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Something strange going on here...
Steve k,9,d.c,i, "Bill Turner" wrote in message ... On Sun, 16 Nov 2003 16:14:04 +0200, "oh2baw" wrote: Although the calibration is quite inexact, it's always possible to listen to the GD-meters frequency on the receiver. __________________________________________________ _______ Here's another fun thing you can do while listening to your GDO on a receiver. Tune the receiver carefully when the GDO is at or near the dip. You will find it is oscillating on two separate (but close) frequencies. One from it's own tank circuit, one from the tank circuit under test. Probably the tank circuit under test isn't oscillating in the true sense of the word, but rather "ringing" due to the power induced into it. I would not have believed it if I hadn't heard it myself. -- Bill, W6WRT |
Something strange going on here...
Steve k,9,d.c,i, "Bill Turner" wrote in message ... On Sun, 16 Nov 2003 16:14:04 +0200, "oh2baw" wrote: Although the calibration is quite inexact, it's always possible to listen to the GD-meters frequency on the receiver. __________________________________________________ _______ Here's another fun thing you can do while listening to your GDO on a receiver. Tune the receiver carefully when the GDO is at or near the dip. You will find it is oscillating on two separate (but close) frequencies. One from it's own tank circuit, one from the tank circuit under test. Probably the tank circuit under test isn't oscillating in the true sense of the word, but rather "ringing" due to the power induced into it. I would not have believed it if I hadn't heard it myself. -- Bill, W6WRT |
Paul Burridge wrote:
Hi gang, I've never had a lot of luck with GDMs for some reason. Even with a decent meter, it seems such a drag tuning across such a vast range looking for a tiny, easily-missed dip which you have to screw out of the meter by forcing the sensing coil so far into the circuit concerned you practically break the circuit board. Am I alone in finding this potentially invaluable device practically useless in practice? Is there a more viable alternative? p. I've built several GDO's over the years. My favorite circuit is the old standby using a split stator variable capacitor and a shunt fed coplets oscillator. This capacitor can be 100-250 pf per section. My favorite tube is the 6CW4 nuvistor, though a 6DV4 might be a better choice at VHF. I put a TO-5 transistor heat sink over the tube fearing it might run hot! I use a cathode resistor of about 150-330 ohms (un-bypassed) and connect a bnc connector to the cathode via a 1000pf capacitor to steal a little signal to drive my frequency counter. With the counter set for low level input I get good drive up to about 100mhz or so. My coil socket is an RCA (phono) connector and the coils use a matching plug. A better idea might be to use a 3 pin DIN plug and socket, this allows for a ground connection to go to a coil center tap. If you add the center tap the circuit will work below 1.5mhz with proper coils, otherwise it will not oscillate below this frequency! (Having coils that go to 455khz would be nice to test if stages). Power supply used two 6.3v 300ma filament transformers back to back with a bridge rectifier. You can find transformers small enough to fit the supply into a box small enough to house the instrument, yet not too big to be hand hold-able. The coils can be wound on 1/4" or 3/8" plastic water line intended for ice maker use. These will fit into small places. Use of peaking chokes in the plate and filament lines can help eliminate "drop outs" in frequency coverage. Sensitivity can be adjusted with a plate pot, or put the pot in the meter circuit. I have used sub-mini meters salvaged from old jap tape recorders (vu-meters). These are typically around 500ua sensitivity. See my web site at www.qsl.net/wa2mze. |
Paul Burridge wrote:
Hi gang, I've never had a lot of luck with GDMs for some reason. Even with a decent meter, it seems such a drag tuning across such a vast range looking for a tiny, easily-missed dip which you have to screw out of the meter by forcing the sensing coil so far into the circuit concerned you practically break the circuit board. Am I alone in finding this potentially invaluable device practically useless in practice? Is there a more viable alternative? p. I've built several GDO's over the years. My favorite circuit is the old standby using a split stator variable capacitor and a shunt fed coplets oscillator. This capacitor can be 100-250 pf per section. My favorite tube is the 6CW4 nuvistor, though a 6DV4 might be a better choice at VHF. I put a TO-5 transistor heat sink over the tube fearing it might run hot! I use a cathode resistor