HT220 Frequency Synthesizer

originally published 1976
by Dale Heatherington, WA4DSY
edited for webpage use by Michael Wright, K6MFW
edited for mistakes by Walter Dilley, KD7DNY
, 6-28-02


The following instructions pertain to the new board marked with the number 051776. There have been several improvements made over the old board. Most noteable is the reduction of receiver birdies and a 12 mA reduction of the power drain. Only 2 receiver birdies remain; 146.000 and 147.100 MHz. The transmitter spurious outputs are at least 5 dB better and the broad band white noise is down much more. The white noise reduction also resulted in improved receiver sensitivity. I still don’t recommend using a high power amplifier with the synthesized HT220 but if you feel the need to do so here is the formula to compute the 2 major spurs so you will at least know where your QRM is.

Upper spur = F + ((F/9) - 15.5555) for simplex
F + ((F/9) - 15.4888) for -600 KHz
F + ((F/9) - 15.6888) for +600 KHz
Lower spur =F - ((F/9) - 15.5555) for simplex
F - ((F/9) - 15.4888) for -600 KHz
F - ((F/9) - 15.6888) for +600 KHz
where F = operating frequency in MHz
example:F = 146.520 MHz simplex
lower spur = 145.796
upper spur = 147.244

Another major improvement was the elimination of the two CD4011 ICs. Many people found it was hard to find CD4011s that would work right in the original circuit. That is no longer a problem since there are no CD4011s in the new circuit. Some people have ask why I used 9.10222 MHz for the reference frequency instead of 4.551111 MHz so the CD4020 reference divider would not be pushed to it’s limit. The answer is simple.... High accuracy crystals in the HC 18/U holder below 6 MHz are not available.

A word about IC sockets: You can use low profile sockets if you bend the leads that need to be soldered to the top side of the board out to the side so you can solder to them. Be careful that they don’t touch anything they should not.




Parts List
Z1CD4059AEProgrammable divider
Z2CD4046AEPhase look loop
Z3CD4020AE14 stage divider
Q1, Q2, Q3,
Q4, Q9, Q10, Q11   
MPS3704 or 2N4401NPN transistor
Q5, Q12MPS3703 or 2N4403   PNP transistor
Q6, Q7, Q840673RCA dual gate MOS-FET
D11N759or any 12 volt 400mw zener diode
D2MV2107 or MV2106Motorola EPICAP tuning diode (22pF at 4V)
D3, D4, D51N914 or 1N4148diode
S176A10Grayhill 10 unit DIP switch
R1, R3, R10
R20, R35, R42
1000 ohm1/4 watt 5% resistor
R2150 ohm1/4 watt 5% resistor
R4, R12, R18,
R19, R23, R24,
R25, R26, R27,
R28, R29, R30,
R31, R32
100K ohm1/4 watt 5% resistor
R5, R7, R18,
R21, R36, R38
10K ohm1/4 watt 5% resistor
R6, R1415K ohm1/4 watt 5% resistor
R8, R92.2K ohm1/4 watt 5% resistor
R11180K ohm1/4 watt 5% resistor
R1322K ohm1/4 watt 5% resistor
R15100 ohm1/4 watt 5% resistor
R16, R17220 ohm1/4 watt 5% resistor
R22820 ohm1/4 watt 5% resistor
R332.7K ohm1/4 watt 5% resistor
R34220K ohm1/4 watt 5% resistor
R373.3K ohm1/4 watt 5% resistor
R39, R40470K ohm1/4 watt 5% resistor
R4147K ohm1/4 watt 5% resistor
C1-C75-25 pF trimmerErie 518-000-5-25 or equivalent
size: 0.220" dia. x 0.170" high
C8, C15, C2710 uF at 20 VDCSprague 183DR106X002D or equiv.
sub-miniature tantalum capacitor (radial leads)
C9, C160.01 uF at 100 VDCDisc ceramic capacitor (0.300" max dia.)
C10, C17, C21
C24, C25, C29
100 pFDisc ceramic capacitor (0.300" max dia.)
C11, C22330 pFDisc ceramic capacitor (0.300" max dia.)
C20, C3310 pFDisc ceramic capacitor (0.300" max dia.)
C2333 pFDisc ceramic capacitor (0.300" max dia.)
C30, C31, C3222 pFDisc ceramic capacitor (0.300" max dia.)
C18, C280.001 uFDisc ceramic capacitor (0.300" max dia.)
C12, C190.1 uF at 35 VDCSprague 182DR104D4035H or equiv.
Sub-miniature tantalum capacitor (radial leads)
C13, C14, C261 uF at 25 VDCSprague 16D105X9025BC2 or equiv.
Sub-miniature tantalum capacitor (axial leads)
L12.7 uH2.7 micro henry choke or
40 turns of #36 magnet wire
wound on the body of a 1 meg ohm
1/2 watt carbon resistor.
X113.68888 MHzcrystal
X215.62222 MHzcrystal
X315.55555 MHzcrystal
X415.48888 MHzcrystal
X59.10222 MHzcrystal

