Digital Frequency Display Applications Notes
Digital Frequency Display
DFD will operate to 45 MHz LO with 2V p-p or more drive.
If you have trouble with input frequencies above 20MHz it
is recommended that you bypass, or short out, the 1K input resistor. (see schematics)
At 40MHz it normally takes 450mv p-p.
Bypassing the resistor it takes only 225mv p-p.
Use an external coupling capacitor as
small as possible to minimize capacitive loading of the local oscillator.
The back to back diodes are there
because some radios present high voltage transients when switching bands which will lock
up the DFD requiring it to be turned off and back on to unlock.
Several customers have reported success
getting local oscillator signals from tube radios by wrapping a few turn of insulated wire
around the tube.
Make sure the insulation can take the heat.
20db gain 30MHz bandwidth preamplifier.
provides useful gain to 50Mhz.
For mounting inside radio.
Can be powered from an
- AC source (labeled Vac) such as filiment transformer in radio.
- If power for the backlit display is to be taken from Vdc when AC is supplied to Vac then
the capacitor from Vdc to gnd should be increased to about 100ufd to 250ufd depending on
current supplied to backlit.
- DC source (labeled Vdc) of 8 to 18 volts or
- DC voltage (labeled 5V) of 5 to 24 volts. (LM78L05 removed)
IN capacitor would typically be in the order of 5pf to100pf. (As small as
possible while providing sufficient coupling)
OUT capacitor would be on the order of 100pf depending on type of cable used to
carry signal to counter.
.01 ufd capacitors are supplied with the kit but the input capacitor should be kept as
small as possible if connected across the oscillator tank circuit.
.01 is OK if signal is taken from a buffered signal source.
Circuit board and parts layout.
Board is 1" X 2" finished size.
This item is available as a
Click above for ordering
$6 plus $1.50 shipping (no shipping if ordered with a DFD kit)
I5TDJ developed the following very sensitive preamplifier which replaces the
This circuit gives 50mv sensitivity to 50MHz. The output of the 74AC00 drives the PIC
directly bypassing the 74HC4046.
Piero used a DFD with the IC-202S two meter rig. It triples a 15MHz VXO to 45 MHz and then
triples it again to 145MHz. Piero took the signal after the first tripler and set DFD to
multiply by 3. The signal was taken from the junction of R25, R26 and C39 through a 100pf
capacitor and about 12" of RG174U.
For those applications that do not require this much gain I played with just the final
stage of I5TDJ's design.
Using a 2N3904 this little amplifier gave a gain of about 15 at 6MHz and dropped to
about 3 at 50MHz.
It has a rather low input impedence so for those who need to decrease the load on their
VFOs an emitter follower can be added.
It is probably best to put this preamplifier in the radio to buffer the
radio from the cable capacitance.
The 5V DC can be borrowed from the DFD at the output of the 78L05.
I purchased your DFD and it works great. Although to get it to work with my Kenwood
TS-820. I had to add an amp to the front end to beef up the VCO signal from my radio. I
thought you would like to know that I used a Signetics NE5205 wide band 20db amp. I used
two 1000pf caps, one on the input and one on the output. Also a .01 bypass cap. I taped
into the 5volts on your board and I was in business! I did a bench test and the DFD will
convert a 50mvp-p signal up to 47Mhz. Which is well above where my radio VCO tops out at.
I thought you would like this info and find it useful, if you didn't already know about
this chip. Thanks for bringing my radio up to modern standards.
Regards, Steve KE6WOH
You can also use an NE5204. These devices have a 50 ohm input
impedence. An emitter or source follower can be added to the input to increase the input
How to use the optional backlit display module.
Direct conversion units are simple. Connect to the LO (VFO) output and
adjust the IF offset to zero. The NE/SE602 is a popular device in such units. The counter
should be connected to pin 7 (the emitter of the oscillator transistor)
Single conversion superhets are common in QRP
transceivers and older vacuum tube receivers. When connecting to a vacuum tube unit a
resistor attenuator may be required. Put a resistor from the input to ground on the Zt
pads provided on the PCB. Connect the unit through a series resistor and capacitor to the
oscillator plate or cathode (if unbypassed).