of about 150-330 ohms (un-bypassed) and connect a bnc connector to the cathode via a 1000pf capacitor to steal a little signal to drive my frequency counter. With the counter set for low level input I get good drive up to about 100mhz or so. My coil socket is an RCA (phono) connector and the coils use a matching plug. A better idea might be to use a 3 pin DIN plug and socket, this allows for a ground connection to go to a coil center tap. If you add the center tap the circuit will work below 1.5mhz with proper coils, otherwise it will not oscillate below this frequency! (Having coils that go to 455khz would be nice to test if stages). Power supply used two 6.3v 300ma filament transformers back to back with a bridge rectifier. You can find transformers small enough to fit the supply into a box small enough to house the instrument, yet not too big to be hand hold-able. The coils can be wound on 1/4" or 3/8" plastic water line intended for ice maker use. These will fit into small places. Use of peaking chokes in the plate and filament lines can help eliminate "drop outs" in frequency coverage. Sensitivity can be adjusted with a plate pot, or put the pot in the meter circuit. I have used sub-mini meters salvaged from old jap tape recorders (vu-meters). These are typically around 500ua sensitivity. See my web site at www.qsl.net/wa2mze. |
On Mon, 17 Nov 2003 13:55:19 +0000, Paul Burridge
wrote: On Sun, 16 Nov 2003 22:33:38 GMT, (J M Noeding) wrote: Maybe you have a different model. Mine purports to cover up to 360Mhz, but there is *no way* AFAICS that the thing would be capable of anything remotely close to that upper range limit, on cursory inspection of the internals. I'm glad I went for the wholesale re-build but still find it hopelessly impractical to use on minature, in-circuit components. Can only agree very much to your comment "there is *no way* AFAICS that the thing would be capable of anything remotely close to that upper range limit", whether the upper limit is 280 or 360MHz becomes less important when it actually don't operate properly above 140MHz But I don't see that an instrument intended to cover down to 500kHz could have practical variable capacitor for UHF, so somewhere should be another construction. Perhaps an idea to look at DL7QY microwave dipmeter.... I might consider copying some of his details to a web page 73 Jan-Martin LA8AK Instruments: http://home.online.no/~la8ak/m1.htm (and -.m2.htm) Homebrew instruments: http://home.online.no/~la8ak/m3.htm Homebrew audio instruments: http://home.online.no/~la8ak/m31.htm -- remove ,xnd to reply (Spam precaution!) |
On Mon, 17 Nov 2003 13:55:19 +0000, Paul Burridge
wrote: On Sun, 16 Nov 2003 22:33:38 GMT, (J M Noeding) wrote: Maybe you have a different model. Mine purports to cover up to 360Mhz, but there is *no way* AFAICS that the thing would be capable of anything remotely close to that upper range limit, on cursory inspection of the internals. I'm glad I went for the wholesale re-build but still find it hopelessly impractical to use on minature, in-circuit components. Can only agree very much to your comment "there is *no way* AFAICS that the thing would be capable of anything remotely close to that upper range limit", whether the upper limit is 280 or 360MHz becomes less important when it actually don't operate properly above 140MHz But I don't see that an instrument intended to cover down to 500kHz could have practical variable capacitor for UHF, so somewhere should be another construction. Perhaps an idea to look at DL7QY microwave dipmeter.... I might consider copying some of his details to a web page 73 Jan-Martin LA8AK Instruments: http://home.online.no/~la8ak/m1.htm (and -.m2.htm) Homebrew instruments: http://home.online.no/~la8ak/m3.htm Homebrew audio instruments: http://home.online.no/~la8ak/m31.htm -- remove ,xnd to reply (Spam precaution!) |
J M Noeding wrote:
On Mon, 17 Nov 2003 13:55:19 +0000, Paul Burridge wrote: On Sun, 16 Nov 2003 22:33:38 GMT, (J M Noeding) wrote: Maybe you have a different model. Mine purports to cover up to 360Mhz, but there is *no way* AFAICS that the thing would be capable of anything remotely close to that upper range limit, on cursory inspection of the internals. I'm glad I went for the wholesale re-build but still find it hopelessly impractical to use on minature, in-circuit components. Can only agree very much to your comment "there is *no way* AFAICS that the thing would be capable of anything remotely close to that upper range limit", whether the upper limit is 280 or 360MHz becomes less important when it actually don't operate properly above 140MHz But I don't see that an instrument intended to cover down to 500kHz could have practical variable capacitor for UHF, so