VFO || VFO Stabilization || VFO Tuning with a CD ROM
The receiver covers the first 100 KHz of the 40 and 20 meter bands, and all of the 30 and 17 meter bands.
The reason for the 100 kHz bandwidth is for smooth tuning with the tuning capacitor without a reduction drive.
The bandpass filters tune 4.4MHz to 20MHz. The 40/30 NBPF (Narrow Bandpass Filter) tunes 4.4MHz to 14.5MHz. The 20/17 NBPF tunes 10.6MHZ to 20MHz.
Because of the wider bandwidth of the SSB band, a reduction drive for the main tuning capacitor helps a lot in tuning the bands. Consider adding one if making the following modifications.
The 40 Meter SSB band is 7.150 to 7.300 MHz and the 20 Meter SSB band is 14.150 to 14.350 MHz.
The VFO frequencies for the 40 meter SSB band will be 10.695 MHz (7.150 MHz) to 10.847 MHz (7.300 MHz). The VFO frequencies for the 20 meter band will be 10.605 MHz (14.150 MHz) to 10.805 MHz (14.350 MHz).
Crystal Filter Modification
There is one modification needed in the 3.547 MHz crystal filter:
____The 3.547 MHz crystal filter capacitors should be changed from 100pf to 39pf. This widens the bandwidth of the filter for SSB reception.
VFO Modifications without the Frequency Stabilizer
Two modifications are made to the VFO:
____Remove the 39pf coupling capacitor to the main tuning cap and replace it with a wire.
____Remove the 68pf capacitor next to the 10.545 relay and put in the 39pf capacitor.
Removing the 39pf capacitor and replacing it with a wire widens the bandspread in the 40/20 bands to a little over 250 kHz, so all the SSB portion for the bands can be received. Replacing the 68pf with the 39pf capacitor allows the VFO to reach the higher frequencies of the SSB bands.
Now set the the VFO frequencies for the 10.545 relay and 10.455 relay as follows:
____The frequency with the 10.545 relay (40 meter SSB, one LED on) is 10.695 MHz. Make sure the main tuning capacitor is all the way closed. Set the frequency using the 10.545 yellow trim capacitor. Frequency should range from 10.697 to 10.970 MHz.
The top of the band, 7.300 MHz is at 10.847 MHz, so almost the whole band could be covered by setting the frequency with the tuning cap all the way open to 10.847 MHz. The first 25 kHz of the band would not be received.
____The frequency with the 10.455 relay (20 meter SSB, both LEDs on) is 10.605 MHz. The tuning capacitor is all the way closed. Set the frequency using the 10.455 yellow trim capacitor. Frequency should range from 10.605 to 10.862 MHz.
VFO Modifications with the Frequency Stabilizer
There are no capacitor changes with the frequency stabilizer. The changes already made when installing the stabilizer will reach the required VFO frequencies.
Set the VFO frequencies for the 10.545 relay and 10.455 relay as follows:
____The frequency with the 10.545 relay (40 meter SSB, one LED on) is 10.695 MHz. Make sure the main tuning capacitor is all the way closed. Set the frequency using the 10.545 yellow trim capacitor. Frequency should range from 10.697 to 10.955 MHz.
The top of the band, 7.300 MHz is at 10.847 MHz, so almost the whole band could be covered by setting the frequency with the tuning cap all the way open to 10.847 MHz. The first 60 kHz of the band would not be received.
____The frequency with the 10.455 relay (20 meter SSB, both LEDs on) is 10.605 MHz. The tuning capacitor is all the way closed. Set the frequency using the 10.455 yellow trim capacitor. Frequency should range from 10.605 to 10.850 MHz.
Modifying the Narrow Bandpass Filters (NBPF)
The NBPFs can be modified to include any section of the HF spectrum with simple addition of capacitors, or by changing the coil inductance (number of windings).
There are unlabeled holes in the bandpass filters for the addition of a capacitor for lowering the range of the filter, or for fine tuning. A signal generator and some way of measuring the output of the filters will be needed to set the filters for other frequencies.
The filters were patterned after the article in the September/October 2000 issue of QEX called "Narrow Band-Pass Filters for HF", by William Sabin. This article goes into great detail on how to design these filters, plus information on how to input the figures into the ARRL Radio Designer.