- Single-Board QRP SSB Transceiver for 20 or 75
Meters (April 1997 QST)
- by Dave Benson
- Connect to collector of Q13 (VFO buffer)
- 20 Meters (8 MHz IF, 6.2 MHz VFO
- Set IF offset to 8 MHz ADD
- 75 Meters (9.83 Mhz IF, 6.2 MHz VFO)
- Set IF offset to 9.83 Mhz SUBTRACT
- Connect across R46 (output of Q1 source follower for VFO)
- Data I have did not specify VFO frequency range but it makes sense that is in the 4 MHz
- Set IF offset to 10 MHz ADD
- MFJ-9040 ( and other MFJ-90XX) by K6ZRL
- Connect across 'VFO Drive' adjust potentiometer R41 on the top side of the PCB.
- coax cable should be less than 12"
- put a 1K resistor in series with the center wire of the coax to reduce loading effects.
- Set IF to 12MHz.
- May need to tweek this for each radio.
- Install +/- Jumper (pin 13 of U2)
- SWAN 700-CX
- Connect to accessory socket pin 8, pin 9 is ground.
- IF is 5.5MHz
- On 80 and 40 meters VFO is above th RF frequency and below it on all other bands
- add/subtract input to DFD should be subtract for 80 and 40, and add for the other bands.
- This can be accomplished using a little toggle switch mounted with the DFD.
- SWAN 240 three band rig
- For the 240 I suggest off the plate or cathode of the oscillator tube 6AU6.
IF offset = 5173.5KHz (which requires a DFD1A to get 500Hz resolution)
on 20MTRS RF=VFO+IF
on 40MTRS RF=VFO-IF
on 80MTRS RF=VFO-IF
so you would have to put a toggle switch on the add/sub jumper of the DFD1A.
- SWAN 250 6 Meter rig
- Connect DFD input to Q2 emitter. (this is the VFO before tripling to 40MHz)
- Set unit to PRESCALE mode
- IF is 10.698 MHz. With zero input frequency (input shorted), adjust offset to 3.566 MHz
- Adjust prescale until display reads 10.698MHz.
- Set unit to ADD
- GE SUPERADIO III
- I'm using the DFD with a GE Superadio III. The Superadio is a $49.95 AM/FM battery/ac
(with internal power transformer) receiver and is a favorite of AM dx'ers. It has a tuned
RF stage for AM, a very large loopstick antenna, a large speaker plus a tweeter, and a
wide/narrow IF selectivity switch. What it lacks is frequency calibration; my dial is off
by maybe 10%. Looking at a web page for Superadio buffs, I find this is a universal
complaint. (Radio Shack sells an Optimus "High Performance" radio that appears
to be the same Chinese-made set).
- I use the DFD only for AM. The only place to get more than about 20 mv of LO signal is
at the collector of the LO transistor, Q15 (middle pin). This is right across the tuning
varactor but the signal is still only about 1v pk. To minimize the capacitive load I built
a dead bug style source follower using a 2N4416 (1.1k source resistor to ground, 100k
resistors from the gate to ground and from the gate to Vcc, Drain to Vcc, 10pf capacitor
from the gate to the base of Q15). Vcc (4.3V) is from the hot end of R16. Ground is the
cold end of R17 (330 Oms). The supply voltage and ground for the DVD (10V) are across C55.
(These are convenient places to make connections w/o removing the radio's circuit board).
- There wasn't room inside the Superadio to mount the DFD so I put a small "D"
connector on the back.
- Thanks to Jon for supplying this information.
- FAQ for the SUPERADIO
- REALISTIC DX-160
From the top inside locate R12, a 1500 ohm resistor, located on the right edge of the
printed circuit board. Connect a 68pf capacitor at the end of R12 going to Q4 (other
end is grounded). Connect the other end of the capacitor to a length of RG174 coax
sufficient to reach your DFD.
- I'm not sure what the IF is for this receiver but I strongly suspect 455KHz. You
can use that to start and verify by tuning in an AM broadcast station of known frequency
and adjusting the offset and +/- jumper to display the correct frequency.
