A synthesizer-controlled 70cm-transceiver
for 9600bd packet radio

Holger Eckardt, DF2FQ, Lorenzstr. 4, 81737 Munich



1.1 Problems of 9k6

Until today, more than 10 years after the publication of the G3RUH-modem, there exists not a single radio from the large amateur radio manufactures, which is suitable for operation at 9600 bit per second with a sufficiently good quality. What are the problems?

The modulation used for 9k6 packet radio data communication typically is frequency modulation, like it is used for voice radio as well. However, in contrast to voice radio, there are some particularities. The most important is that the frequency response of the transmission channel, i.e. the channel from the modulation input of the transmitter to the demodulator output of the receiver, must be linear over a large range.

In the ideal case it should be constant from 0 Hz to approx. 6kHz. A lower cutoff frequency of 5 or 10 Hz doesn't affect the signal significantly. The higher the lower cutoff frequency is, the better must be the signal-noise-ratio to achive still a sufficiently error-free transmission. Exceeding a boundary of about 50 Hz, no useful transmission is possible any more.

However not only the frequency response must be linear, also the time, which the signal needs to cross the channel must be more or less constant in relation to the frequency. This is called group delay time ("Gruppenlaufzeit"). This somewhat abstract term shall be expained with an example: The spectrum of a square wave signal contains many harmonic frequencies. If the individual parts of the spectrum need different times to cross the channel, the signal at the end of the transmission will be highly distorted, and accordingly difficult to decode. As we use squarewave-like sequences for our data transmission, delay time distortions will strongly be apperent. Human voice on the other hand, due to its spectrum with just few harmonics, almost isn't affected by delay problems.

Another point is the time, which the radio needs to switch between transmit and receive. Using 1200bps-technology, 100ms switchover time were still a useful value (which nevertheless many radios hardly achieved). But at 9600bps this period of time already causes a severe slowdown of data throughput. If one considers, that e.g. an acknoledge frame only needs 25ms, it's obvios, that even a radio with 100ms switchover delay spends most time not with the data transmission, but with waiting for the synthesiser of the radio. Some of the radios used today need 300ms switchover time.

Besides the primary points enumerated above, there are some second order effects, which currently are far less taken into account in today available transceivers than the pure transmission parameters. One example is the transient spectrum at cyclic keying. This means transmissions outside the operation frequency, which are made during the transition from receive to transmit. As at 9k6 operation the switchover cycles are much shorter than at voice transmission, this can considerably impair adjacent stations. Which devasting spectra are even emitted by commercial radios of newest production, DF2FQ will show in a comprehensive test report on aptility for data transmission of amateur-transceivers during this year. In this context he'll also describe the measuring processes.


1.2 A simple solution


Because of the insufficient technology of the commercial providers of ham equipment, unfortunatly synthesizer controlled equipment has been discredited a lot. This would not have been necessary, as one can see at the example of professional data radio technology, which partially has to fullfill much stronger requirements (e.g. for DECT radios a TX/RX-switchover time of 20us is demanded), and still offers high-grade solutions.

Amateur transceivers, especially those intended for self construction, have to fullfill different constraints than DECT transceivers. The criterias which DF2FQ had in mind during the development of the device presented in this text were, besides an excellent usefullness for 9k6 data transmission, a high reliability of reconstruction, and a favorable price. Simple constructions also requires, that the radio can be adjusted without special measuring devices. A special difficulty was to get along without SMD-components. The predominant part of modern components nowaydays isn't available anymore in 'traditional' (non-smd) technology. However, the utilisation of SMD-technolgy would surely have been a large potential source of error in the assembly.

Still, the assembly of the transceiver is not intended for an unexperienced beginner. The construction requires good experience with the assembly of electronic circuits. Also tuning and setting up require good knowledge of RF technology, especially to estimate what might be wrong, if the device isn't working at first.