somewhere should be another construction. Perhaps an idea to look at DL7QY microwave dipmeter.... I might consider copying some of his details to a web page 73 Jan-Martin LA8AK Instruments: http://home.online.no/~la8ak/m1.htm (and -.m2.htm) Homebrew instruments: http://home.online.no/~la8ak/m3.htm Homebrew audio instruments: http://home.online.no/~la8ak/m31.htm -- remove ,xnd to reply (Spam precaution!) You can use a capacitor in series with the variable capacitor to reduce the tuning range, but you would need a switch to select the high or low capacitance range, and to move the coil connection to the active components. That, or use a dual capacitor, build two circuits, and select one oscillator at a time. -- I say, the boy is so stupid that he tried to make a back up copy of his hard drive on the Xerox machine! Michael A. Terrell Central Florida |
J M Noeding wrote:
On Mon, 17 Nov 2003 13:55:19 +0000, Paul Burridge wrote: On Sun, 16 Nov 2003 22:33:38 GMT, (J M Noeding) wrote: Maybe you have a different model. Mine purports to cover up to 360Mhz, but there is *no way* AFAICS that the thing would be capable of anything remotely close to that upper range limit, on cursory inspection of the internals. I'm glad I went for the wholesale re-build but still find it hopelessly impractical to use on minature, in-circuit components. Can only agree very much to your comment "there is *no way* AFAICS that the thing would be capable of anything remotely close to that upper range limit", whether the upper limit is 280 or 360MHz becomes less important when it actually don't operate properly above 140MHz But I don't see that an instrument intended to cover down to 500kHz could have practical variable capacitor for UHF, so somewhere should be another construction. Perhaps an idea to look at DL7QY microwave dipmeter.... I might consider copying some of his details to a web page 73 Jan-Martin LA8AK Instruments: http://home.online.no/~la8ak/m1.htm (and -.m2.htm) Homebrew instruments: http://home.online.no/~la8ak/m3.htm Homebrew audio instruments: http://home.online.no/~la8ak/m31.htm -- remove ,xnd to reply (Spam precaution!) You can use a capacitor in series with the variable capacitor to reduce the tuning range, but you would need a switch to select the high or low capacitance range, and to move the coil connection to the active components. That, or use a dual capacitor, build two circuits, and select one oscillator at a time. -- I say, the boy is so stupid that he tried to make a back up copy of his hard drive on the Xerox machine! Michael A. Terrell Central Florida |
On Fri, 21 Nov 2003 21:29:19 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote: I've built several GDO's over the years. My favorite circuit is the old standby using a split stator variable capacitor and a shunt fed coplets oscillator. This capacitor can be 100-250 pf per section. My favorite tube is the 6CW4 nuvistor, though a 6DV4 might be a better choice at VHF. I put a TO-5 transistor heat sink over the tube fearing it might run hot! I use a cathode resistor of about 150-330 ohms (un-bypassed) and connect a bnc connector to the cathode via a 1000pf capacitor to steal a little signal to drive my frequency counter. With the counter set for low level input I get good drive up to about 100mhz or so. My coil socket is an RCA (phono) connector and the coils use a matching plug. A better idea might be to use a 3 pin DIN plug and socket, this allows for a ground connection to go to a coil center tap. If you add the center tap the circuit will work below 1.5mhz with proper coils, otherwise it will not oscillate below this frequency! (Having coils that go to 455khz would be nice to test if stages). Power supply used two 6.3v 300ma filament transformers back to back with a bridge rectifier. You can find transformers small enough to fit the supply into a box small enough to house the instrument, yet not too big to be hand hold-able. The coils can be wound on 1/4" or 3/8" plastic water line intended for ice maker use. These will fit into small places. Use of peaking chokes in the plate and filament lines can help eliminate "drop outs" in frequency coverage. Sensitivity can be adjusted with a plate pot, or put the pot in the meter circuit. I have used sub-mini meters salvaged from old jap tape recorders (vu-meters). These are typically around 500ua sensitivity. See my web site at www.qsl.net/wa2mze. Thanks. Do you reckon there's any benefit in using toobz over FETs, then? -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Fri, 21 Nov 2003 21:29:19 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote: I've built several GDO's over the years. My favorite circuit is the old standby using a split stator variable capacitor and a shunt