A Digital VFO for Vintage Transmitters
by Greg Latta, AA8V

VFO Front View

Select A Link Below:
 QST Article - January 2014
 Introduction
 Basic N3ZI Synthesizer
 Power Supply
 Two Transistor Buffer Amplifier
 1 to 2 Step Up Voltage Transformer
 RIT Keying Circuit
 Complete Block Diagram of the VFO
 Keying Amplifier
 Connecting the VFO Output to a Vintage Transmitter
 Connecting the Complete System to a Vintage Transmitter
 Using the VFO with a Vintage Transmitter
 Spotting the VFO in the Receiver:
 Vintage Operating Procedures: Answering a CQ
 Vintage Operating Procedures: Calling CQ
 Front View
 Rear View
 Inside View

QST Article - January 2014:
I originally described this VFO in the January 2014 issue of QST Magazine. Though this web site greatly expands on the magazine article, I still suggest that you read the article as it will give you a good, concise view of the project. If you don't have the back issue of QST magazine, you can go to the ARRL QST Site and read the article on-line. (You must be an ARRL member to read the on-line version.) Not an ARRL member? You can get a 90 day guest account or join here.

Introduction:
I love old gear, especially tube gear. As a novice, I started by building an Ameco AC-1 transmitter. Then I moved up to a Knight Kit T-60. Both of these were crystal controlled, and it always seemed like someone was already on the only crystal frequency I had. I got my general ticket after a couple of years, and I longed for a VFO, but couldn't afford one.

I did eventually get my hands on a used Heathkit HG-10 VFO, but it wouldn't properly drive either of my transmitters. It wasn't until I got hold of a Heathkit DX-40 transmitter that true VFO capability for me became a reality. Wow! What a difference. Of course, that VFO wasn't perfect. It drifted a bit and had some chirp, but that was acceptable and sure beat the heck out of operating with crystals.

A lot of great crystal controlled transmitters are still out there, like the Eico 720, the DX-40, DX-60, and many others, but they sit on the shelf because their owners don't have any crystals, or are unwilling to put up with the difficulties of operating with crystal control. There are also many VFO controlled transmitters out there, like the Johnson Viking Ranger, but their VFOs are old and unstable, or were unstable to begin with. These sit on the shelf too, wasting away. What is needed is a modern VFO that can be used with these transmitters. This VFO was my answer to that call.

The N3ZI Synthesizer:
The heart of the VFO is a synthesizer from N3ZI called the "Super DDS". You must purchase and build this first. This is a great circuit and is very reasonably priced. N3ZI has done a beautiful job designing the synthesizer. It is hard to imagine anything better at such a price. The synthesizer can output any frequency from 0.1 to 32 MHz in 1 Hz steps. It tunes very much like an analog VFO and comes with an LCD display if you buy the "Deluxe" kit. It has 12 memories and an RIT function (very important, since I use this for keying). For my VFO, I used the standard (or serial) LCD display. Though not backlit, the characters are large and easy to read.
Click Here To Purchase an N3ZI Super DDS

When you build the synthesizer, don't use the board mounted up/down switches.. Instead, use your own push button switches (normally open, momentary contact) and connect them to the circuit board with wires so they can be mounted on the front panel. Also connect the rotary encoder to the circuit board with wires. Wired connections give you more freedom when mounting the circuit board and related components. Below is a photo of the completed synthesizer board:

Synthesizer Board
N3ZI Synthesizer

Power Supply:
You will need some sort of power supply to power the N3ZI synthesizer board, the buffer amplifier, and the keying circuit. You could use a 12V wall wart, but I hate wall warts, since they sometimes lack proper filtering, and prefer to use an internal power supply. The entire VFO draws only 90mA, so just about any 12V power supply will do. However, the power supply must be well filtered. I used a surplus 16V transformer from my junk box, a full wave bridge rectifier, 1000uf capacitor filter, and a 7812 voltage regulator for my power supply. The current draw on the supply is so small that it is not necessary to heat sink the 7812 regulator. Though my transformer is a 16V transformer, any small transformer with an output of 12V - 17V should also work. See the schematic diagram and photo below:

Power Supply Schematic Diagram
Power Supply Schematic Diagram


Power Supply
VFO Power Supply
The transformer is on the left, and the bridge rectifier, capacitor, and regulator at the lower right.