Table 1 shows the specifications of the transceiver (mean values of five assembled radios, the deviation of the data lies, if at all, within few dB)

General
Frequency range 430,000... 439,975 MHz
Channel steps 25/12,5 kHz
TX - RX - switchover time < 25ms
Power supply 7 ...14V, 60mA RX, maximum 2,5A TX
Size 145x75x22mm
Receiver
sensitivity digital -120dBm (=0.22uV) for BER<10-4 (measured with DF9IC-modem)
sensiticity analog -118dBm for 20dB SINAD (CCITT)
frequency response 1Hz ... 7000Hz bei -3dB
intermodulation -54dB (3-tone measurement)
neighbouring channel selection > 56dB
mirror frequency suppression > 60dB 1.mirror, > 51dB 2.mirror
Transmitter
output power 1.5WO at 7V, 6.5W at 12V
frequency response 1Hz ... 15000 Hz bei-3dB
Klirrfaktor < 1.5%
spurious transmissions -66dBc (1. harmonic), < -75dBc otherwise
transient neighbour channel interference <-40dB
Table 1, the technical data




2. The circuit in details

The circuit diagram is distributed onto four pages. Image 1 shows the synthesizer and the modulation-circuit, image 2 the receiver, and image 3 the transmitter. On the fourth page the control is presented.

2.1 The synthesizer

The central part of the synthesizers is the VCO, it is used both for transmitter and receiver. A Helix circuit is used as oscillator coil, which ensures low oscillator noise and resistance against ("Mikrofonie"). For modulation and tuning separate Varicaps are used, which simplifies the injection of the NF-signal.

The VCO oscillates on half the transmitting frequency, for obtaining a better decoupling from the PA. After the VCO follows a frequency doubler, after that a buffer amplifier. The attenuation circuit between those also is intended for decoupling oscillator and PA. The collector path of the buffer contains a notch circuit for filtering the remaining signal of the VCO frequency.

The VCO is controlled by the wellknowen synthesizer sircuit by Fujitsu. As the charge pump of the IC provides far to little current for the fast switch over time we need, there is a push-pull-amplifier at the output of the phase comparator. This amplifier feeds the low-impedance loop filter.

If the modulation signal were only fed into the VCO, the frequency response would be insufficient for packet opertion. Below the loop cutoff frequency of the loop filter, the deviation would decrease with 6dB/octave. at the loop resonance frequency it would have large ("Überschwinger"). As we are using a loop cutoff frequency of 700Hz, there would be no recognisable modulation signal at 10Hz. Thus also the reference oscillator is modulation with the NF. The frequency response at this point is complementary to that at the VCO injection point. Together this results in a fully linear frequency response.

The signal of the reference oscillator is simultaneous used as LO signal for the second receiver mixer. As the reference frequency must be divisible by 25kHz, this yields in a value of 450kHz for the second IF instead of the usual 455kHz. According to this, an apropriate IF-filter is used.


2.2 The receiver

The receiver uses two dual-circuit helix filters for the input selection, one before and one after the initial stage. This yields an excellent band selection. People living near 'bundle' radio or 'Modacom'-base stations [commercial radio system in Germany operating close to the 430-440Mhz amateur band], will know to respect this advantage.

The mixer is realized with a Dual - Gate - Fet, the matching of the high-impedance gate is done using printed inductors.

The IF signal is taken from the drain. Because of the high requirements on the group delay time performance, the trimmer for the adjustment of the quartz filter unfortunately is inevitable.

After the quartz filter follows an amplifier stage, and after this a Motorola IF-IC. This component contains, beside the second mixer, a limiter amplifier, the demodulator, a RSSI-circuit and an OpAmp. Latter is used here as a 2-pole low pass filters for the suppression of the IF. The filter for the second IF has, compared with the more common CFW-type, a filtering caracteristic which is a few dB inferior, however it has a considerably better group delay time.

The RSSI-output (radio signal strength indicator) emits a current which is, over a dynamic range of approx. 60dB, proportionally to the logarithm of the input voltage. With the help of one transistor it becomes a fast DCD-signal (Data Carrier Detect), which is especially usefull for operation over multimode-nodes. The activation level is adjustable within the RSSI-range using a potentiometer.


2.3 The transmitting part

Like in most directly modulated synthesizer-transceivers, the transmitter is not very complicated. The VCO-signal is amplified in the driver stage to approx. 30mW. This signal is fed into a PA-module. which at 12 volts supply voltage creates an output power of about 7W. After the low pass filter and the PIN-diode switch, which serves as a transmit-receive switch, about 6W are available. The transistor, which switches the power supply of the transmittor, is fed by a constant current source, which itself using a RC-combination creates a linear ramp with a time constant of 5ms. By this slow activation of the transmitter one avoids disturbances in the neighbor channels caused by hard switch-on edges. A 5V - voltage regulator supplies all parts of the circuit besides the driver and the PA. The transmitting part is fed by the unregulated supply voltage.