Filter values are obtained by downloading NBPF.ZIP from the ARRL site at http://www.arrl.org/files/qex/ The kit uses the top coupled filters. The file has all the information for modifying the filters using the ARRL Radio Designer. The article provides a lot of additional information, but the NBPF.ZIP file contains everything needed to work with the filters.
Wide traces and lots of ground plane are provided for modifications to be soldered below the board.
Modifying VFO Frequencies
The Tesla Oscillator has a range of approximately of 9MHz to slightly above 14 MHz with almost the same drive output.
When modifying the oscillator for lower frequencies, capacitors can be soldered to the ground plane and traces underneath the board. Frequencies below 9 MHz require raising the value of the feedback capacitors.
Setting up the oscillator for higher frequencies means taking out one or more capacitors and replacing them with smaller ones.
The most critical ones are the feedback capacitors, the 39pf and the 82pf, that connect to the gate of the 2N5486. They should be the best NPO capacitors that you can obtain.
This circuit is well documented in the July 1997 issue of QEX, "Meet the Vacker: The Simple, Stable VFO You've Been Looking For", by Mark L. Meyer, WU0L. This excellent article on the Vacker gives the formulas for changing the VFO to any frequency that is desired. He explains in great detail how to stabilize the VFO.
The diagram is the exact one in the article. The receiver VFO is very similar. This VFO is set up for operation on 4.570 MHz.
Quoting from the article, Page 8, QEX, July 1997, "You can easily modify the values of the inductor and the capacitors for any frequency you wish. Just multiply the values listed by the ratio of your desired frequency to this design frequency of 4.570 MHz. For example, if you wish to have a frequency of 3.500 MHz, you multiply C1-C7 and L1 by 1.3 (4.57/3.5 = 1.3). Frequencies higher than 4.75 MHz will result in a multiplier value less than one. Then all your components will have values smaller than those shown in Fig 1." Continuing on Page 9, "Nothing is critical in the values for the resonant frequency. All you have to do is get close with C1, C2, C34, C4, C5, and L1 and then start trimming."
The receiver VFO was designed for three different frequencies. The procedure was to set the inductance of the coil at the average of the frequencies. For example, the kit uses 14 MHz and 10 MHz. The coil's inductance was set for a VFO design frequency of 12 MHz.
The capacitors were calculated for a design frequency of 14 MHz. The capacitors were installed, along with a small ceramic trim cap between the coil and ground, so the ceramic trim cap could tune through the target frequency.
On the first leg of the VFO, switched by the first relay, a 68pf, a 47pf, and a yellow trim capacitor is added to reach 10.545 MHz. On the second leg, switched by the second relay, a yellow trim capacitor is added to reach 10.455 MHz.
The highest frequency (14MHz) works with the 4.000 MHz crystal filter.
Subtracting 4 Mhz (crystal filter frequency) from the 14 MHz VFO frequency, the receiver will work at 10 MHz (30 meters). Adding 4 MHz to the 14 MHz VFO frequency, the receiver will work at 18 MHz (17 meters).
The same band imaging techniques that were used to set up the amateur radio bands can be used for the SWL bands. Another example, using a 13 MHz VFO, subtracting 4 will give you the 9 MHz SWL band, then adding 4 will give you the 17 MHz SWL band.
The second and third VFO frequencies are calculated with the 3.547 MHz crystal filter.
Adding the 3.547 MHz crystal filter frequency to a 10.453 VFO frequency will yield 14 MHz (20 meters). Subtracting 3.547 MHz from a 10.547 VFO frequency equals 7.000 MHz (40 meters).
Modifying Receiver Frequencies
To reach different bands the IR receivers, the phototransistor and photodiode, at the VFO can be replaced by SPST switches. Then any combination of crystal filter, VFO frequency, and bandpass filter can be used.
To disable the IR switching, just put a black cap over the lens of the Phototransistor and Photodiode controlling the VFO frequencies. The Phototransistor is between the VFO amplifiers, and the Photodiode is next to the 10.455 yellow trimmer. I used heat shrink tubing, folded over and then heated with a cigarette lighter.
Tie one side of the SPST switches to the "A" terminal of the photodiode and the "E" terminal of the phototransistors, which is the gate lead of the switching MOSFET/MESFET. The other side of the switch ties to 12 Volts. Remember that the 10.545 relay on the VFO must be on for the 10.455 relay to have power.
SPST switches for the bandpass filters and crystal filters are installed on the board, and are switching to ground.
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