Buffer Amplifier:
Though the N3ZI synthesizer is a great circuit, the output is quite low, about 225mV peak-to-peak, or 88mV RMS. This is fine if you want to use the synthesizer as a signal generator, but it is far to low to drive vintage transmitters, which typically require up to 10 or 20 Volts peak-to-peak. To use the synthesizer with a vintage transmitter, the output must be amplified.

It turns out that it is very easy to build a two transistor amplifier using a couple of 2N2222 transistors. The amplifier circuit below yields a peak-to-peak output of about 5V, which may be enough to drive some vintage transmitters. N3ZI's original circuit, on his web site, is basically the same as the one below. I have instituted the changes he suggests to maximize the output of the buffer amplifier up to 7 MHz. It rolls off above 7 MHz, but still gives useful output even up to 14 MHz.

Buffer Amplifier Schematic
Two Transistor Buffer Amplifier

My buffer amplifier is wired on a small piece of perf /vector board, which also holds the RIT inverter circuit. See the picture below:

Buffer Amplifier and RIT Inverter
Perf/Vector Board With Buffer Amplifier and RIT Inverter.
The buffer amplifier is on the left (red wires) and the RIT Inverter is on the right (black and green wires).

I have a switch on the back panel of the VFO which can be used to bypass the buffer amplifier when the VFO is used as a signal generator and the extra output isn't needed. The red switch can be seen at left in the photo above. A block diagram of the bypass switch wiring is shown below:

Buffer Amplifier Bypass Switch Wiring
Buffer Amplifier Bypass Switch Wiring

1 to 2 Step Up Voltage Transformer:
The output with the two transistor amplifier is about 5 volts peak-to-peak, and this is sufficient to drive many vintage transmitters. However, some transmitters, such as my Eico 720, need more drive. To drive such transmitters, you will need to use a 1 to 2 voltage (1 to 4 impedance) step up transformer.

The output of the two transistor amplifier is low impedance, and it is possible to get more output by using a transmission line transformer to step up the voltage. It is very easy to make a 1 to 2 voltage (1 to 4 impedance) step-up transformer and it is pretty much foolproof. All you need is some small wire and the proper toroidal core.

My transformer is wound on an FT-50A-61 ferrite toroid core (AL=75 mH/1000 turns). Cores are available on line from several sources such as hamfests or on-line at Amidon Associates or Palomar Engineers:

www.amidoncorp.com
palomar-engineers.com

Do not use a powdered iron core. These won't provide enough inductance. The core is not critical in size or material as long as it is a ferrite core with an AL value of at least 75 mH/1000 turns. If you have an unknown core, it is easy enough to try it out. If it works, fine. If not, you will need to try another core.

I used #24 enameled magnet wire to wind my transformer, but just about any insulated wire that is small enough so that 11 bifilar turns can be wound on the toroid will work. Double the wire up, and then carefully wind 11 turns on the toroid, keeping the wire neat and parallel to itself. The first pass through the toroid counts as turn number one. If you wind an extra turn or two the transformer will still work fine. Strip the ends of the four leads. To strip enameled wire, burn about 1/2" with a lighter and sand off the ash. Use an ohmmeter to identify the two windings and then wire up the transformer as shown in the schematic and photo below:

1 to 2 Voltage Transformer
1 to 2 Voltage Step Up Transformer

I built my transformer into a pill bottle I got from my local pharmacy. For the input to the transformer, I mounted a female RCA jack on the top of the pill bottle. This is visible at left in the photos below. I used a short length of RG-174 coax with a male RCA plug on the end ("pigtail") for the output from the transformer. This is visible at top center in the photo below. This piece of coax must be kept as short as possible. The value of the blocking capacitor is not critical. Anything from 0.001uf up to 0.01uf will work fine.