fed coplets oscillator. This capacitor can be 100-250 pf per section. My favorite tube is the 6CW4 nuvistor, though a 6DV4 might be a better choice at VHF. I put a TO-5 transistor heat sink over the tube fearing it might run hot! I use a cathode resistor of about 150-330 ohms (un-bypassed) and connect a bnc connector to the cathode via a 1000pf capacitor to steal a little signal to drive my frequency counter. With the counter set for low level input I get good drive up to about 100mhz or so. My coil socket is an RCA (phono) connector and the coils use a matching plug. A better idea might be to use a 3 pin DIN plug and socket, this allows for a ground connection to go to a coil center tap. If you add the center tap the circuit will work below 1.5mhz with proper coils, otherwise it will not oscillate below this frequency! (Having coils that go to 455khz would be nice to test if stages). Power supply used two 6.3v 300ma filament transformers back to back with a bridge rectifier. You can find transformers small enough to fit the supply into a box small enough to house the instrument, yet not too big to be hand hold-able. The coils can be wound on 1/4" or 3/8" plastic water line intended for ice maker use. These will fit into small places. Use of peaking chokes in the plate and filament lines can help eliminate "drop outs" in frequency coverage. Sensitivity can be adjusted with a plate pot, or put the pot in the meter circuit. I have used sub-mini meters salvaged from old jap tape recorders (vu-meters). These are typically around 500ua sensitivity. See my web site at www.qsl.net/wa2mze. Thanks. Do you reckon there's any benefit in using toobz over FETs, then? -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Sat, 22 Nov 2003 13:50:38 GMT, "Michael A. Terrell"
wrote: You can use a capacitor in series with the variable capacitor to reduce the tuning range, but you would need a switch to select the high or low capacitance range, and to move the coil connection to the active components. That, or use a dual capacitor, build two circuits, and select one oscillator at a time. in practice - a not so good idea! -- remove ,xnd to reply (Spam precaution!) |
On Sat, 22 Nov 2003 13:50:38 GMT, "Michael A. Terrell"
wrote: You can use a capacitor in series with the variable capacitor to reduce the tuning range, but you would need a switch to select the high or low capacitance range, and to move the coil connection to the active components. That, or use a dual capacitor, build two circuits, and select one oscillator at a time. in practice - a not so good idea! -- remove ,xnd to reply (Spam precaution!) |
Paul Burridge wrote:
On Fri, 21 Nov 2003 21:29:19 -0500, Kenneth %wa2mze%@%bellsouth%.%net% wrote: I've built several GDO's over the years. My favorite circuit is the old standby using a split stator variable capacitor and a shunt fed coplets oscillator. This capacitor can be 100-250 pf per section. My favorite tube is the 6CW4 nuvistor, though a 6DV4 might be a better choice at VHF. I put a TO-5 transistor heat sink over the tube fearing it might run hot! I use a cathode resistor of about 150-330 ohms (un-bypassed) and connect a bnc connector to the cathode via a 1000pf capacitor to steal a little signal to drive my frequency counter. With the counter set for low level input I get good drive up to about 100mhz or so. My coil socket is an RCA (phono) connector and the coils use a matching plug. A better idea might be to use a 3 pin DIN plug and socket, this allows for a ground connection to go to a coil center tap. If you add the center tap the circuit will work below 1.5mhz with proper coils, otherwise it will not oscillate below this frequency! (Having coils that go to 455khz would be nice to test if stages). Power supply used two 6.3v 300ma filament transformers back to back with a bridge rectifier. You can find transformers small enough to fit the supply into a box small enough to house the instrument, yet not too big to be hand hold-able. The coils can be wound on 1/4" or 3/8" plastic water line intended for ice maker use. These will fit into small places. Use of peaking chokes in the plate and filament lines can help eliminate "drop outs" in frequency coverage. Sensitivity can be adjusted with a plate pot, or put the pot in the meter circuit. I have used sub-mini meters salvaged from old jap tape recorders (vu-meters). These are typically around 500ua sensitivity. See my web site at www.qsl.net/wa2mze. Thanks. Do you reckon there's any benefit in using toobz over FETs, then? Probably not, but there isn't any disadvantage to using a tube, except for having to use an AC operated supply. If you need a portable grid dipper (to check antennas up on the tower for example) then you want a solid state battery operated unit (which sure beats 100 feet of extension cord dragging behind you when you climb the tower!). Not all fets function well into the vhf range. If you want to build a solid state dipper using fets make sure you get vhf units, not ones intended for audio use. There are circuits using fets that simulate negative resistance (like tunnel diodes) which make for an interesting oscillator since no coil taps are needed and a single gang cap can be used. |