ATLAS 210/215 Using DFD1 (A custom chip is available DFD1-Atlas)
- DFD1-Atlas is programmed for automatic band selection when the radio is
operated in the amateur bands only. Display outside amateur bands may not be
- ATLAS 210
- Pins 2 and 3 of external VFO input connector, gnd on pin 4. An 18pf capacitor couples
VFO output to DFD using RG-174.
- Thanks to W4MCD for this data.
- Rudy King, who is willing to provide details, firstname.lastname@example.org , really neat internal mounted DFD1A
with backlit display option.
Atlas210 with Digital frequency Display
Atlas210 DFD on 10mtrs
Atlas210 DFD view of installation from rear
Atlas210 Transceiver with DFD
Atlas210 preamp board installation
Atlas210 cover removed showing DFD installation
- Tentec Triton
- Connect as shown above and put a SPST switch across the DFD add/subtract jumper pins.
- Triton VFO is:
- 80 mtrs.......12.5-13MHz (set DFD to subtract)
- 40 mtrs.......16-16.5MHz (set DFD to subtract)
- 20 mtrs....... 5-5.5MHz (set DFD to add)
- 15 mtrs.......12-12.5MHz (set DFD to add)
- 10 mtrs.......19-21MHz (set DFD to add
- Set IF offset to 9Mz.
The oscillator pre-mix type unit mixes a VFO with a crystal oscillator.
Either the sum or difference frequency of that is filtered and sent to the RF mixer. The
pre-mix output frequency is the main LO frequency. The IF offset should be set to the
units IF frequency. ADD/SUBTRACT depends on if the main LO is above the RF frequency
(SUBTRACT) or below (ADD). In some units it may be a function of which band it is set on.
In that case it is necessary to have a switched input to the ADD/SUBTRACT input of the
- Ten-Tec Omni DFD1
- Set IF offset to 9 Mhz
- The main LO frequencies and ADD/SUBTRACT modes are
- 160 Meters 10.8-11.3 MHz (SUBTRACT)
- 80 Meters 12.5-13 MHz (SUBTRACT)
- 40 Meters 1.0-16.5 MHz (SUBTRACT)
- 10MHz band 19.0-19.5 MHz (SUBTRACT)
- 20 Meters 5.0-5.5 MHz (ADD)
- 15 Meters 12.0-12.5 MHz (ADD)
- 10 Meters A 19.0-19.5 MHz (ADD)
- 10 Meters B 19.5-20.0 MHz (ADD)
- 10 Meters C 20.0-20.5 MHz (ADD)
- 10 Meters D 20.5-21.0 MHz (ADD)
- There is a 9 pin Molex connector on the back.
- Connect the DFD input to pin 5 (VFO output) and pin 8 (ground).
- 8VDC power for the DFD is available on pin 2.
- There is a 12 pin Molex connector on the back which has connections to an uncommitted
wafer on the band switch.
- Connect pins 5, 6, 7 and 8 togather and connect to the ADD/SUBTRACT input of the DFD.
- Connect pin 1 (ground) to pin 4 (switch common)
- Drake R-4A/B DFD1 or
DFD1A Set IF offset to 5.645 Mhz
- High side injection is always used (set DFD to SUBTRACT)
- Connect DFD input to the connector on back of the unit intended to drive a T-4 or T-4X
- NOTE there is a bandpass filter tuned by the PRESELECT knob on the
output of the LO mixer. DFD may not read right unless the PRESELECT is set at least close
to correct for the frequency of operation.
- K5DKZ's R-4A with DFD1 built
This type uses a frequency translation phase lock loop to add the VFO to the crystal
oscillator. This is better than the pre-mix approach because it lacks the spurious
frequencies generated by the LO mixer.
- TS-820 DFD1
- Set IF offset to 8.83 Mhz
- The LO is always high-side (above the RF frequency). This makes the maximum LO frequency
38.83 MHz. With 2V p-p or more the DFD can be connected to the LO output at pin 4 of the
RF unit (X44-1150-001). This signal will have to be routed out of the unit. I suggest
using high impedence shielded cable such as used for RCA type video cables between VCR and
- A preamp may be required as shown in this application
This type is a crystal controlled band switching converter in front of
a tunable IF.