Notated Transformer Picture
Notated Picture of the 1 to 2 Voltage Step Up Transformer
Click for a close up view.

Below is a picture of the completed transformer mounted inside the pill bottle:
Transformer in Pill Bottle
Completed Transformer Mounted Inside A Pill Bottle
Click for a close up view.

Do not try to extend the cable (pigtail) to the transmitter. This is high impedance and must be kept very short. In fact, it is perfectly acceptable to mount the transformer inside the transmitter for best performance. I decided to do this with my Viking Ranger transmitter by mounting the transformer inside the external VFO input I added to the Ranger.

RIT Keying Circuit:
The VFO must be disabled or somehow moved off frequency during receive periods. Otherwise, it will interfere with reception. The N3ZI synthesizer board contains an RIT (Receiver Incremental Tuning) input (pin J3-8) that, when connected to ground, shifts the VFO frequency by an amount equal to the RIT offset. If the RIT offset is large enough (for example, 1MHz, which is what I use), the new frequency will be well outside the receiver passband, and the VFO won't interfere with reception.

We could connect a switch to the RIT input. When it is open, the VFO would be on frequency, and when closed, off frequency. We could then open the switch every time we wanted to transmit, but that is very inconvenient. We could also use an extra pair of contacts on the transmit/receive relay to handle the switching, but in both cases the VFO is continuously on while transmitting. That is fine when running AM phone, but when running CW it makes it very difficult to monitor our sending. The best solution is to use the key itself to bring the VFO on and off frequency. That is, we want the key itself to key the VFO.

First, we need an inverter circuit that will protect the RIT input in case we connect it to something we shouldn't, and also invert the RIT input so that closing a switch (like the key) will bring the VFO on frequency, but keep the VFO off frequency when the switch is open. This is easy to do using one 4.7kohm resistor and a 2N2222 transistor as shown below:

RIT Inverter Circuit
RIT Inverter Circuit
Click for a close up view.

The RIT input is on pin J3-8 of the synthesizer board. For normal operation, the RIT keying (SPOT) switch is closed. Q5, which is normally turned on, then pulls the RIT input low and puts the VFO off frequency. When the input to Q5 is brought to ground, Q5 turns off, the RIT input goes high, and the VFO goes on frequency.

Opening the RIT Keying (SPOT) switch lets the RIT input go high and brings the VFO on frequency regardless of what is present at the input of Q5. This allows the operator to "SPOT" or listen to the VFO in the station receiver, regardless of what is present at the RIT keying input.

I mounted my RIT inverter circuit on the same little piece of perf board that I used for the two transistor buffer amplifier. See the picture below:

Buffer Amplifier and RIT Inverter
Perf/Vector Board With Buffer Amplifier and RIT Inverter.
The buffer amplifier is on the left (red wires) and the RIT Inverter is on the right (black and green) wires..

Complete Block Diagram of the VFO:

A complete block diagram of the VFO (less the power supply) showing how the synthesizer board, RIT keying circuit, and buffer amplifier are interconnected is shown below:

VFO Block Diagram Buffer Amplifier Circuit RIT Keying Circuit N3ZI Synthesizer Board
Complete Block Diagram of VFO
Click on any section of the diagram for more information on that part of the VFO.

You can also click here for an inside view the complete VFO showing the buffer amplifier/RIT circuit perf board, synthesizer board, and power supply.

Keying Amplifier:
Now that we have a way to key the VFO, we need a circuit that will allow the key to control both the VFO and transmitter at the same time. The problem is that some vintage transmitters have a negative voltage at the key, and others have a positive voltage at the key. These voltages are sometimes quite high (several hundred volts), and sometimes large currents (perhaps 250mA) need to be keyed. Our circuit must be able to handle all of these situations.

The circuit below does the job nicely. Though some folks don't like to use a battery, it greatly simplifies the circuit. You just have to remember the battery is there and replace it once in a while!

