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

Introduction

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.

Good Luck
Dale

Drawings

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Schematic (247K, size 17x22)

Parts Layout (293K, size 17x22)

Transmitter Interface (91K, size 12x10)

Receiver Interface (114K, size 10x14)

Power Supply Interface (118K, size 13x10.5)

Parts List

Z1	CD4059AE	Programmable divider
Z2	CD4046AE	Phase look loop
Z3	CD4020AE	14 stage divider
Q1, Q2, Q3, Q4, Q9, Q10, Q11	MPS3704 or 2N4401	NPN transistor
Q5, Q12	MPS3703 or 2N4403	PNP transistor
Q6, Q7, Q8	40673	RCA dual gate MOS-FET
D1	1N759	or any 12 volt 400mw zener diode
D2	MV2107 or MV2106	Motorola EPICAP tuning diode (22pF at 4V)
D3, D4, D5	1N914 or 1N4148	diode
S1	76A10	Grayhill 10 unit DIP switch
R1, R3, R10 R20, R35, R42	1000 ohm	1/4 watt 5% resistor
R2	150 ohm	1/4 watt 5% resistor
R4, R12, R18, R19, R23, R24, R25, R26, R27, R28, R29, R30, R31, R32	100K ohm	1/4 watt 5% resistor
R5, R7, R18, R21, R36, R38	10K ohm	1/4 watt 5% resistor
R6, R14	15K ohm	1/4 watt 5% resistor
R8, R9	2.2K ohm	1/4 watt 5% resistor
R11	180K ohm	1/4 watt 5% resistor
R13	22K ohm	1/4 watt 5% resistor
R15	100 ohm	1/4 watt 5% resistor
R16, R17	220 ohm	1/4 watt 5% resistor
R22	820 ohm	1/4 watt 5% resistor
R33	2.7K ohm	1/4 watt 5% resistor
R34	220K ohm	1/4 watt 5% resistor
R37	3.3K ohm	1/4 watt 5% resistor
R39, R40	470K ohm	1/4 watt 5% resistor
R41	47K ohm	1/4 watt 5% resistor
C1-C7	5-25 pF trimmer	Erie 518-000-5-25 or equivalent size: 0.220" dia. x 0.170" high
C8, C15, C27	10 uF at 20 VDC	Sprague 183DR106X002D or equiv. sub-miniature tantalum capacitor (radial leads)
C9, C16	0.01 uF at 100 VDC	Disc ceramic capacitor (0.300" max dia.)
C10, C17, C21 C24, C25, C29	100 pF	Disc ceramic capacitor (0.300" max dia.)
C11, C22	330 pF	Disc ceramic capacitor (0.300" max dia.)
C20, C33	10 pF	Disc ceramic capacitor (0.300" max dia.)
C23	33 pF	Disc ceramic capacitor (0.300" max dia.)
C30, C31, C32	22 pF	Disc ceramic capacitor (0.300" max dia.)
C18, C28	0.001 uF	Disc ceramic capacitor (0.300" max dia.)
C12, C19	0.1 uF at 35 VDC	Sprague 182DR104D4035H or equiv. Sub-miniature tantalum capacitor (radial leads)
C13, C14, C26	1 uF at 25 VDC	Sprague 16D105X9025BC2 or equiv. Sub-miniature tantalum capacitor (axial leads)
L1	2.7 uH	2.7 micro henry choke or 40 turns of #36 magnet wire wound on the body of a 1 meg ohm 1/2 watt carbon resistor.
X1	13.68888 MHz	crystal
X2	15.62222 MHz	crystal
X3	15.55555 MHz	crystal
X4	15.48888 MHz	crystal
X5	9.10222 MHz	crystal

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. http://www.icmfg.com

Specifications

Frequency range: 146.000 to 147.995 MHz transmit and receive

Channel spacing: 5 KHz

VCO frequency

Transmit 16.2222 - 16.4438 MHz
Receive 14.3555 - 14.5772 MHz (for 16.8 MHz rec. IF)

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.

     * 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.
      
   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.
   
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.

Test and Alignment

Test equipment needed:

• 20 MHz frequency counter
  
• 15 volt 100 mA DC power supply
  
• VTVM

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.

C1   Receive frequency adjust
C2   +600 KHz offset frequency adjust (transmit)
C3   Simplex frequency adjust (transmit)
C4   -600 KHz frequency adjust (transmit)
C5   Reference oscillator frequency adjust
C6   VCO range adjust (receive)
C7   VCO range adjust (transmit)

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