2.4 The control

A not completely unimportant part of the transceiver is the control. There a micro controller of the type PIC 16F84 is used. It takes over several tasks. The PTT-signal must be examined, the PLL IC must be reprogrammed on transmit-to-receive and vice-versa switch over, and receiver and transmitter must be turned on and off in a controlled manner. Besides the the user interface is controlled, if e.g. the channel should be changed, or a new frequency be programmed. The necessary program is stored in the chip itself.


3.1 Setup

The complete circuit is on a board with the dimenstion 72x144mm and fits in a standard tinplate cases. Image 7 shows the component placemnt diagram. The layout-top is on image 5, the bottom side on image 6. On the upper side of the board there are only wireings, the most of it forms a ground plain. Without a completely contaced card it will be difficult to get the device working properly, even if one solders the over 100 contacts by hand. If you still want to do this, remember to solder all ground pins of the components on the top AND bottom side.

Table 2 shows the part list. It is advisable to mark each soldered component in the list, to avoid forgetting any component. The VMOS - transistor T11 (BS170) is not placed in before adjustment (see below). During assembly, one should pay attention, that the components are positioned close to the board, and that the wires are cut short on the bottom side. No IC sockets are allowed be used, execpt for IC1. For this IC however a socket is recommended, to simplify an exchange of the software version.

Four of the coils must be self-manufactured. These are marked with 3W3D in the circuit diagram. This means three turns, 3mm diameter. You use a 0,5mm strong isolated copper wire for these coils, from which you wrap three turns on a 3mm thick drill. The wire should be tin-plated before soldering. The PA-module becomes the only component mounted on the bottom side of the board. The distance between the installation flags and the board must be 4mm.

Before installing the board in the case, holes in the side parts and the lower cover must be drilled according to drawing image 8. Then the casing gets assembled and the board gets fitted into it. For the screws in the middle of the board, the 5mm distance-holders are used, for mounting the PA-module the 4mm ones. Then the board gets fastend with M2,5x10-screws to the bottom part of the case. When everything fits, the side parts are solderd together at the edges, the board gets soldered to the case about every 15mm with a soldering point.

The last two parts to assemble are the BNC-socket and the feed-through capacitor for the power supply. They get mounted from outside through the appropiate holes and soldered to the side part of the case. On image 11 you can see a photo of the assembled transceiver.

If you want to design the transmitter for permanent operation, a cooling fan with 5K/W or less is needed. For the usual packet radio operation with 30% transmitting cycle a 2-3mm thick aluminium plate covering half of the case is enough.