If your vintage transmitter has a negative voltage at the key (i.e. it is grid block keyed) you don't need Q3. If your vintage transmitter has a positive voltage at they key (i.e. it is cathode keyed) then you must use Q3. Q3 is an NPN transistor that must be able to handle the positive voltage and current that must be keyed by your transmitter. This varies greatly from transmitter to transmitter. If Q3 can handle about 200V and 250mA, than it will probably work OK in most situations.. I used an NPN power transistor in a TO-3 case that was originally intended as the horizontal output transistor in a TV set. These can typically handle higher voltages and currents.

Note that the keying input to the amplifier is negative with respect to ground. This is no problem with a mechanical key or bug, but if you use a keyer be sure to use the negative keyer output that was designed for vintage, grid-block keyed transmitters.

Keying Amplifier Schematic
Keying Amplifier Schematic
Click for a close up view.

The key is connected to the input of the amplifier, and a cable is run from the Q4 output to the RIT keying input on the VFO. Grid block keyed transmitters should be connected to the "Center Negative" key jack. Cathode keyed transmitters should be connected to the "Center Positive" jack.

The operation of the keying amplifier is straightforward. Under key up conditions, the 1N4005 diode prevents any negative voltage at the "Center Negative" key jack from flowing back through the battery and transistors Q3 and Q4. Under key up conditions, Q3 and Q4 are turned off. When the key is closed, any transmitter connected to the "Center Negative" jack is keyed. Q3 and Q4 are turned on because the negative lead of the battery is connected to ground through the diode and the closed key. Q3 then keys any transmitter connected to the "Center Positive" jack, and Q4 keys the VFO, bringing it on frequency.

I chose to mount my keying amplifier in a small minibox separate from the VFO. That would make things easier whenever I wanted to use the VFO as a signal generator in my shop. Originally intended for use only with my cathode (positive) keyed Eico 720 transmitter, I used a short pigtail rather than a 1/4" jack and cord to connect to the key jack on the transmitter. I later added keying for grid-block (negative) keyed transmitters.

Keying Amplifier
Keying amplifier mounted in a small minibox.

In the photo above, the key plugs into the 1/4" jack on the right side of the box. For positive keyed transmitters, I decided to use a pigtail with a 1/4" plug on it rather than a jack. Negative keyed transmitters plug into the 1/4" jack on the front of the box, and the RIT keying output is the RCA jack on the front of the box. When I get around to it, I plan to add a switch on the battery to turn it off when not in use. (In the past I have accidentally closed the contacts on my bug overnight and ruined the battery.) The switch would prevent such occurrences.

Connecting the VFO Output to the Transmitter:
Connecting the VFO to the transmitter varies from one transmitter to the next. If your transmitter has a separate VFO input, then try that first. Some transmitters may also require you to throw a switch or use a jumper as well. For example, my Eico 720 has a VFO input on the back, but it also has a slide switch that must be set to VFO to work properly.

Lacking a VFO input, you must use the crystal socket. Check the transmitter schematic. In many/most transmitters, such as my 6CL6 transmitter or Johnson Viking Ranger, one side of the crystal socket is connected to ground. In such a case, connect the hot lead from the VFO to the ungrounded side of the crystal socket, and connect the ground lead from the VFO to the grounded side of the crystal socket.

In some transmitters, neither side of the crystal socket is grounded. In these cases, one side of the crystal is connected to the grid of the oscillator tube, either directly or through a coupling capacitor. Connect the hot lead from the VFO this side of the crystal socket and the ground lead of the VFO, though a 0.001uf capacitor, to the other side of the crystal socket. The coupling capacitor prevents any DC that might be on the crystal socket from being shorted to ground. If that doesn't work, try connecting the ground lead of the VFO directly to ground, rather than the other side of the crystal socket. You will have to experiment to see what works in these situations.

The actual physical connection to the crystal socket can be a problem. In the crudest/simplest case, it can be made by simply stuffing the wires into the socket pins. But this is not a very poor solution, and should not be used in the long term. If the crystal socket is part of an octal tube socket, then you can purchase an octal plug to make the connection or use the base from a defective octal tube. Octal socket plugs are available from Antique Electronic supply. Click on the link below:

Octal Tube Base:
Antique Electronic Supply Part Number: P-SP8-476

If you have a defective octal tube, put it in a bag and break the envelope with a hammer or vise, clean out the base, and then unsolder the wires from the base pins. Be sure to wear safety goggles when working with glass.