Paul Burridge wrote:
On Fri, 21 Nov 2003 21:29:19 -0500, Kenneth %wa2mze%@%bellsouth%.%net% wrote: I've built several GDO's over the years. My favorite circuit is the old standby using a split stator variable capacitor and a shunt fed coplets oscillator. This capacitor can be 100-250 pf per section. My favorite tube is the 6CW4 nuvistor, though a 6DV4 might be a better choice at VHF. I put a TO-5 transistor heat sink over the tube fearing it might run hot! I use a cathode resistor of about 150-330 ohms (un-bypassed) and connect a bnc connector to the cathode via a 1000pf capacitor to steal a little signal to drive my frequency counter. With the counter set for low level input I get good drive up to about 100mhz or so. My coil socket is an RCA (phono) connector and the coils use a matching plug. A better idea might be to use a 3 pin DIN plug and socket, this allows for a ground connection to go to a coil center tap. If you add the center tap the circuit will work below 1.5mhz with proper coils, otherwise it will not oscillate below this frequency! (Having coils that go to 455khz would be nice to test if stages). Power supply used two 6.3v 300ma filament transformers back to back with a bridge rectifier. You can find transformers small enough to fit the supply into a box small enough to house the instrument, yet not too big to be hand hold-able. The coils can be wound on 1/4" or 3/8" plastic water line intended for ice maker use. These will fit into small places. Use of peaking chokes in the plate and filament lines can help eliminate "drop outs" in frequency coverage. Sensitivity can be adjusted with a plate pot, or put the pot in the meter circuit. I have used sub-mini meters salvaged from old jap tape recorders (vu-meters). These are typically around 500ua sensitivity. See my web site at www.qsl.net/wa2mze. Thanks. Do you reckon there's any benefit in using toobz over FETs, then? Probably not, but there isn't any disadvantage to using a tube, except for having to use an AC operated supply. If you need a portable grid dipper (to check antennas up on the tower for example) then you want a solid state battery operated unit (which sure beats 100 feet of extension cord dragging behind you when you climb the tower!). Not all fets function well into the vhf range. If you want to build a solid state dipper using fets make sure you get vhf units, not ones intended for audio use. There are circuits using fets that simulate negative resistance (like tunnel diodes) which make for an interesting oscillator since no coil taps are needed and a single gang cap can be used. |
On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote: Not all fets function well into the vhf range. If you want to build a solid state dipper using fets make sure you get vhf units, not ones intended for audio use. There are circuits using fets that simulate negative resistance (like tunnel diodes) which make for an interesting oscillator since no coil taps are needed and a single gang cap can be used. Right. I don't like fets, execpt as switches, RX RF amplifier and in PA. We had a discussion in a radio club among designers 30 years ago if it was any reason for using fets in oscillators instead of bipolars, but couldn't really find any good reason, except they were fancy. It is also a circuit using a jfet and pnp-combination which works in a negative impedance mode, but usually found on lower frequencies. Used it for some time to check resonnant frequency of unknown tuned circuits with a counter, but the problem is to control oscillation level. It worked fine for 50% of the applications while the counter indicated some higher (or lower) frequency for some circuits, so it was abandomed. Believe I have seen it mentioned for griddipmeters, too. It seems to be a reason why PNP UHF transistors are used in most UHF tuners found here, usually BF479, BF679, BF979. I've copied some of the text for DL7QY's dip meter covering up to 1400MHz on http://home.online.no/~la8ak/m3.htm -JM -- remove ,xnd to reply (Spam precaution!) |
