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VCXO frequency isn't high enough
Hello all,
I was playing around and saw that my junk box had all the parts so I started tossing this together: http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Higher capicitance does result in even lower frequency. I made the 20uH inductor by wrapping 17 turns on an old amidon FT50-61 ferrite toroid. I don't have any way to measure the inductance, but by my calculations it should be approximately 20uH. Is the ferrite saturating and messing things up? I see that touching the 20uH inductor it will cause the frequency to shift down as much as 4-5kHz. I believe this is direct conversion and that the crystal should exactly match the desired station (3581kHz) I want to recieve, that's right isn't it? Maybe this is close enough? I have to wait til tonight to see if I can actually hear anything. I mostly tinker with PIC chips so this is pretty much out of my league. I just wanted to throw it together and listen to the nightly report. It seems to be working as I can touch the 7.8uH coil (40 turns on a T50-2 toroid) and I get increased noise out of pins 4 and 5 on the NE602 (hey it's what I had in the box ;-). I haven't added the 386 yet, I just wanted to test the front end first. I'm cramming it all on a RS 276-259 pc proto board, but the layout is coming along pretty well for not planning it out. ;-) Thanks for helping. |
VCXO frequency isn't high enough
On Mar 13, 12:02 pm, "Anthony Fremont" wrote:
Hello all, I was playing around and saw that my junk box had all the parts so I started tossing this together:http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Higher capicitance does result in even lower frequency. I made the 20uH inductor by wrapping 17 turns on an old amidon FT50-61 ferrite toroid. I don't have any way to measure the inductance, but by my calculations it should be approximately 20uH. Is the ferrite saturating and messing things up? I see that touching the 20uH inductor it will cause the frequency to shift down as much as 4-5kHz. I believe this is direct conversion and that the crystal should exactly match the desired station (3581kHz) I want to recieve, that's right isn't it? Maybe this is close enough? I have to wait til tonight to see if I can actually hear anything. I mostly tinker with PIC chips so this is pretty much out of my league. I just wanted to throw it together and listen to the nightly report. It seems to be working as I can touch the 7.8uH coil (40 turns on a T50-2 toroid) and I get increased noise out of pins 4 and 5 on the NE602 (hey it's what I had in the box ;-). I haven't added the 386 yet, I just wanted to test the front end first. I'm cramming it all on a RS 276-259 pc proto board, but the layout is coming along pretty well for not planning it out. ;-) Thanks for helping. It's interesting that I don't see that oscillator circuit arrangement at http://www.ace.ual.es/~jgazquez/icons/ne602.pdf I guess you're attempting to drag the xtal off freq? Why not just use a tank? www.telstar-electronics.com |
VCXO frequency isn't high enough
Telstar Electronics wrote:
On Mar 13, 12:02 pm, "Anthony Fremont" wrote: It's interesting that I don't see that oscillator circuit arrangement at http://www.ace.ual.es/~jgazquez/icons/ne602.pdf I guess you're attempting to drag the xtal off freq? Why not just use a tank? Thanks for the reply. I should have been more clear, the desired frequency is 3581kHz so yes I need to pull the crystal up in frequency about 1.5kHz. Right now I'm only able to get to just under 3580. It seems that much inductance should have more effect than moving it up only about 500Hz, but this isn't my bag. By tank I guess you mean to rid myself of the crystal and use an ordinary LC circuit? I think I can come up with something simple, but I'd rather just get this working if at all possible. Do you have any idea what impedance the 602 wants to see on it's oscillator? I have a couple more chips and I could try and cobble something together if I knew what impedance to target. |
VCXO frequency isn't high enough
Try going smaller than 10 pF (you can put a 10 pF fixed capacitor in series
with the trimmer). |
VCXO frequency isn't high enough
"Anthony Fremont" wrote in message ... Hello all, I was playing around and saw that my junk box had all the parts so I started Get rid of the series choke and just use the small value variable capacitor to go higher in frequency. I believe the inductance will lower the frequency--the combination works to give an increased VXCO range if I am not mistaken. Pete k1zjh |
VCXO frequency isn't high enough
In message , Uncle Peter
writes "Anthony Fremont" wrote in message ... Hello all, I was playing around and saw that my junk box had all the parts so I started Get rid of the series choke and just use the small value variable capacitor to go higher in frequency. I believe the inductance will lower the frequency--the combination works to give an increased VXCO range if I am not mistaken. Pete k1zjh If the series capacitor is very small, the oscillation will stop. In these circumstances, I have successfully pulled VHF crystals (3rd, 5th and 7th overtone, between 70 and 200MHz) HF by adding an inductor IN PARALLEL with the crystal. The value will be somewhat less than that required to neutralise (ie parallel resonate with) the parallel capacitance of the crystal. The lower the inductance, the higher the frequency. Ian. -- |