I have developed a special version of the DFD (DFD2)which
will measure three different frequency simultaneously, the crystal OSC, the VFO and the
BFO. It will then compute the carrier frequency of the RF for LSB, USB, AM and the zero
beat carrier frequency of CW. It will automatically determine the operation mode as a
function of BFO frequency and display LSB, USB, CW or AM.
There are custom chips for the following which offer jumper selectable
10/100Hz resolution and display using either "MHz" or dummy zeros to fill out 8
digits. ie: 14.234.56MHz or 14.234.560 (10Hz resolution) or 14.234.5 MHz or 14.234.500
- FT-101 (using DFD2-101)
- CONNECTING TO THE FT-101 TRANSCEIVER
- Cabling options:
- 1) You may use the remote VFO plug (octal) if: a. You are sure that you, or anyone else
that you may sell the rig to in the future, will never want to install a Yaesu remote VFO;
b. You have a spare octal, male plug; c. You are willing to work in a very restricted
space. 2) You may bring cables out of the rear of the rig and terminate them with RCA or
BNC connectors at the Digital Frequency Display unit if: a. You are not concerned with
affecting the resale value of the FT-101 by drilling holes in the rear panel; b. You want
to avoid soldering to an octal plug in a very congested location. 3) You may install RCA
or BNC chassis mount jacks on the rear panel if: a. You want the modification to appear as
inconspicuous as possible; b. You dont want cables that cannot be disconnected at
the rear panel; c. You are willing to very accurately measure and drill mounting holes in
a very small space.
- Yaesu FT-101 signals
- The BFO signal (3.1793 mhz) is approximately a 3 volt (peak-to-peak) signal that may be
tapped in either of two different places. It appears on pin 6 of board 1184A and is
carried by a short piece of coaxial cable to pin 5 of board 1183A. Each board has a
convenient grounded pin for shield connection, but board 1183A is easier to reach with a
small soldering iron.
- After routing the new cable (or mini-coax, which ever you have chosen) from the rear
panel to the area of the board chosen, solder a .01 disk ceramic capacitor to the center
conductor of the cable. Then solder the free lead of the capacitor to the chosen pin of
the board edge connector and the shield to the nearest grounded pin on the edge connector.
- The Local Oscillator signal (approximately 6 mhz above the displayed signal, or 8 to 36
mhz) is approximately a 3 volt (peak-to-peak) signal that is available at the test point
near the top edge of board 1181A.
- You have 2 cabling options here. The first option is to route the cable from the rear
panel toward the front of the transceiver, and the openings around the tuning dial. Use
these openings to pass the cable to the top of the chassis. While viewing the FT-101 from
the normal operating position in front of the rig, route this cable up over the tuning
shaft and to the right of the chassis. Solder a .01 disk ceramic capacitor to the center
conductor and a small ground lug to the shield. Solder the free lead of the capacitor to
the test point at the top of board 1181A, and attach the ground lug under the adjacent
controls mounting nut and lock-washer.
- CAUTION. This method of installation means that you must disconnect the ground lug in
order to remove the board from its edge connector.
- AN ALTERNATIVE installation method is to install the blocking capacitor (.01 disk
ceramic) on the board between the test point and unused pin 15. This will allow easy
removal of board 1181A. However, it requires soldering the coax to the edge connector for
board 1181A in a very congested area. In this alternative installation the cable stays on
the underside of the chassis and is soldered to pin 15, with the shield is soldered to pin
18 of the edge connector for board 1181A.
- The VFO signal (approximately 9 mhz) is about a 1 volt (peak-to-peak) signal available
at pin 11 of board 1180A. After routing the cable from the rear of the chassis, solder a
.01 disk ceramic capacitor to the center conductor. Solder the free capacitor lead to pin
11 and the shield to pin 10 of board 1180A.
- (The VFO signal also appears on the remote VFO adapter (octal) plug. In some versions of
FT-101s, this signal does not have enough amplitude to operate the DFD reliably.
Your mileage may vary.)
- Thanks to Stan for supplying this information.
- Collins S line
- 75S Receivers (using C75S or DFD2-S)
- Connect a cable from the BFO Test jack to the BFO input.
