Denys Roussel, F6IWF
A Low-Cost HF SSB/CW Transceiver
VHF Communications 4/1997
A year and a half after the first prototype was put into operation, a further
transceiver has now been completed.
The real work consisted of designing printed circuit boards, carrying out the
mechanical assembly, and keeping an eye open for the possibilities for
Some problems were recognised during the second assembly, and some necessary
improvements were made which provided for a higher degree of reproducability
1. CHANGES TO THE BLOCK DIAGRAM
The single significant change is the additional stage at the power amplifier
The reason for this is that the level at the mixer output was previously too
low, and was insufficient to control the PA correctly and thus obtain a good
inter-modulation interval, IM3.
The MIC level can still be increased somewhat, which gives a sufficient level at the mixer output, which in turn improves the inter-modulation products. An increase of 6dB in the amplification keeps the IMD to an acceptable value.
2. CIRCUIT CHANGES
The circuit changes are explained below in relation to the corresponding
sections of Part-2 (issue 3/96). The corresponding circuit diagrams are
shown with changes and component values.
2.1. The Mixer (Section 6.1.)
The mixers were previously matched to 50 Ohms. The mixers now operate at
100 Ohms which firstly reduces the stress on the high-frequency phase shifter
and secondly saves a transformer (Fig.2).
Space was provided on the board for an optional balun transformer and a
balance potentiometer (22 W).
This allows better carrier suppression to be achieved in transmission
operation -50dB is possible.
These changes are not really needed for the second assembly, as carrier
suppression of 35dB, which has already been achieved by other improvements, is sufficient.
2.2. The High-Frequency Pre-Amplifier (Section 6.2.)
Here the new assembly created instability. A tendency to oscillate arose in
the 600 - 700 MHz range. The effect sounded like microphony if the C408 capacitor was touched.
The C413 SMD capacitor gives better de-coupling on the basis of T401 (Fig.3).
A ferrite bead can be provided for additional damping on the basic connection.
2.3. Low-Frequency Pre-Amplifier and Mixer-Driver (Section 6.3.)
There were no circuit changes here. The component values are shown in Fig.4.
2.4. VFO and High-Frequency Phase Shifters (Section 6.4.)
The VFO was completely re-designed, thanks to F5OYV. The circuit (Fig.5) became somewhat more universal, with an improved output level and additional options: RIT, tuning through Varicap, frequency drift monitoring, and an additional voltage regulator.
The cost of the additional stage was balanced out by the fact that a bipolar transistor was used in the oscillator instead of the J310.
It is not advisable to use Varicap tuning without a crystal-stabilised circuit for frequency drift compensation.
All components are housed on a small printed circuit board, only 36mm x 46mm.
The TTL-IC 7400 is replaced on the phase shifter by a type 2N2222 transistor. This increases the sensitivity and reduces the costs.
The outputs to the mixers were alternated between Q1 and Q2. These outputs are not fed back to their inputs in IC 901, which increases the power for the mixer.
In case of any changes in the sidebands, space was provided on the small printed circuit board so that the positions of the resistors R903 - R904 could be exchanged. The wiring shown corresponds to that for the lower sideband.
The high-frequency phase shifter is mounted on a plug, to provide for a possible expansion to a multi-band system.
2.5. Low-Frequency Phase Shifter (Section 6.5.)
The circuit has been expanded by three potentiometers and a CMOS switch 4066, in preparation for a multi-band version (Fig.6).
These additional potentiometers are necessary, since the zero settings (image frequency suppression) are different for each band.
Since these functions are not required as yet, only P201 is fitted, and a bridge is placed between pins-8 and 9, instead of IC 205.
2.6. The Low-Frequency Section (Section 6.6.)
A voltage regulator IC 78L08 was used instead of the stabilisation circuit for +8.5 Volts (Fig.7).
The D307 diode creates a better insulation between the microphone and the 301C IC during receive.