On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote: Not all fets function well into the vhf range. If you want to build a solid state dipper using fets make sure you get vhf units, not ones intended for audio use. There are circuits using fets that simulate negative resistance (like tunnel diodes) which make for an interesting oscillator since no coil taps are needed and a single gang cap can be used. Right. I don't like fets, execpt as switches, RX RF amplifier and in PA. We had a discussion in a radio club among designers 30 years ago if it was any reason for using fets in oscillators instead of bipolars, but couldn't really find any good reason, except they were fancy. It is also a circuit using a jfet and pnp-combination which works in a negative impedance mode, but usually found on lower frequencies. Used it for some time to check resonnant frequency of unknown tuned circuits with a counter, but the problem is to control oscillation level. It worked fine for 50% of the applications while the counter indicated some higher (or lower) frequency for some circuits, so it was abandomed. Believe I have seen it mentioned for griddipmeters, too. It seems to be a reason why PNP UHF transistors are used in most UHF tuners found here, usually BF479, BF679, BF979. I've copied some of the text for DL7QY's dip meter covering up to 1400MHz on http://home.online.no/~la8ak/m3.htm -JM -- remove ,xnd to reply (Spam precaution!) |
J M Noeding wrote:
On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth %wa2mze%@%bellsouth%.%net% wrote: Not all fets function well into the vhf range. If you want to build a solid state dipper using fets make sure you get vhf units, not ones intended for audio use. There are circuits using fets that simulate negative resistance (like tunnel diodes) which make for an interesting oscillator since no coil taps are needed and a single gang cap can be used. Right. I don't like fets, execpt as switches, RX RF amplifier and in PA. We had a discussion in a radio club among designers 30 years ago if it was any reason for using fets in oscillators instead of bipolars, but couldn't really find any good reason, except they were fancy. It is also a circuit using a jfet and pnp-combination which works in a negative impedance mode, but usually found on lower frequencies. Used it for some time to check resonnant frequency of unknown tuned circuits with a counter, but the problem is to control oscillation level. It worked fine for 50% of the applications while the counter indicated some higher (or lower) frequency for some circuits, so it was abandomed. Believe I have seen it mentioned for griddipmeters, too. It seems to be a reason why PNP UHF transistors are used in most UHF tuners found here, usually BF479, BF679, BF979. I've copied some of the text for DL7QY's dip meter covering up to 1400MHz on http://home.online.no/~la8ak/m3.htm -JM -- remove ,xnd to reply (Spam precaution!) The negative resistance fet circuit I mentioned used a pair of jfets, one P channel and one N channel with the sources tied together and the gates of each transistor tied to the drain of the other. In this way each transistor acts as the self bias resistor of the other. The resulting voltage/current curve of the combo exhibits a region where the current decreases as the voltage increases, looking much like the 'kink' in the plate circuit curves of an old time tetrode tube (such as the 24A). The major disadvantage of this circuit for a gdo is that it is such a strong oscillator that you don't get much of a dip (you need to sample the output and feed it to a rectifier and meter) unless you load the coil down with a shunt resistor....and you need a different value for each range. Also P channel fets with good vhf performance are as rare as hen's teeth. |
J M Noeding wrote:
On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth %wa2mze%@%bellsouth%.%net% wrote: Not all fets function well into the vhf range. If you want to build a solid state dipper using fets make sure you get vhf units, not ones intended for audio use. There are circuits using fets that simulate negative resistance (like tunnel diodes) which make for an interesting oscillator since no coil taps are needed and a single gang cap can be used. Right. I don't like fets, execpt as switches, RX RF amplifier and in PA. We had a discussion in a radio club among designers 30 years ago if it was any reason for using fets in oscillators instead of bipolars, but couldn't really find any good reason, except they were fancy. It is also a circuit using a jfet and pnp-combination which works in a negative impedance mode, but usually found on lower frequencies. Used it for some time to check resonnant frequency of unknown tuned circuits with a counter, but the problem is to control oscillation level. It worked fine for 50% of the applications while the counter indicated some higher (or lower) frequency for some circuits, so it was abandomed. Believe I have seen it mentioned for