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"[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote:
VFO Coil Magic! TCXO's and crystal i.f. filters!
Here is a Huff & Puff vfo coil idea and a comment for using Temperature Controlled Xtal Oscillators TCXO's.
The following interesting coil design is considered here for Huff & Puff Stabilizers as the vfo coil and can be tapped: for use in a Hartley Oscillator configuration as well as tapped for multiple band selection use. It offers many features desirable not only for the finished product but for R&D prototyping where the coil is good in the shop for general study of the performance of the device being designed. A version can be desinged in a box with a tuning capacitor added and a band selection switch included which will come in handy around the radio shack where design and testing is going on.
This is a very stable coil design that offers high Q and temperature stability by being able to dissipate heat better. It is commonly used in regenerative receiver designs as the oscillator coil and is used up into HF in those designs also. You might know them as spider web coils or flat (disk like) spiral windings. Such regenerative receivers have been used for amateur CW and SSB work. You can tap the coil in numerous places to change the inductance and use it over numerous bands. It has proved to be effective in all around use in the past. And, self shielded against local noise, which is another attribute worth considering.
A current coil form can be had for around $3.00 from Antique Electronic Supply. {Spider Web Coil Form.}
Coating the coil when finish with a good High Q dope is advised to prevent vibration from effecting the coil turns and to also prevent the turns from moving under thermal expansion; however since the coil dissipates better such thermal effects are reduced. Nylon or Silk insulated Litz {German: Lightening} wire is available since it is a good known performer in these coils.
If the flat spiral winding is mounted so that the broadside of the coils diameter is positioned up right; then the coil is mounted as an up right heat sink for good dissipation. I would consider such a coil for use in a highly stable, general coverage vfo for use in direct conversion SDR radio.
If used as a single band coil the efficiency of the coil is at its best. Having built such coils and receivers using them; I know that they also appear very good in the selectivity area. The regenerative receiver works in the similar fashion as the quadrature mixer in that, the signal is mixed on the fundamental with an offset tuning for the audio sideband. However this is not phase shifted into two audio sidebands as in SDR.
One or both sidebands as in AM detection are the amazing attributes of a one or two tube regenerative receiver. To have been used like this, in staying up on frequency in CW and SSB work have made coils like this very usefully in custom designs over time. In the early radio days this fashion of coil was the only means of stability and sharp selectivity they had. So it was standard in many radio vfo's and also worked excellently in the preselector stage of the front end of the receiver.
If you have no room to mount it up right then you can mount it horizontally, it still is a good heat dissipater this way just less efficient as compared to the up right position, however it is more efficient at dissipation than other coil designs either way it is mounted. Hence very thermal stable.
Consider mounting also the vfo circuit board in an upright position leaving room on both sides for heat dissipation. Likewise the main tuning capacitor needs to be such as the types used in linear amplifiers with wide plate spacings and with thick plates to remain rigged as well as have better dissipation from the plates larger spacings. This will be more stable with thermal changes. ALso, if you use a large capacitor and I advice you do so, you will be satisfied with the results. Make sure the lengths of hook up wire are good diameter and a hard sort of wire that does not move under vibration. Flat strips of copper work good in this case they will not change position much under thermal expansion. Also, make sure that the Capacitor has a metal shield between it and your hand as you tune the dial because your hand capacitance will effect tuning and once you remove you hand from the knob the tuning will change slightly. So remember this also. If possible use a fiber
glass rod as an extension between the tuning knob shaft and the capacitor shaft to isolate the hand capacitance even further.
In high frequencies a large diameter insulated wire size such as #28 to #26 can be used in such a coil. An 11 meter 1/4 wavelength long insulated wire can be wound in about a 4 to 6 inch diameter spiral disc coil. This makes a 1/4 wave resonator for 11 meters. All of these spiral disc coil designs are self shielded against local noise.
Now, for more help in PLL designs, consider that the local clock oscillator also be a TCXO type so that it is thermally stable. The oscillator is also self shield and all in all these are some of the better ideas. TCXO's come only in 20 MHz to date but should by now be higher at ICM. So they may have a 24 MHz one. A 20 MHz TCXO may work fine in the Slow-Tune Fast Stabilizer.
Using a simple thermal compensation circuit can help the vfo be slow in drift and more able to stay on the phase locked frequency even with a moderately locked loop. Tuning should be fine in resolution and the PLL needs to unlock on its own when the vfo is tuned but; locks back fast on its own also. This later description defines the idea of the Slow-Tune Fast Stabilizer.
Adjustment of the vfo's thermal compensation network should be done without the use of the PLL section being on. Watch the vfo over an hour without both the thermal compensation and the PLL on and note the thermal drift in Hz or KHz with a frequency counter. Turn on the thermal compensation and adjust either for positive or negative frequency drift. Vfo's tend to naturally drift up frequency as they warm up. Some tweeking may have to be done and then the best method of adjustment will be figured out with more use by people.
Finally, you can get crystal filters as wide as +/- 50 kHz @ 60dB from International Crystal Manufacture (ICM). If you are looking into single conversion or even a dual conversion SDR receiver then consider this also.
I hope that this PLL VFO device will lead to some good radio design and developments. As things are discovered, perfections in the design will occur. I think that if such things as this exist then at least the concerns for a stable but inexpensive receiver can be developed for Quadrature Detection using general coverage direct conversion if one so chooses. Or if you prefer single conversion or even dual conversion I think we have some tools for such designs.
The receivers can be both simple and yet something entirely new in concepts. I am at work on the circuits. So there are ideas here and I like many of them.
I would use a good ceramic, ferrite slug tuned coil in a vfo also. When you get serious on the design then rinky dink overly miniturized parts are your problem. Use a bigger coil and a bigger mass for a tuning capacitor. If your receiver comes out looking like a piece of test equipment next to your computer well that just makes every thing look more scientific around the radio shack or shop.
A HF frequencies of the high sort, if you use a broadcast variable capacitor then you will have to use a series capacitor inline with it to reduce the effects of the tuning capacitance range down into a range suited for the higher frequencies. This also reduces the capacitace drift of the broadcast varible capacitor. I mean a 480 pF varible with a series capacitor of 75 to 150 pF to reduces its capacitance. Use a polystyrene thermally stable capacitor here. If possible use all polystyrene caps in the vfo section around the main tuning capacitor and coil.
An S-Meter can be used in these receivers however they are sampling a wide 96 KHz swath of band and so their use will be more for looking at the band energy in the region tuned to. Likewise you can use stereo Vu Meters in the audio chain of the I Q outputs to view the tuned to modulation.
KA9RZA
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