Capasitors
Part Value Package Part Value Package
C1 33p C2 C39 10n C25
C2 10n C25 C40 10n C25
C3 1uF ELKO C41 3p3 C25
C4 47n C5 C42 100p C25
C5 10n C25 C43 5p6 C25
C6 1p5 C25 C44 5p6 C25
C7 1n C25 C45 2p2 C25
C8 1n C25 C46 100p C25
C9 10n C25 C47 2u2 ELKO
C10 10uF ELKO C48 1n C25
C11 4u7 ELKO C50 100p C25
C12 100p C25 C51 100p C25
C13 10n C25 C52 47n C5
C14 1p C25 C54 100p C25
C15 33p C25 C55 100p C25
C16 33p C25 C56 10uF ELKO
C17 100p C25 C57 100p C25
C18 1p C25 C58 47n C5
C19 100uF ELKO C59 4p7 C25
C20 33p C25 C60 100p C25
C21 .1uF C5 C61 1uF ELKO
C22 10uF ELKO C62 10p C25
C23 10uF ELKO C63 1uF ELKO
C24 10n C25 C64 100p C25
C25 47n C5 C65 100p C25
C26 47n C5 C66 47n C5
C27 47n C5 C67 10n C25
C28 10p C25 C68 10n C25
C29 10n C25 C69 100p C25
C30 1p8 C25 C70 30p CT5
C31 8p2 C25 C71 5p6 C25
C32 33p C25 C72 10p C25
C33 47n C5 C73 10u ELKO
C34 100p C25 C74 47n C5
C35 100p C25 C75 .1uF C5
C36 10uF ELKO C76 8p2 C25
C37 10n C25 C77 100p C25
C38 10uF ELKO C78 470p C25
C79 47n C5      
Resistors
Part Value Package Part Value Package
R1 100 R10 R33 18k R10
R2 1M R10 R34 82 R10
R3 1M R10 R35 10k R10
R4 1M-tr R_TR_ST R36 470 R10
R5 680 R10 R37 2k2 R10
R6 3k3 R10 R38 2k2 R10
R7 10k R10 R39 1k R10
R8 4k7 R10 R40 100 R10
R9 4k7 R10 R41 25k-tr R_TR_STM
R10 10k R10 R42 100k R10
R11 3k3 R10 R43 100 R10
R12 100k R10 R44 150 R10
R13 10k R10 R45 220 R10
R14 100 R10 R46 18k R10
R15 10k R10 R47 470 R10
R16 15k R10 R48 150 R10
R17 100 R10 R49 39 R10
R18 180 R10 R50 470 R10
R19 47k R10 R51 470 R10
R20 100 R10 R52 100 R10
R21 1k R10 R53 100k-trim R_TR_STM
R22 6k8 R10 R54 10k R10
R23 2k7 R10 R55 10k R10
R24 10k R10 R56 270 R10
R25 10k R10 R57 10k R10
R26 33k R10 R58 100 R75
R27 10k R10 R59 100 R10
R28 4k7 R10 R60 330 R10
R29 33 R75 R61 100 R10
R30 56k R10 R62 10k R10
R31 220 R10 R64 1M R10
R32 82 R10      
Inductors
Part Value Package Part Value Package
L1 514630 HELIX L10 .33uH SP10
L2 .33uH SP7 L11 3W3D SP5
L3 .33uH SP7 L12 3W3D SP5
L4 .33uH SP7 L13 .33uH SP10
L5 3W3D SP5 L14 3.3uH SP7
L6 511765 HELIX L15 .33uH SP10
L7 511765 HELIX L16 .1uH SP7
L8 3.9uH SP7 L17 3W3D SP5
L9 455kHz,sw BANDFI L18 1uH SP10
Semiconductors
Part Value Package Part Value Package
D1 BB204 TO92 T2 BC557 TO92
D2 BB405 D5 T3 BFR91 SOT103
D3 1N4148 D7 T4 BC547 TO92
D4 BA479 D5 T5 BF980 SOT103
D5 BA479 D5 T6 BFG65 SOT103
D8 BA479 D7 T7 BFR96 SOT103
D9 ZF5V1 ZD T8 BD140 TO139
D10 BB204 TO92 T9 BFR91 SOT103
IC1 PIC16F84 DIL18 T10 BFR91 SOT103
IC2 MB1504 DIL16 T11 BS170 TO92
IC3 MC3371 DIL16 T14 BC547 TO92
IC4 78L05 TO92 T15 BF255 TO92
PMOD M67749M PWR-MOD T16 BC557 TO92
T1 BC547 TO92 T17 BS170 TO92
Other
Part Value Package Part Value Package
FI1 21U15A HC45U P3 ANT PIN
FI2 CFUS450D CFU P4 +UB PIN
Q1 20.95MHz HC49/U X1 2X07/90 RM2.54
Q2 ZTB655 QS X2 2X05/90 RM2.54
Table 2, part list



3.2 Tuning

The transeiver has 9 tuning points, which sounds quite much, but still the tuning is simple. Nevertheless a few measuring instruments are needed:



As first one connects a supply voltage of 7.. 12 volts to the device. If there are no short-circuits on the board, it will draw a current of approx. 60mA. After two minutes warmup-time you connect the frequency counter to Pin 2 from IC3 (MC3371). The frequency should be close to 20.950MHz. Now you must tune the frequency to exactly that point using R4. You should consider, that each Hz offset on the reference frequency results in an error of 12Hz on the operation frequency. Because of the large coupling capacitors, the reaction to tuning of R4 is quite slow.

