Receiver Range
SSB Modifications
Modifying NBPF/VFO Frequencies

Board 1 || VFO || TV & FM, BC Filters || RF Amplifier
Bandpass Filters || First Mixer & Amplifier || Crystal Filter

VFO || VFO Stabilization || VFO Tuning with a CD ROM


40 and 20 Meter SSB Setup || Modifying the Narrow Bandpass Filters (NBPF) || Modifying VFO Frequencies
Adding an Outboard DDS VFO or Discrete VFO

Receiver Range

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.


40 and 20 Meter SSB Setup


Crystal Filter Modification
VFO Modifications without the Frequency Stabilizer
VFO Modifications with the Frequency Stabilizer

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.

Back to "Mounting/Testing Board 1 - Setting the VFO Frequencies"


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.

Back to "Mounting/Testing Board 1 - Setting the VFO Frequencies"

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."

Diagram of the VFO used in the receiver.

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.

Adding an Outboard DDS VFO or Discrete VFO

Adding an HF range DDS VFO will make a general coverage receiver from 7 MHz to about 18.2 MHz. The Bandpass filters continuously cover that range.


Connecting the output of an Outboard DDS VFO to the input of the VFO Amplifiers

The power to the on board VFO needs to be disconnected so that its signal does not interfere with the new outboard DDS VFO.

Also, if you have a Stabilizer Input connected to the Stabilizer box between the VFO Amplifiers, remove the connection from the Stabilizer box.

The 7805 5 volt regulator needs to be removed or the output lead cut so that 5V is removed from the Vackar VFO. Cutting the output lead, the one at the bottom leading to the VFO, and pushing it away from the rest of the lead will allow one to bend it back and reconnect it in case you decide to use the Vackar VFO again.

Or a small SPDT switch can be added to make it easier to switch back and forth with the on board and outboard VFOs.

A miniature SPDT switch can also be added below at the .01 capacitor if one wants to switch between the VFOs.


The bottom lead of the .01 capacitor that goes to Gate 1 of the First MOSFET amplifier is unsoldered from the board and lifted to provide access to connect to the Outboard VFO as shown in the picture above. Unsoldering this leads disconnects the Vacker VFO from the VFO Amplifier chain.

Second, a wire loop is inserted into two of the Ground holes in the box labeled Gnd as shown above.


The center lead of the coax from the DDS VFO is connected to the unsoldered lead of the .01 capacitor going to Gate 1 of the First MOSFET amplifier.

The Ground lead of the coax is connected to the wire loop inside the Ground box as shown in the picture above.



The DDS VFO shown is Hans Summer's QRP Labs VFO/Signal Generator
Uses SI5351a with ~3 V P-P Square Wave Output


The DDS VFO shown is the Simplified PIC-EL Kit using an AD9850
The output shown is typical of most all AD9850/AD9851 DDS VFOs

Power is applied to the receiver and to the outboard DDS VFO (or other outboard VFO).

The brightness of the First Mixer LEDs shows the drive level of the VFO amplifiers. The brightness shown in the picture above shows more than adequate drive to provide very good sensitivity for the receiver.

The mixer will work when the LEDs are lit at any level, even barely turned on. The LEDs are a drive level indicator showing when the VFO has adequate signal level to make the mixer work. If you add a VFO that does not light up the LEDs, then an additional amplifier needs to be added to the outboard VFO to raise the output to light up the First Mixer LEDs.

Notice that there is a 'Gain Adjust' trimmer pot to the left of the First MOSFET Amplifier. This trimmer adjusts the drive level to the First Mixer. Adjusting this trimmer for maximum output is near mid range of the trimmer and results in a 6V P-P signal at the First Mixer input. Maximum brightness of the First Mixer LEDs shows the maximum output.

VFO || Receiver Range/SSB Mods/Modifying NBPF/VFO Frequencies || VFO Stabilization || VFO Tuning with a CD ROM

Board 1 || VFO || TV & FM, BC Filters || RF Amplifier || Bandpass Filters || First Mixer & Amplifier || Crystal Filter

Send E-Mail || Amateur Radio Receivers || Electroluminescent Receiver

Last Update: 6/11/2018
Web Author: David White, WN5Y

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