Crystals are 0.002%, 20 pF load cap. calibrated at room temp. HC 18/U holder. Order type HA-M1 from International Crystal Mfg Co.



Frequency range: 146.000 to 147.995 MHz transmit and receive

Channel spacing: 5 KHz

VCO frequency

Power requirements: 12 to 15 volts at at 29 mA

Spurious outputs: 45 dB below carrier

Transmitter offsets for repeaters: -600 KHz, +600 KHz, and simplex

Input/Output lead description

  1. to +600 KHz lug on transmit offset switch
  2. to simplex lug on transmit offset switch
  3. to -600 KHz lug on transmit offset switch
  4. to common lug on transmit offset switch
  5. Audio input. Connect to wiper of the IDC control in the HT 220.
  6. PTT (+14.6 transmit, gnd for receive). Connect this lead to point M in the transmitter power distribution circuit of the HT 220.
  7. VCO drive for the receiver. Connect to emitter of receive 1st osc. transistor. *
  8. VCO drive for the transmitter. Connect to base of 1st tripler transistor. *
  9. Power input from battery.
     GND. Connect a wire between the synthesizer ground and the HT 220 ground.

     * See interface diagrams for details.


Construction Hints

Because of the high parts density of this circuit, the procedures outlined below should be followed.

  1. Use a low wattage soldering iron with a small tip and #24 or smaller gauge solder.
  2. Since the holes in the circuit board are not plated thru, you must solder the parts on both the top and bottom of the board where necessary.
  3. The order in which the parts are installed is important because some parts overlap others. Install all the feed-thru wires first. Next install the dip-switch on the bottom of the board and solder it in place. Be sure it will be right side up when the board is installed in the radio. Put in all of the transistors next. Q1, 3, 4, 8, 9, 10 and 12 need one or more leads soldered to the top of the board. The remaining parts can be installed in any order but the three ICs should be put in last. IC sockets can NOT be used because some pins need to be soldered on top of the board.
  4. The VCO coil is a 2.7 uH inductor. You can wind it yourself if you prefer.

    1. Select 1/2 watt carbon resistor greater than 1 meg-ohm with a straight smooth body.
    2. Cut a shallow notch in each end with a triangle file as illustrated.
      2.7 uH inductor illustration
    3. Cut a 19 inch piece of #36 magnet wire and remove 1 inch of insulation from each end.
    4. Wind 40 turns close spaced in a single layer on the resistor body and solder both ends of the coil to the resistor leads.
    5. Coat the finished coil with lacquer to secure the windings.
    6. Install the coil on the circuit board as illustrated below.
    inductor installation illustration
  5. The transmit offset switch is a 3 position rotary switch that replaces the original "PL" switch. Some PL switches can be modified for 3 positions by removing a stop in the switch. Check the top of your switch for a pin which could be moved or removed.
  6. The synthesizer can only be installed in the long omni housing with the privateline board and all other accessories removed. The extention sleeve must be modified by cutting out the antenna sheath and all other mounting tabs around the edges. The antenna cannot be fully collapsed when the synthesizer is installed. A rectangular hole must be out in the back for the frequency select switch. These modifications can be done with an Xacto knife and a mototool.
  7. Wiring between the radio and the synthesizer should be done with #26 or #28 gauge stranded wire.
  8. The parts near the VCO coil should be glued down to prevent vibration from modulating the VCO.
synthesizer block diagram