For the next step, you have to select a frequency as close as possible to the center of the 70cm band. How to do this is described in the -The user interface-. The frequency must be equal to that of the unmodulated carrier from the HF generator (or from handheld transceiver, or ham living close to you, etc). One connects the digital volt meter to the RSSI-output of the board. The DCD-potentiometer R53 must be in center position. Without signal a voltage of approx. 0.5 to 1 volts should be measured. According to the intensity of the input signal this value rises by a certain amount. Now you should tune alternately L6 and L7 to a maximum output level. If the input volatage is greater than -60dBm, the RSSI-voltage will reach approx. 3.5V, and will not increase any further. Therefore in this case, you have to reduce the input level in order to be able to continue the tuning.

For adjusting to the demodulator circuit you also need the unmodulated carrier [leave it on during all of the following process!]. You must connect a sinodial generator (e.g. from image 9) and set it to 300mV output voltage. It is important that T11 is not connected, as this transistor normally switches of the modulation input while receiving. At the NF-OUT Pin you now must be able to see signal with approx. 1Vss amplitude. Adjust L9 for maximal amplitude, C70 is adjusted for an ideal sinodial shape. You can view this best on a dual-input oscilloscope, where the second channel is connected directly to the sinus generator.

The receive path is now fully adjusted, now its time to adjust the transmitter. To do so, it is not necessary to switch the transceiver to transmit mode. All you have to do is to switch the signal generator to square wave. You now should see a square-like signal on the oscilloscope, which still is connected to the NF-OUT pin. The unmodulated carrier still must be at the receiver input! Now you turn R41 until the signal shows an optimal square wave. Now you mustn't forget to insert the T11. [-- T11 is not necessary if your TNT/Modem itself switches off its modulation signal while receiving]

Finally the transmitter is switch to transmit, and the power of the output signal is controlled with a watt meter.


4. The user interface

Considering the normal usage of the transceivers, the user interface is rather expedient than comfortable, which mainly first keeps down the cost of the parts.

Beside antenna and supply voltace connection the device has two sockets, a 10-pin (X2) and a 14-pin (X1) socket. The pinout is shown in table 3. For both sockets there exist plugs for flat ribbon cables. X1 serves for setting the frequency, on X2 are situated all signals for the connection to the modem.

X1 X2
1 D0 2 n.c. 1 GND 2 +5V
3 D1 4 n.c. 3 DCD 4 PTT
5 D2 6 n.c. 5 GND 6 MOD
7 D3 8 TXD 7 GND 8 NF-OUT
9 n.c. 10 RXD 9 n.c. 10 RSSI
11 PTT 12 n.c.        
13 GND 14 +5V        
Table 3, pinpout of the plugs X1 and X2


4.1 Frequency input

As you can see in the specifications, the transceiver covers the complete 70cm-band in the 25kHz-steps [support for 12.5kHz steps in the newer version], you can use any arbitrary relay shift (or none at all). The device contains a memory for 10 channel pairs for transmitting and receiving. The data is stored in a EEPROM, so the memory content is saved after turning off the supply voltage.

The current channel is selected by the Pins D0 to D3 on the plug X1. The channel is coded in BCD-code and can e.g. set with a BCD-switch or with plug-in jumpers. When using a BCD switch, the common switch must be connected to Pin 13 (ground). D0 is the lowest, D1 the highes order bit. The jumpers are connected over opposite pins, e.g. 1 and 2, 3 and 4, etc. [This does NOT hold for D3!] Although n.c. really means -no connection-, these Pins are pulled to ground by the PIC during normal operation, and thus can be used as ground connection for the jumpers.

To enable the transceiver to transmit or receive on a channel, you at first have to program the corresponding frequency previously. This is done via the serial interface with the help of a computer with V24 interface and a terminal program. You connect the TXD-pin of the Interface (at MSDOS-computers e.g. COM1 or COM2) to the RXD-Pin of the transceivers (Pin 10) and the RXD-pin of the computer to the TXD-Pin (Pin 8).