VCXO frequency isn't high enough
"Anthony Fremont" wrote in message ... Hello all, I was playing around and saw that my junk box had all the parts so I started tossing this together: http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Higher capicitance does result in even lower frequency. I made the 20uH inductor by wrapping 17 turns on an old amidon FT50-61 ferrite toroid. I don't have any way to measure the inductance, but by my calculations it should be approximately 20uH. You have a capacitor and inductor in series. Capacitive reactance = -j/(wC) Inductive reactance = +jwL The net reactance [jwL-j/(wC)] may be capacitive (negative) or inductive (positive). Reducing the inductor makes it less inductive i.e. more capacitive. Reducing the capacitor also makes the net reactance more capacitive. You turned your trimmer all the way down to 10pF and it wasn't enough, so try taking a few turns off the inductor. |
VCXO frequency isn't high enough
On Tue, 13 Mar 2007 12:02:38 -0500, Anthony Fremont wrote:
The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Use less inductance or replace it with a wire link. 73, Ed. EI9GQ. -- Linux 2.6.18 Remove 'X' to reply by E-Mail |
VCXO frequency isn't high enough
Eamon Skelton wrote:
On Tue, 13 Mar 2007 12:02:38 -0500, Anthony Fremont wrote: The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Use less inductance or replace it with a wire link. I did this and now I'm up to 3580.050; an increase of about 70Hz or so.. I don't understand how this was designed to work. What is making the crystal oscillate above its design frequency? I thought the inductor had something to do with it, obviously not. Is it being operated in series mode instead of parallel? I tried putting small cap in series with the adjustable one and that just greatly narrowed the tuning range and only slightly raised the frequency. It seemed to be getting unstable at that point as my old frequency counter started showing variations of quite a few hertz each update instead of remaining steady. It could be the amplitude is too low and the counter is missing ticks. |
VCXO frequency isn't high enough
"Anthony Fremont" wrote in message ... Eamon Skelton wrote: On Tue, 13 Mar 2007 12:02:38 -0500, Anthony Fremont wrote: The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Use less inductance or replace it with a wire link. I did this and now I'm up to 3580.050; an increase of about 70Hz or so.. I don't understand how this was designed to work. What is making the crystal oscillate above its design frequency? I thought the inductor had something to do with it, obviously not. Is it being operated in series mode instead of parallel? I tried putting small cap in series with the adjustable one and that just greatly narrowed the tuning range and only slightly raised the frequency. It seemed to be getting unstable at that point as my old frequency counter started showing variations of quite a few hertz each update instead of remaining steady. It could be the amplitude is too low and the counter is missing ticks. If you want a VXCO, you use the capacitor and inductor to achieve maximum range of the crystal frequency. An inductor in series will lower the frequency. A capacitor in series will raise the frequency. Eliminate the coil and just use the series trimmer to see how how high the crystal frequency can be raised before it becomes unstable or quits oscillating. As Ian suggested, you can add an inductor ACROSS the crystal to raise the frequency. Think of the crystal as being a parallel LC circuit (over simplification) and you'll be better able to visualize how this works. Try a few other color burst xtals in the circuit for the heck of it. You may not be able to pull the crystal that far. |
VCXO frequency isn't high enough
Anthony Fremont wrote:
Telstar Electronics wrote: On Mar 13, 12:02 pm, "Anthony Fremont" wrote: It's interesting that I don't see that oscillator circuit arrangement at http://www.ace.ual.es/~jgazquez/icons/ne602.pdf I guess you're attempting to drag the xtal off freq? Why not just use a tank? Thanks for the reply. I should have been more clear, the desired frequency is 3581kHz so yes I need to pull the crystal up in frequency about 1.5kHz. Right now I'm only able to get to just under 3580. It seems that much inductance should have more effect than moving it up only about 500Hz, but this isn't my bag. By tank I guess you mean to rid myself of the crystal and use an ordinary LC circuit? I think I can come up with something simple, but I'd rather just get this working if at all possible. Do you have any idea what impedance the 602 wants to see on it's oscillator? I have a couple more chips and I could try and cobble something together if I knew what impedance to target. You need to reduce the inductor.. that is causing a down swing in your freq. -- "I'm never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5 |
VCXO frequency isn't high enough
Jamie wrote:
You need to reduce the inductor.. that is causing a down swing in your freq. Thanks, I took it out and the frequency increased by only about 70Hz, but it did increase. :-) Do you know of anything else I can do to increase the frequency by about another couple of kcs? |
VCXO frequency isn't high enough
"Anthony Fremont" wrote in message
... Hello all, I was playing around and saw that my junk box had all the parts so I started tossing this together: http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Higher capicitance does result in even lower frequency. I made the 20uH inductor by wrapping 17 turns on an old amidon FT50-61 ferrite toroid. I don't have any way to measure the inductance, but by my calculations it should be approximately 20uH. Is the ferrite saturating and messing things up? I see that touching the 20uH inductor it will cause the frequency to shift down as much as 4-5kHz. I believe this is direct conversion and that the crystal should exactly match the desired station (3581kHz) I want to recieve, that's right isn't it? Maybe this is close enough? I have to wait til tonight to see if I can actually hear anything. I mostly tinker with PIC chips so this is pretty much out of my league. I just wanted to throw it together and listen to the nightly report. It seems to be working as I can touch the 7.8uH coil (40 turns on a T50-2 toroid) and I get increased noise out of pins 4 and 5 on the NE602 (hey it's what I had in the box ;-). I haven't added the 386 yet, I just wanted to test the front end first. I'm cramming it all on a RS 276-259 pc proto board, but the layout is coming along pretty well for not planning it out. ;-) Thanks for helping. maybe the Q of you inductor is too low, try an air core, needs lots more turns, a trimmer core as a core is good too. Also you should test it with the crystal shorted, then when you get close to the frequency you want put the crystal back in. Colin =^.^= |
VCXO frequency isn't high enough
Anthony Fremont wrote:
Jamie wrote: You need to reduce the inductor.. that is causing a down swing in your freq. Thanks, I took it out and the frequency increased by only about 70Hz, but it did increase. :-) Do you know of anything else I can do to increase the frequency by about another couple of kcs? try a different Xstyle, it sounds like your at the limits.. it's possible the chip has too much capacitance on the osc legs, i suppose you could try a stand alone osc with a high freq transistor to lower the cap. use a buffer on the stage to drive the chip . Also, have you considered the board mounting? you could have to much cap via your board construction. -- "I'm never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5 |
VCXO frequency isn't high enough
On 13 Mar, 17:52, "colin" wrote:
"Anthony Fremont" wrote in message ... Hello all, I was playing around and saw that my junk box had all the parts so I started tossing this together: http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Higher capicitance does result in even lower frequency. I made the 20uH inductor by wrapping 17 turns on an old amidon FT50-61 ferrite toroid. I don't have any way to measure the inductance, but by my calculations it should be approximately 20uH. Is the ferrite saturating and messing things up? I see that touching the 20uH inductor it will cause the frequency to shift down as much as 4-5kHz. I believe this is direct conversion and that the crystal should exactly match the desired station (3581kHz) I want to recieve, that's right isn't it? Maybe this is close enough? I have to wait til tonight to see if I can actually hear anything. I mostly tinker with PIC chips so this is pretty much out of my league. I just wanted to throw it together and listen to the nightly report. It seems to be working as I can touch the 7.8uH coil (40 turns on a T50-2 toroid) and I get increased noise out of pins 4 and 5 on the NE602 (hey it's what I had in the box ;-). I haven't added the 386 yet, I just wanted to test the front end first. I'm cramming it all on a RS 276-259 pc proto board, but the layout is coming along pretty well for not planning it out. ;-) Thanks for helping. maybe the Q of you inductor is too low, try an air core, needs lots more turns, a trimmer core as a core is good too. Also you should test it with the crystal shorted, then when you get close to the frequency you want put the crystal back in. Colin =^.^= My W1AW receiver uses the crystal with just a 5-47 pf variable capacitor...no inductor, and it nets right on frequency. Try shorting the inductor and see if this gets you closer to the required frequency. Not all color burst crystals were created equal. If you have another crystal you might want to try it. Your ferrite will not be saturating at the small amount of signal you are sending through it as part of an SA-602 oscillator. Arv - K7HKL _._ |
VCXO frequency isn't high enough
On Tue, 13 Mar 2007 18:00:15 -0500, "Anthony Fremont"