Another option is to find a defective FT-243 crystal or one that is not in the ham bands. This can be disassemble and the crystal can be removed. A hole can then be drilled in the top for a cable to pass through and the cable can be connected to the pins.

Many years ago, connectors were made for plugging television sets into a wall socket using twin lead. These will also work, but are hard to find now.

Important: regardless of how you make the connection, if you are using the 1:2 transformer you must keep the leads from the transformer to transmitter as short as possible.

I made adaptors for my various transmitters that all go to a female RCA connector through very short wires. I can then have a male RCA plug on my 1:2 transformer and the cable from my VFO. I can then easily use the VFO with or without the 1:2 transformer and with any of my vintage transmitters or my 6AG7 VFO amplifier.

Connecting The Complete System to a Vintage Transmitter

The diagram below shows how to connect the VFO and keying amplifier to a typical transmitter:

Overall Block Diagram Digital VFO 1:2 Voltage Step-Up Transformer Keying Amplifier
Overall system diagram showing how to connect the VFO and keying amplifier to a vintage transmitter.
Click on any section of the diagram for more information on that part of the VFO.

To connect the VFO and keying amplifier to a vintage transmitter, you must make the following connections:

1. The key, bug, or keyer (center negative) is connected via shielded cable to the keying input of the keying amplifier.

2. The appropriate keying output from the keying amplifier (center positive for cathode keyed transmitters such as the Eico 720, center negative for grid-block keyed transmitters, such as the Johnson Viking Ranger) is connected via a shielded cable to the key jack of the transmitter.

3. A shielded cable runs from the RIT output of the keying amplifier (Q4) to the RIT input of the VFO (Q5).

4. A cable made of RG-58U coax cable is connected between the buffered output of the VFO and the crystal socket or VFO input of the transmitter. Do not use audio cable for this connection. If the 1:2 transformer is used, it can be connected to the transmitter with wires, twin lead, or coax cable, but it should be kept as close as possible to the transmitter input.

Using the VFO with a Vintage Transmitter:

Once you have the system connected as shown above, it is time to get the transmitter on the air.

The first thing to do is to set the RIT offset on the synthesizer to about 1MHz.. This is done as follows:
1. Be sure that the "Display RIT" switch is off.
2. Press, and while holding, the tuning knob activate the up/down push buttons until the display shows "dF" (for difference in Frequency).
3. Release the tuning knob and rotate the tuning knob for the desired offset. This is not critical, but I typically use a setting of 1MHz.
4. After you have set the desired offset, simply wait and the display will return to normal.

Next, you want to set the desired tuning step. For CW, I find that 10Hz is fine on 80m and 40m. You may wish to decrease it further (to 5Hz or 2Hz) if you regularly operate on any of the higher frequency bands. To set the tuning step size, do the following:
1. Press, and while holding, the tuning knob activate the up/down push buttons until the display shows "SP" (for SteP)
2. Release the tuning knob and rotate the tuning knob for the desired offset. This is a matter of personal preference, and you can experiment here, but I typically use a setting of 10Hz on 80m and 40m, and 5Hz or 2Hz on the higher frequency bands..
3. After you have set the desired step size, simply wait and the display will return to normal.

As a preliminary test, choose one of the VFO memories and set it to a frequency of your choice, say 7MHz. Tune your receiver to the same frequency, and open the "SPOT" switch. You should be able to hear the signal in your receiver. Close the "SPOT" switch and, if the system is connected properly to the keying amplifier, the signal should go away. Press the key and you should again here the signal in the receiver. If the RIT keying is working properly, you will be able to copy your keying in the receiver. If you turn on the "Display RIT" switch, the display should show the VFO frequency while the key is down, and the VFO frequency plus the RIT offset when the key is up.