The serial port of the computer is configured to 1200 bits per second, 8 bits, no parity, two stop bits, no local echo, no data procotol. Now you can program channels with a simple syntax. The character sequency which you enter must be:

Cnrrrttt[RETURN]

Thereby is C the upper-case C on the keyboard (HEX 43), n represents the memory position to be programmed between 0 and 9. rrr represents the receive frequency and ttt the transmit frequency. The number to be entered is the channel number counted from 430,000 MHz in 25kHz steps, it must always be entered in three digits, even if the first number is a 0. The input is completed with RETURN. The input can't be corrected, i.e. the backspace key will not work. If you made a mistake, press RETURN and start over once more from the beginning. Channel numbers over 399, according to 439,975Mhz are ignored. An Example:

You want to programm the channel 0 with the receive frequency 438,100 MHz and the transmit frequency 430,500 MHz. The input sequency you have to type then is C0324020. If you want to the programm channel 8 with the simplex frequency 434,125 you type in C8165165.

Is the RXD pin doesn't provide a real +/- 12V level, it can happen on some computers that the echo of the type charactors will not work. Nevertheless the communication from the PC to the transceiver still works.


4.2 The modem signals

NF input and output are compatible to most 9k6 - modems. The NF output level of a signal with 3kHz shift is 1Vss, at the transmitter you need 300mVss to create a deviation of 3kHz. Some modes also deliver NF signal during receive operation, thus the input is switched off while receiving. [T11]

The transceivers delivers a fast DCD-signal (Pin 3). It is derived from the RSSI voltage. If an input signal arrives, which lies over the threshold adjustable with R53, it rises from a level of 0V to 5V. The delay is approx. 5ms. If the Poti is in the left-most position, this function is switched off.

As parameter for TX-Delay a value of 'T4', i.e. 40ms, has shown to work reliable. Indeed 'T3' should also work, but DF2FQ has noticed, that the timing of Z80-TNCs is not very accurate, and a value of 'T3' sometimes results in a TX-delay close to 20ms, which is too little for this transceiver.

The analog parts of many 9k6 - modems itself require a very long time to switch from transmitting on receive. The reason are the coupling capacitors at the operational amplifiers. Often it helps to put the modem on permanent NF output to solve this problem. [In my experiance, the solution to most of these problems is a small modification to the transceiver. Connecting the supply voltage of the MC3371 permanently to +5V instead of swithing it off while transmitting avoids the large voltage step on the NF-OUT line which causes the problems with the coupling capacitors]


5. voice communication and other things that are possible

With little additional expense the transceiver can also be used for voice radio. All you need is a microfone amplifier (1 transistor) and a NF-amplifier (e.g. LM386) for the loudspeaker. The Squelch is derived from the DCD-signal derived, R53 can be fed to the outside as Squelch-Poti via X2. (image 10)

For packet radio operation at 1200 Bps this transceiver is applicable without modification. If you want to use it for higher speed, e.g. for 19200 Bps use, you must install broader IF-filters in the receiver, at the transmitting part nothing changes. Due to the increased NF bandwith, the sensitivity will decrease. Experiments with this have not yet been made.

A more comfortable user interface is in development. It'll contain a LCD display, a rotable knob for frequency setting, some memories, and perhaps a scanning feature. For this extensions some pins have been reserved on the plug. With this equipment this device is comparable to the products of the Japanese amateur radio industry.


6. Final remark

The circuit published here may be reconstructed by everybody for personal use. Commercial utilization, including the utilization of parts of the circuit, requires permisson from the author. Above all, this publication is made without consideration of possible rights of third parties, which have to be checked separatly in the individual case. The same holds for this article, for which the right of publication, including partial publication, remains at the author. For damages caused by the use or the reconstruction of the circuit published hare, no responsibility is taken. With proper assembly, the circuit fulfills all requirements of the new European regulation for amateur radio devices ETS 300-684 as well as the EMV-regulation EN 55022, as far as it is applicable here. However the decive has not been officially certificated according to these regulations. So according to the law, this radio must not be sold as commercially manifactured radio. Kits however are available. For information about the transceiver and for technical questions DF2FQ can be reached on packet radio (DF2FQ@DB0PV), or by email (holger.eckardt@vlsi.com), or also by letter (return-postage!).

Schematics

Synthesizer
Receiver
Transmitter
Controller


Text translated from the german text by DF2FQ.
Remarks in [sqare brackets] are comments of the translator.
HTML translation by LC3VAT