wrote: Eamon Skelton wrote: On Tue, 13 Mar 2007 12:02:38 -0500, Anthony Fremont wrote: The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Use less inductance or replace it with a wire link. I did this and now I'm up to 3580.050; an increase of about 70Hz or so.. I don't understand how this was designed to work. What is making the crystal oscillate above its design frequency? I thought the inductor had something to do with it, obviously not. Is it being operated in series mode instead of parallel? I tried putting small cap in series with the adjustable one and that just greatly narrowed the tuning range and only slightly raised the frequency. It seemed to be getting unstable at that point as my old frequency counter started showing variations of quite a few hertz each update instead of remaining steady. It could be the amplitude is too low and the counter is missing ticks. --- Your color burst crystal is ground to ring at 3.579545 MHz, and if you try to run it at another frequency you'll run into drive and tempco problems which will make it worthless. -- JF |
VCXO frequency isn't high enough
Jamie wrote:
Anthony Fremont wrote: Thanks, I took it out and the frequency increased by only about 70Hz, but it did increase. :-) Do you know of anything else I can do to increase the frequency by about another couple of kcs? try a different Xstyle, it sounds like your at the limits.. it's possible the chip has too much capacitance on the osc legs, i suppose you could try a stand alone osc with a high freq transistor to lower the cap. use a buffer on the stage to drive the chip . Also, have you considered the board mounting? you could have to much cap via your board construction. Looks like you are quite likely correct. Without the inductor and with a 10pF cap in place of the variable cap, I can get the frequency up to just over 3580kHz. I did some looking around and it appears that if I were to parallel another crystal, I could get a bigger spread on frequency. Or I could just try a different crystal too. I think I will tinker on a breadboard to see how high in frequency I can get the crystal to vibrate. |
VCXO frequency isn't high enough
Charles Schuler wrote:
Try going smaller than 10 pF (you can put a 10 pF fixed capacitor in series with the trimmer). I did this and it will go slightly higher in frequency, but it gets a tad unstable (could be the counter too, I wish my new scope would get here). I guess I'm at the limits of that particular crystal. |
VCXO frequency isn't high enough
Ian Jackson wrote:
In message , Uncle Peter Get rid of the series choke and just use the small value variable capacitor to go higher in frequency. I believe the inductance will lower the frequency--the combination works to give an increased VXCO range if I am not mistaken. If the series capacitor is very small, the oscillation will stop. In these circumstances, I have successfully pulled VHF crystals (3rd, 5th It hasn't stopped, but it has gotten weak or unstable (I can't be sure right now) with capacitance below 10pF. and 7th overtone, between 70 and 200MHz) HF by adding an inductor IN PARALLEL with the crystal. The value will be somewhat less than that required to neutralise (ie parallel resonate with) the parallel capacitance of the crystal. The lower the inductance, the higher the frequency. Are you referring to the capacitance inside the crystal, or the 100pF external cap? Do you have a ballpark number on the inductance value I should try? |
VCXO frequency isn't high enough
Andrew Holme wrote:
"Anthony Fremont" wrote in message ... Hello all, I was playing around and saw that my junk box had all the parts so I started tossing this together: http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Higher capicitance does result in even lower frequency. I made the 20uH inductor by wrapping 17 turns on an old amidon FT50-61 ferrite toroid. I don't have any way to measure the inductance, but by my calculations it should be approximately 20uH. You have a capacitor and inductor in series. Capacitive reactance = -j/(wC) Inductive reactance = +jwL The net reactance [jwL-j/(wC)] may be capacitive (negative) or inductive (positive). I understand that, but I don't use values like j and w, just 2, f and PI. ;-) Reducing the inductor makes it less inductive i.e. more capacitive. Reducing the capacitor also makes the net reactance more capacitive. You turned your trimmer all the way down to 10pF and it wasn't enough, so try taking a few turns off the inductor. I took it out, and it's still too low in frequency. :-( Back to the drawing board I guess. |
VCXO frequency isn't high enough
Uncle Peter wrote:
"Anthony Fremont" wrote in message Use less inductance or replace it with a wire link. I did this and now I'm up to 3580.050; an increase of about 70Hz or so.. I don't understand how this was designed to work. What is making the crystal oscillate above its design frequency? I thought the inductor had something to do with it, obviously not. Is it being operated in series mode instead of parallel? I tried putting small cap in series with the adjustable one and that just greatly narrowed the tuning range and only slightly raised the frequency. It seemed to be getting unstable at that point as my old frequency counter started showing variations of quite a few hertz each update instead of remaining steady. It could be the amplitude is too low and the counter is missing ticks. If you want a VXCO, you use the capacitor and inductor to achieve maximum range of the crystal frequency. An inductor in series will lower the frequency. Ok, just like an antenna A capacitor in series will raise the frequency. 