil I made in parallel and see what happens. Hopefully it won't short the oscillator and kill my 15 year old NE602, I only have two spares.
do you need that much ? what freq you need ? whats the colourburst crystal freq ?
no they are involved in setting the frequency too, in order for you circuit to work it needs to resonate, with the 2 100pf the input where the crystal is looks like a capacitor with some negative impedance, the circuit with your crystal, inductor, and trimmer must be inductive, it then forms a resonant ciruit with the capacitance of the input. usualy the crystal would just be operated so that it looks inductive.
the negative part of the input impedance must be stronger than the loss in the tuned circuit. this is affected by the ratio of the 2 100pf capacitors.
a crystal can apear to be a very high inductance at resonance, at the point where you want to operate it probably has very high inductance indeed. you can determnine the eqv inductance by using the equivalent internal inductance and capacitance. you need to find the mutual capacitance of the crystal wich is hard to find man specs wich tel you this but it is often something like 14ff for example. (0.014pf) you can then work out the eqv series inductance for it to resonate with the std load wich may be 20pf.
you can then work out what inductance the crystal will apear to have at the frequency you want. and hence the series capacitance you need. you might find the inductance is so high that you need less than 1pf or it has become capacitive.
Depends on what sort of signal you are trying to receive.
To receive a single our double sideband signal (or, in a very touchy way, an AM one), you want your direct conversion receiver's local oscillator exactly on the transmitter frequency.
But to receive a CW signal, you want your local oscillator 700 Hz or so above or below the transmit frequency. That will cause you to hear a 700 Hz audio tone for the CW.
Unless I'm mistaken, 3.581 MHz is in the CW portion of the 80 meter band. If you can get to 1 KHz below the transmitter, you should be able to copy CW as 1KHz audio. If you can only get to 1.5 KHz below, you'll get 1.5 KHz audio... not ideal listening, but probably workable.
Your trimmer cap will block any DC flow to ground. If you are really paranoid then put a 0.1 mfd in series with the crystal.
If you do accidentally ground the crystal input terminal on that SA602, it only pulls the base of a transistor to ground and thus turns it off. That shouldn't hurt anything.
While you may not be able to purchase new "602's" now (they are long ago declared obsolete) the Phillips SA-612 is the same unit and is readily available from a number of outlets.
I "need" an increase of about .0546%. Isn't that about 546ppm?
3581.5kHz to zero beat with the desired signal.
3579.545kHz.
Ok, I had to read that a few times to get it. A period between ".....100pf" and "the input..." there would have been quite helpful. ;-)
So they do act like a voltage divider of sorts and shunt some of the oscillator output to ground and some back to the input.
Cool, a way to figure out just how high you can pull it and how to attain a certain frequency. I'll probably stick to tinkering though. ;-) Thanks allot for the detailed explanation. :-)
Doh, I see that now. For some reason I was thinking that they crystal had both pins connected to the oscillator on the 602, and not with one leg grounded as the circuit has it. Having yet another senior moment I guess. Only about the tenth one so far this week.
I saw that 612 part when I was poking around on the net. A fellow ham gave me three NE602s about 15 years ago. I had fogotten about them and recently found them. I even have a couple of tuning cap vernier drives he gave me.
aha ok, thats a fair bit, but maybe within range, as as been said at a certain frequency the crystal becomes a complete open circuit.
Yeah I kinda got lost in my own explanation myself. didnt have much time to explain.
this circuit is an emiter folower wich has no voltage gain, so actually they operate in the opposite way wich is kinda confusing but let me explain ... consider a typical tuned circuit with LC and a tap in the L, driving the circuit at the tap acts as a step up, but the crystal is the inductor wich make it difficult to put a tap here, but at resonance the 2 capacitors can act in the same way and provide a voltage step up.
but you can see there is a curent loop involving all these components in series, this is what sets the frequency.
If you decrease the capacitor accross the 2 pins of the ic this will increase the voltage accros it and so give more drive, as wel as increase the frequency.
another way to look at it is if you consider that ground is at the base then the capacitors are in fact a voltage divider wich feed into the emiter of a comon base amplifier.
The pulling range is usually equal to the motional capcitance over the crystal self capacitance, so for 14ff and 20pf this gives 700ppm, but actually at this extreme its unusable in this circuit as its required to be inductive.
as said by some1 else the farther you pull it the worse the performance.
It is, I mistakenly thought they did the 80 meter voice announcement down there. The voice bulletin is actually on 3990kHz.
Since I have most of what I need (tuning caps, vernier and toroids/wire) I started looking around at different VFO circuits. I'm trying to find something that will span from 3500 to 4000kHz and still be fairly simple. I could then inject this into pin 6 of the 602 and be done with it. :-)
Right. I was hoping to be able to "get on the other side" of the signal to have a second chance at avoiding possible QRM.
My guess is the project's original author hadn't planned on that... most likely, they'd planned on rubbering the crystal only far enough to get a reasonable audio tone from the RF-LO combination.
Obviously you can get more range out of a VFO, though building nice VFO's isn't simple.
Even the tuning caps can be a pain. I spent a while as a teenager knocking alternate plates out of old 365 pf AM broadcast caps to try to make some suitable for 40 meters - or you can use a series capacitor. Beware hand capacitance when you go to tune it. Today, varacter tuning is another option - stable regulated supply and a multi-turn pot.
Another thing you might do is google the "poundshop" (dollar store) receiver projects. Those are little KHz-IF varactor tuned auto- scanning FM radios that people have been modifying into direct conversion ham band receivers.
I think you are getting close to the limits :-) You might get a few Hz more by reducing the value of the 2 100pF capacitors. Try another crystal or a ceramic resonator.
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