- Some 75S-3 receivers do not have a BFO Test jack. If your receiver is one of those slip
a wire with an eyelet over pin 8 of V8 and plug the tube back in. With the tube pins
- you, the pins are counted clockwise from the wide space.
- Remove the nut from the screw between the power transformer and the filter capacitor,
and put the ground lug over the
- screw and replace the nut. Connect a gray cable from the RCA connector to the BFO input.
- Connect a cable from the VFO Output to the VFO input.
- Connect the black cable from the XTAL Osc. Output to the XTAL input.
- Plug the cable from the power supply into the PWR input.
- KWM-2 (using C75S or DFD2-S)
- Fabricate two RCA jack adapters similar to shown above. The center to a wire loop
that will fit firmly over a tube pin.
- Remove V15 and the loop over pin 9 and plug the tube back in.
- Ground the shell at the right front corner screw on the final amplifier shield. Connect
a cable from the RCA
- connector to the C75S/DFD2-S BFO input.
- Remove V6 loop over pin 3 and plug the tube back in.
- Thread the ground strap and the RCA connector through the tube shield and replace the
- Remove the nut from the screw beside V13 and put the ground lug over the screw and
replace the nut.
- Connect the cable from the RCA connector to the C75S/DFD2-S HFO input.
- Connect a third cable from the EXTERNAL VFO jack on the rear panel to the VFO input.
- If you are using an external VFO you will need a T connector.
- Connect appropriate power to the C75S/DFD2-S power jack. Anything from 9 to 18
volts DC or 6.3V AC.
- JUMPER OPTIONS
- There are two plug on jumpers on the PC board inside the unit.
- To access these jumpers on the C75, remove the screw from the bottom of the unit and
slide the chassis out of the cabinet.
- They are small black plastic jumper plugs that are plugged on four pins on the PCB.
- The top jumper selects either 10Hz resolution (jumper on) or 100Hz resolution
- The bottom jumper selects how the frequency is displayed
- Bottom jumper ON
- 10Hz resolution would display as
12.345.670 USB AS SHIPPED
- 100Hz resolution would display as
- Bottom jumper OFF
- 10Hz resolution would display as
- 100Hz resolution would display as
12.345.6 MHz USB
- ADJUSTMENT (Using C75S)
- The unit has been adjusted at the factory and will probably not need readjusted. If
adjustment is required, remove the
- screw in the bottom of the cabinet and slide the chassis to the rear just enough
to reach the controls.
- Tune to the highest frequency you will use and peak the PRESELECTOR.
- Set the mode switch to LSB. If the display does not read LSB, turn the BFO control CW
until the mode reads correct,
- then turn it a little more.
- Set the mode switch to USB and repeat the above.
- If the frequency is not reading correctly, turn both the VFO and XTAL controls fully
CCW. The display will be reading the
- BFO frequency when the mode switch is set to SSB, or 455 if the mode is AM. The 455 is
in software, there is no BFO in
- AM mode.
- Turn either control until the display reads a different frequency and is stable, then
turn it a little more.
- Turn the other control until the display reads the correct frequency, then turn it a
- Try some other bands to be sure the display reads correctly on all bands. If a band is
found that is not reading correctly,
- turn the XTAL control CW a little more.
- KWS-1 (Using DFD2-KWS)
- The hook-up is straight forward,
- BFO input connected to V104, pin 8 ,
- VFO to VFO Injection jack J203 and
- HFO to Crystal Injection jack J205.
- SB-102(using DFD2-301)
- Hello Neil! I received the DFD2 kit, programmed
for a Heath SB-301,
- in the mail yesterday to be used on my Heath SB-102. Foresaking all
- other household chores, I assembled the kit. It works like a champ.
- For other person's info, these are the connections I made to the Heath
- SB-102. For the BFO connection, I used Pin 9 on Tube V-13C which is the
- product Detector. For the VFO, I used Pin 7 on Tube V-12A which is the
- Second Receiver Mixer. These can easily be accessed fron the bottom of
- the Circuit board. The HFO is a little harder to access. I used Pin 7
- on Tube V-11 which is the First Receiver Mixer. It is almost impossible
- to access from the bottom, but the way the tube sockets are constructed,
- it is easy to solder the lead from the top on the tube socket. These
- are all Cathode connections, which you recommended. Naturally I used
- shielded cable for all connections.