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. Eliminate the coil and just use the series trimmer to see how how high the crystal frequency can be raised before it becomes unstable or quits oscillating. 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. As Ian suggested, you can add an inductor ACROSS the crystal to raise the frequency. Think of the crystal as being a parallel LC circuit (over simplification) and you'll be better able to visualize how this works. Can you suggest a value to start with? Try a few other color burst xtals in the circuit for the heck of it. You may not be able to pull the crystal that far. 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. |
VCXO frequency isn't high enough
John Fields wrote:
On Tue, 13 Mar 2007 18:00:15 -0500, "Anthony Fremont" wrote: Eamon Skelton wrote: On Tue, 13 Mar 2007 12:02:38 -0500, Anthony Fremont wrote: The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Use less inductance or replace it with a wire link. I did this and now I'm up to 3580.050; an increase of about 70Hz or so.. I don't understand how this was designed to work. What is making the crystal oscillate above its design frequency? I thought the inductor had something to do with it, obviously not. Is it being operated in series mode instead of parallel? I tried putting small cap in series with the adjustable one and that just greatly narrowed the tuning range and only slightly raised the frequency. It seemed to be getting unstable at that point as my old frequency counter started showing variations of quite a few hertz each update instead of remaining steady. It could be the amplitude is too low and the counter is missing ticks. --- Your color burst crystal is ground to ring at 3.579545 MHz, and if you try to run it at another frequency you'll run into drive and tempco problems which will make it worthless. 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. :-) |
VCXO frequency isn't high enough
Arv wrote:
My W1AW receiver uses the crystal with just a 5-47 pf variable capacitor...no inductor, and it nets right on frequency. Try shorting Story of my life. ;-) the inductor and see if this gets you closer to the required frequency. Not all color burst crystals were created equal. If you have another crystal you might want to try it. That seems to be the common consensus. I suspect my crystal is just too good. ;-) Your ferrite will not be saturating at the small amount of signal you are sending through it as part of an SA-602 oscillator. Thanks, I know very little about these things. Arv - K7HKL _._ |
VCXO frequency isn't high enough
"John Fields" wrote in message
... On Tue, 13 Mar 2007 18:00:15 -0500, "Anthony Fremont" wrote: Eamon Skelton wrote: On Tue, 13 Mar 2007 12:02:38 -0500, Anthony Fremont wrote: The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. Use less inductance or replace it with a wire link. I did this and now I'm up to 3580.050; an increase of about 70Hz or so.. I don't understand how this was designed to work. What is making the crystal oscillate above its design frequency? I thought the inductor had something to do with it, obviously not. Is it being operated in series mode instead of parallel? I tried putting small cap in series with the adjustable one and that just greatly narrowed the tuning range and only slightly raised the frequency. It seemed to be getting unstable at that point as my old frequency counter started showing variations of quite a few hertz each update instead of remaining steady. It could be the amplitude is too low and the counter is missing ticks. --- Your color burst crystal is ground to ring at 3.579545 MHz, and if you try to run it at another frequency you'll run into drive and tempco problems which will make it worthless. 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 Colin =^.^= |
VCXO frequency isn't high enough
On Mar 13, 12:02 pm, "Anthony Fremont" wrote:
Hello all, I was playing around and saw that my junk box had all the parts so I started tossing this together:http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency 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. Steve |
VCXO frequency isn't high enough
colin wrote:
Its only a pull of ~100ppm, this should be easily pullable for most fundamental xtals. I'd like to get close to 500ppm if possible. 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 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. |
VCXO frequency isn't high enough
Anthony Fremont wrote:
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 s/.well resistance at this point.// It's just inductive reactance, I need more coffee. ;-) 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. Should I be afraid to do this? Does it need something to block DC current? |
VCXO frequency isn't high enough
"Anthony Fremont" wrote in message
... colin wrote: Its only a pull of ~100ppm, this should be easily pullable for most fundamental xtals. I'd like to get close to 500ppm if possible. do you need that much ? what freq you need ? whats the colourburst crystal freq ? 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 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. 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. Colin =^.^= |