Read the transmitter manual to see what VFO/crystal frequencies you are supposed to use with your transmitter. With many transmitters, the VFO/crystal frequency will be in the 80m band (3.5MHz) for 80m, and it will be in the 40m band (7MHz) for all other bands. However, some transmitters, such as the Viking Ranger, use a VFO/crystal frequency of 160m (1.8MHz) for 160m and 80m, and a 40m frequency (7MHz) for all of the other bands. You must remember that your output frequency will be the VFO frequency or an integer multiple of it. For instance, if you are on the 15m band and wish to operate on 21.027MHz, you will have to set the VFO to 1/3 of your desired operating frequency, or 7.009MHz. (This is what you had to do when using crystals or a vintage VFO, and you must still do it here. Such are the joys of operating vintage equipment.)

You can try the system with and without the 1:2 step up transformer. Most transmitters will need the transformer. If the VFO is driving your transmitter properly, your transmitter should behave about the same as when it is used with crystals. However, if you must turn up the drive control much more than normal, or if your grid current is too low, or if the transmitter is unstable, you either don't have enough drive, or you haven't connected the VFO output properly to your transmitter.

I have used this VFO system successfully with my Eico 720 transmitter, my Johnson Viking Ranger, my 6CL6 transmitter, and my 6AG7 amplifier. With all of these I use the 1:2 step up transformer. This VFO is as stable or more stable than modern transceivers. If you follow the calibration procedure in the N3ZI synthesizer manual, you can get the accuracy of the VFO to within 1Hz at 10MHz, which is better than most modern transceivers!

Spotting the VFO in the Receiver:
In a modern transceiver, the transmitter is automatically set to the receiver frequency as you tune the receiver. But in a vintage station you must learn to separately set the the transmitter frequency to the receiver frequency. This is called "Spotting" the transmitter, and it is a very important technique to learn. (Note that spotting the transmitter is NOT the same as zero beating the transmitter!) Properly spotting your transmitter takes a little practice, and if not done properly your transmitter frequency can be as much as 2kHz away from your receive frequency! This will cause you to lose QSOs and/or to cause QRM on the bands.

Spotting the VFO/transmitter involves turning on the VFO with the spot switch (opening the spot switch) and then adjusting the VFO until the signal is heard in the receiver. (You may also need to engage a "spot" setting on the transmitter). The problem is that many older receivers lack single signal selectivity, and it is possible to hear the VFO at two places on the dial, rather than one. You must learn by practice which of those two places is the correct one.

As the VFO is tuned, the audio pitch in the receiver will change, and there will be a particular pitch where the receiver response is the best. For better receivers with "single signal" selectivity, there will be only one setting of the VFO where the response is best. This is obviously the correct setting. Count yourself lucky if you have one of these receivers.

For receivers which lack single signal selectivity there are two settings of the VFO which will be picked up by the receiver. If one is noticeably louder than the other, that is the correct VFO setting. You may have to listen carefully (or look at the S-meter, if the receiver is so equipped) to hear the difference, but once you are aware of it you should be able to set the VFO to the correct frequency without too much difficulty.

For receivers with very poor selectivity (such as regenerative receivers), there will be two places where you can pick up the VFO signal, and there will be no noticeable difference in the receiver response. In such receivers, as you tune through a station from the low frequency side, the pitch will start high and then go down so low as to be inaudible ("zero beat") and then go back up. We say that you can tune in the signal on either the low frequency side of zero beat or the high frequency side of zero beat. The first step in successfully using such a receiver is to always tune the station in on the low frequency side of zero beat. This means that if you tune the receiver to a slightly higher frequency, the audio pitch will go down and vice-versa.

Assuming you have tuned your receiver on the low frequency side of zero beat, adjust the VFO so you can hear the signal in your receiver. If have the correct VFO setting, the audio pitch will go down if you slightly lower the VFO frequency. If the pitch goes up, you have the VFO set to the wrong frequency. In such a case, tune the VFO so the frequency goes down and then back up on the other side of zero beat. When you have the correct VFO setting, the audio pitch will go down if you slightly lower the VFO frequency. (Likewise, if you slightly raise the receiver frequency the pitch should go down. This might sound complicated, but after you do it a few times you will get used to it and then do it without much thought.