- I used the Phono connectors on the back of the Chassis. There is Spare
- A and Spare B already, I unhooked the wire from the ALC connecter (Since
- I don't plan to use an Amplifier) and used that one for the third
- HW-101 (using DFD2-301)
- I used this with a HW-101. Here are the particulars:
- All connections are made at the cathodes of the various oscillator tubes.
- All connections are made through a 27pF cap and RG-174 coax. 27 pf was
- found to provide adequate coupling without loading any stage.
- HFO connection at pin 7 of V11
- VFO connection at pin 7 of V12
- BFO connection at pin 9 of V13
- Bypass (Jumper out) the input .01 and the 1 K res on the HFO input ckt of
- the counter if it will not count above 15 meters.
- Keep cables as short as possible.
- TS-520/S/SE (using DFD2-520 recommended for the TS-520S and SE)
- usable on the TS-520 by tapping signals inside radio
using emitter follower shown below.
- plugs into the three RCA jacks intended to drive the
DG5 digital display on the 520S and SE.
- The HF output of some 520S/SEs may not have enough
output on the 10 meter band.
- IF so
- Try shorting the 1K series resistor on the DFD HFO
- OR add an AADE
preamplifier kit to the HF input
- The TS-520 (not the TS-520S) requires an emitter follower to buffer the
- PRINTED CIRCUIT BOARD
- Thanks to WA4PJP for supplying this information.
- TS-520 (using DFD3 recommended for the plain TS-520 (no S or SE))
Suggest connecting DFD3 to the 3-5MHz VFO at the jack leading to HF LO
mixer as shown above.
This type uses a frequency translation phase lock loop to generate the local
oscillator. The VCO is locked onto the sum or difference of the VFO and a harmonic of the
- Henry Radio Tempo 2020 (using
- Unit already contains MHz and 100KHz digital display.
- By connecting the DFD1 to the VFO (J701 pins 5 and 4)
the bandspread (0-100KHz) is displayed.
- VFO range is 9.038 MHz to 9.138 MHz.. Not sure which way it tunes. Either:
- Set the IF offset to 9.038 MHz SUBTRACT
- or 9.138 MHz SUBTRACT whichever works.
- By connecting DFD to the LO (J103) the DFD will display the total RF frequency.
- Set IF offset to 6.187 MHz SUBTRACT
- Not sure what signal level is but if it is 2 Vp-p or more it will work.
This is a novel design approach intended to provide a stable MHz per band approach
without using phase lock loops. A 1MHz xtal oscillator is run through a diode harmonic
generator to generate harmonics from 3 to 32 MHz. The "MHz oscillator" tunes
continuously from 55.5 to 84.5 MHz. Whenever it is close to one of the harmonics from the
harmonic generator one of the many frequencies will be 52.5 MHz. This is selected out by a
bandpass filter. A level detector on the output of the filter usually drives some sort of
front panel "lock" indicator.
The "MHz oscillator" also mixes with the incoming RF frequency and is passed
through a 1 MHz wide bandpass filter centered around 55 MHz. The output of that filter is
mixed with the 52.5 MHz filter output causing a 1MHz band of frequencies centered at 2.5
MHz at the output of the 2-3 MHz filter.Any positive drift in the MHz oscillator causes a
negative drift in the 52.5 MHz frequency effectively canceling out the drift so far as the
2-3 MHz band is concerned.
A 3.455 to 2.455 MHz VFO tunes the 2-3 MHz into a 455 KHz IF section.
At present the DFD1 must connected to the
3.455-2.455MHz VFO and only the bandspread frequency (0-.9999MHz) will be displayed. The
MHz portion must be read off the front panel when the lock indication is on.
You can also connect DFD3 in the same way
and obtain complete frequency display using the band selector function of DFD3.
- Yaesu FRG-7
- Take signal from TP404 of the IF-AF UNIT
- I'm not sure which way it tunes but either:
- Set IF offset to 3.455 MHz SUBTRACT
- or Set IF offset to 2.455 MHz SUBTRACT
- one of these will provide correct display.
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