Test and Alignment

Test equipment needed:

Note: It is better to do preliminary test and alignment before you connect the synthesizer to the radio. It can prevent trouble later on if you locate any "bugs" now.

Reference oscillator adjustment

  1. Connect 15 volts to the board. + to point I, - to the ground foil.
  2. Connect the frequency counter lead thru a 20 pF capacitor to pin 10 on Z3, the CD4020.
  3. Adjust C5 for 9.10222 MHz.

VCO and offset oscillator adjustments

  1. Connect the frequency counter lead to point H (xmit drive).
  2. Connect the VTVM to the + side of C14 (1 uF).
  3. Set S1 (freq. select switch) to 147.000 MHz.
  4. If the board is not in the radio, connect point F (ptt) to +15 volts. This puts the synthesizer in transmit mode. If the board is in the radio you must key the transmitter.
  5. Connect a temporary jumder wire from point D to point B. This puts the synthesizer in simplex mode.
  6. Turn on the power and adjust C7 for a 5 volt reading on the VTVM. If you get the 5 volt reading at this time all of the circuits are working ok.
  7. Adjust C3 for 16.33333 MHz VCO frequency.
  8. Remove the jumper from point B and connect it to point A. This puts the synthesizer in +600 KHz mode. Adjust C2 for a VCO frequency of 16.4000 MHz.
  9. Remove the jumper from point A and connect it to point C. This puts the synthesizer in -600 KHz mode. Adjust C4 for a VCO frequency of 16.26666 MHz.
  10. Remove the jumper from point F to +15. Connect the jumper from point F to ground. This puts the synthesizer in receive mode.
  11. Adjust C6 for a 5 volt reading on the VTVM. Adjust C1 for a VCO frequency of 14.4666 MHz.

Alignment is now complete and the synthesizer can now be installed in the radio. Some of the trimmer capacitors may need to be touched up after it is in the radio. Below is a list of the trimmers and their function.

NOTE: If you have trouble with any of the test and alignment steps cheek for a 555.555 Hz square wave at pin 3 on Z3, CD4020. Some 4020s won’t work at 9.1022 MHz. If you have an input on pin 10 of this IC but no output on pin 3 you need to try another one.


How to use the binary frequency select switch

This switch consist of 10 spst switches numbered 1 thru 10. In the following discussion an "ON" switch will be called 1 and an "OFF" switch will be called 0. The frequency is entered directly in BCD code.

Switch #1 is the MHz select switch. 0 = 146.XXX MHz, 1 = 147.XXX MHz

Switches #2, 3, 4, 5 are the 100 KHz select switches. They are BCD coded and can select any digit from 0 to 9.

Switches #6, 7, 8, 9 are the 10 KHz select switches. They are also BCD coded and can select any digit from 0 to 9.

Switch #10 is the 5 KHz select switch. 0 = 14X.XX0, 1 = 14X.XX5

Note: The following is in courier font so the numbers should align in proper columns.

Caution, when operating above 147.400 MHz be sure the transmit offset switch is in simplex or -600 KHz. If not the transmitter will transmit at 148 MHz or above.

Photo of DIP switch in back of a HT220 (image from an eBay item).

Feedback is desired (suggestions, comments, errors, gripes, whatever) Michael Wright, K6MFW,

Go the HT220 Page