Failure to properly spot the VFO/transmitter in the receiver is why many modern hams fail to make successful QSOs with vintage gear or regenerative receivers. Failure to properly spot your transmitter frequency can place your transmitter frequency as many as 2kHz or more off from your receiver frequency! This causes QRM and the loss of QSOs.

Vintage Operating Procedures: Answering A CQ:
Operating vintage equipment is a little different than when using modern equipment because it involves spotting the VFO in the receiver. When answering a CQ, tune in the station on the receiver. If you have a receiver with poor selectivity, be sure to tune the signal in on the low frequency side of zero beat. Enable Spot on the VFO and Spot the VFO in the receiver. Be careful to do this properly. Fine tune the pitch of the VFO so the pitch of the VFO is the same as the pitch of the other station If you can't get the pitch close enough, you will need to decrease the VFO step size. (See Using the VFO with a Vintage Transmitter above). No NOT zero beat the VFO! When the VFO is spotted correctly and the pitch of the VFO is the same as the station you are answering, you are on the proper frequency and can can call the other station.

Remember, your operating frequency will be the VFO frequency or an integer multiple thereof, so the VFO may not show your actual operating frequency.

Vintage Operating Procedures: Calling CQ:
When calling CW, you must first tune the receiver to a clear frequency. Listen for a while to make sure the frequency is clear. Spot the VFO in the receiver, being careful to do this properly. Call QRL? and listen carefully for any response. A response of QRL, YES, or C means that the frequency is in use, and you need to find another frequency. Just for good measure, call QRL? again to be sure the frequency is clear. If the frequency is in use, find another one, Spot the VFO again, and repeat. Only call CQ when you are sure the frequency is clear.

Remember, your operating frequency will be the VFO frequency or an integer multiple thereof, so the VFO may not show your actual operating frequency.



A Digital VFO for Vintage Transmitters:

 
Front View:
This is a front view of the digital VFO. The frequency can be set within 1 Hz, but I usually keep the frequency step size to 10Hz, which is more than adequate for CW. The frequency step size can be easily changed to suit your needs.

The large tuning knob is of my own design. Turning the knob sets the frequency, and pushing the knob activates the synthesizer menus.

The black and red push buttons, along with the VFO A/B switch, select one of 12 different memories. The buttons are also used to program the synthesizer.

I use one memory for each band, 80m, 40m, 30m, and 20m, and have six additional memories programmed with the six center channel frequencies of the 60m band.

One of the memory locations is labeled VFO A/B and can actually hold two frequencies. The VFO A/B switch selects which frequency is programmed or used.

The Spot switch turns on (keys) the synthesizer so that the operator can spot the transmitter frequency in the station receiver.

The RIT Display switch causes the RIT frequency to be displayed. As explained elsewhere, I use the RIT to keep the VFO off frequency during receive periods, unless the Spot switch is turned on. Keying the VFO brings it on frequency while the key is down, and takes it off frequency when the key is up. (Unless the Spot switch is thrown.) I typically don't use the RIT Display switch for anything, but it is there in case it is needed.

 Front View
Click on the image for a larger view.

Click here for a super detailed view.


 
Rear View:
One the back of the VFO there are two outputs, one that is buffered/amplified and one that is not. The buffered output is used when driving a transmitter, and the unbuffered output is used when the VFO is used as a signal generator. The switch selects which one you wish to use.

The RIT keying jack is normally connected to the RIT output on the keying amplifier. When grounded, it brings the VFO on frequency. When open, the VFO is off frequency by an amount equal to the RIT offset unless the SPOT function is activated.

 Rear View
Click on the image for a larger view.

Click here for a super detailed view.


 
Inside View:
This is an interior view of the VFO. The synthesizer board is mounted at top center. The power supply is at the bottom, and the perf board containing the buffer amplifier and RIT inverter/keying circuit is at top left.

The red switch on the back panel at left selects either the buffered or unbuffered output.

The rotary encoder, LCD display, and other switches are mounted on the front panel on the right.

 VFO Interior
Click on the image for a larger view.

Click here for a super detailed view.



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