VCXO frequency isn't high enough
"Anthony Fremont" wrote in message
... Anthony Fremont wrote: 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 s/.well resistance at this point.// It's just inductive reactance, I need more coffee. ;-) 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. Should I be afraid to do this? Does it need something to block DC current? the trimmer should block all the dc. im not convinced about the inductor in parallel with the crystal it might work though. |
VCXO frequency isn't high enough
On Mar 13, 12:02 pm, "Anthony Fremont" wrote:
I believe this is direct conversion and that the crystal should exactly match the desired station (3581kHz) I want to recieve, that's right isn't it? Maybe this is close enough? I have to wait til tonight to see if I can actually hear anything. 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. |
VCXO frequency isn't high enough
On 14 Mar, 07:42, "Anthony Fremont" wrote:
Anthony Fremont wrote: 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 s/.well resistance at this point.// It's just inductive reactance, I need more coffee. ;-) 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. Should I be afraid to do this? Does it need something to block DC current? Anthony 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. A datasheet for this device is located at: http://www.nxp.com/pip/SA612AD_01.html Arv - K7HKL _._ |
VCXO frequency isn't high enough
colin wrote:
"Anthony Fremont" wrote in message ... colin wrote: Its only a pull of ~100ppm, this should be easily pullable for most fundamental xtals. I'd like to get close to 500ppm if possible. do you need that much ? I "need" an increase of about .0546%. Isn't that about 546ppm? what freq you need ? 3581.5kHz to zero beat with the desired signal. whats the colourburst crystal freq ? 3579.545kHz. 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 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. 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. 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. ;-) 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. 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. 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. 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. :-) |
VCXO frequency isn't high enough
Arv wrote:
On 14 Mar, 07:42, "Anthony Fremont" wrote: Anthony Fremont wrote: 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 s/.well resistance at this point.// It's just inductive reactance, I need more coffee. ;-) 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. Should I be afraid to do this? Does it need something to block DC current? Anthony 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. 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. 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 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. A datasheet for this device is located at: http://www.nxp.com/pip/SA612AD_01.html Thanks Arv - K7HKL _._ _.. . _. ..... __._ __ __. |
VCXO frequency isn't high enough
On Mar 14, 3:33 pm, "Anthony Fremont" wrote:
what freq you need ? 3581.5kHz to zero beat with the desired signal. But why do you want to zero beat? |
VCXO frequency isn't high enough
"Anthony Fremont" wrote in message
... colin wrote: "Anthony Fremont" wrote in message ... colin wrote: Its only a pull of ~100ppm, this should be easily pullable for most fundamental xtals. I'd like to get close to 500ppm if possible. do you need that much ? I "need" an increase of about .0546%. Isn't that about 546ppm? what freq you need ? 3581.5kHz to zero beat with the desired signal. whats the colourburst crystal freq ? 3579.545kHz. 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. 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 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. 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. 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. ;-) Yeah I kinda got lost in my own explanation myself. didnt have much time to explain. 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. 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. 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. 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. 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. :-) 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. Colin =^.^= |
VCXO frequency isn't high enough
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VCXO frequency isn't high enough
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VCXO frequency isn't high enough
On Mar 14, 4:23 pm, "Anthony Fremont" wrote:
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. 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. |
VCXO frequency isn't high enough
On Wed, 14 Mar 2007 04:58:52 -0500, Anthony Fremont wrote:
I guess I'm at the limits of that particular crystal. 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. 73, Ed. EI9GQ. -- Linux 2.6.18 Remove 'X' to reply by E-Mail |
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