griddipmeters, too. It seems to be a reason why PNP UHF transistors are used in most UHF tuners found here, usually BF479, BF679, BF979. I've copied some of the text for DL7QY's dip meter covering up to 1400MHz on http://home.online.no/~la8ak/m3.htm -JM -- remove ,xnd to reply (Spam precaution!) The negative resistance fet circuit I mentioned used a pair of jfets, one P channel and one N channel with the sources tied together and the gates of each transistor tied to the drain of the other. In this way each transistor acts as the self bias resistor of the other. The resulting voltage/current curve of the combo exhibits a region where the current decreases as the voltage increases, looking much like the 'kink' in the plate circuit curves of an old time tetrode tube (such as the 24A). The major disadvantage of this circuit for a gdo is that it is such a strong oscillator that you don't get much of a dip (you need to sample the output and feed it to a rectifier and meter) unless you load the coil down with a shunt resistor....and you need a different value for each range. Also P channel fets with good vhf performance are as rare as hen's teeth. |
Uncle Peter wrote:
It is kind of hard to get the proper coupling on PCB style coils. Bingo. Those were intended for larger tube circuit designs, not PCB based gear. Daily Double! Dana |
Uncle Peter wrote:
It is kind of hard to get the proper coupling on PCB style coils. Bingo. Those were intended for larger tube circuit designs, not PCB based gear. Daily Double! Dana |
On Sat, 22 Nov 2003 22:02:03 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote: remove ,xnd to reply (Spam precaution!) The negative resistance fet circuit I mentioned used a pair of jfets, one P channel and one N channel with the sources tied together and the gates of each transistor tied to the drain of the other. In this way each transistor acts as the self bias resistor of the other. The resulting voltage/current curve of the combo exhibits a region where the current decreases as the voltage increases, looking much like the 'kink' in the plate circuit curves of an old time tetrode tube (such as the 24A). The major disadvantage of this circuit for a gdo is that it is such a strong oscillator that you don't get much of a dip (you need to sample the output and feed it to a rectifier and meter) unless you load the coil down with a shunt resistor....and you need a different value for each range. Also P channel fets with good vhf performance are as rare as hen's teeth. Interesting. The circuit I rebuilt my old Tradiper with used two dual-gate MOSFETs., 3SK88s I believe they are. It gives a nice fragile ouput which is easily pulled down by an external tuned circuit, thereby giving rise to a good, deep dip. The only problem is the size of the blasted snifffer coils! Whilst this is fine for big old valve type constructions, it *is* totally impractical for today's ever smaller PCB jobs. I guess the time is coming to abandon it in favour of some other method of measurement, probably involving a scope and sig-gen. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Sat, 22 Nov 2003 22:02:03 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote: remove ,xnd to reply (Spam precaution!) The negative resistance fet circuit I mentioned used a pair of jfets, one P channel and one N channel with the sources tied together and the gates of each transistor tied to the drain of the other. In this way each transistor acts as the self bias resistor of the other. The resulting voltage/current curve of the combo exhibits a region where the current decreases as the voltage increases, looking much like the 'kink' in the plate circuit curves of an old time tetrode tube (such as the 24A). The major disadvantage of this circuit for a gdo is that it is such a strong oscillator that you don't get much of a dip (you need to sample the output and feed it to a rectifier and meter) unless you load the coil down with a shunt resistor....and you need a different value for each range. Also P channel fets with good vhf performance are as rare as hen's teeth. Interesting. The circuit I rebuilt my old Tradiper with used two dual-gate MOSFETs., 3SK88s I believe they are. It gives a nice fragile ouput which is easily pulled down by an external tuned circuit, thereby giving rise to a good, deep dip. The only problem is the size of the blasted snifffer coils! Whilst this is fine for big old valve type constructions, it *is* totally impractical for today's ever smaller PCB jobs. I guess the time is coming to abandon it in favour of some other method of measurement, probably involving a scope and sig-gen. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
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