In order to prevent problems in the low-frequency section during switching from transmit to receive, R312 and R338 were provided, which are there in order to retain the potentials of the capacitors, C305 and C330 respectively.
The CW filter has been replaced by a lower-noise circuit version.
A favourably priced 200mA type can now be used as an S-meter.
An 8.2W resistance has been inserted into the power supply circuit of the TBA820M and suppresses the low-frequency feedback to the 12Volt supplies..
2.7. CW Oscillator (Section 6.7.1.)
The oscillator is now fed +8V directly from the low-frequency printed circuit board (Fig.8).
2.8. Transmit / Receive Switching (Section 6.7.2.)
The transceiver now has reverse battery protection, by means of a P600 diode and a 10A fuse (Fig.8).
Switching from battery operation to an external power supply is now done manually through a switch.
PTT-Out has been provided as an additional output.
2.9. Band-Pass Filter (Section 6.8.1.)
The filter is now combined with a 6dB amplifier (Fig.9). Only five additional components are required for the assembly and switching of the stages.
D502 is switched from the transistor current. The emitter resistance has been inserted into the earth connection of TR502.
The transistor is a favourably priced BC238B in a plastic housing, adequate for the short-wave range.
The ferrite bead, as the basic connection for the damping, should not be forgotten.
2.10. Power Amplifier (Section 6.8.2.)
SMD components are now used for power supply switching and for the input stage instead of the wired components previously provided.
Moreover, D603/5 and D606/8 are available as protection against a B/E breakthrough at an extreme Veb value (Fig.9).
Two additional transistors have also been provided in the PA, in order to increase the power by app. 20%.
The transceiver is divided into eight sub-assemblies for assembly:
Low-frequency board as core of TRX
High-frequency phase shifter
Band pass filter
Low pass filter
Transmit / receive switching
3.1. Low-Frequency Board
This is the most complicated sub-assembly.
Mounted on the 176mm x 76mm printed circuit board (Fig.10) are low-frequency phase shifters and low-frequency phase switching (Fig.5), the low-frequency section (Fig.6) and the CW oscillator.
The tight squeeze is necessary to keep the equipment as small as possible (Fig.11).
It was the cost angle which led to the decision to disentangle the printed circuit board on one side only, instead of using a double-sided printed circuit board with feedthroughs, which would have cost 3 to 5 times as much to produce.
In addition, the one-sided version requires considerably more development work, and a quantity of unattractive wire bridges has to be accepted.
So when equipping, we first begin by inserting the wire bridges, which are created using 0.5mm tin-plated copper wire.
In the IC 201/204 range, it is recommended that insulated wire of 0.3 to 0.4mm be used, to avoid short circuits. Don't forget the bridges under the IC's!
Next, the diodes, resistors, IC sockets, trimmers and finally capacitors and transistors are soldered in.
For the IC's, it is recommended that sockets be built in. The sole exception is IC 303, the low-frequency amplifier, which is adequately cooled only if mounted directly onto the PCB.
C210 and R337 are SMD components and are soldered on last, on the tracks side. There was no longer enough room left for any other form of construction.
In addition, three insulated connections also had to be inserted, for which there would otherwise have been no room left:
8V In had to be linked to +8V Out
CW Out had to be linked to CW In, and
Tone Out had to be linked to Tone In.
Particular attention has to be paid to the earth connections with this construction. As already mentioned in the theoretical section, it is particularly difficult to suppress low-frequency feedback / coupling loops in receivers with direct switching and built-in loudspeakers.
The following methods have proved to be particularly suitable:
The printed circuit board must basically be connected to the earth. To do this, the board is screwed down only in the vicinity of the IC 303, with two screws, about 1.5cm apart, and earthed.
The earth pin near R316 (identified as "G") must be connected to the earth screw near C323 by the shortest route, and with an adequate cross-section (app. 1.2 to 1.5mm2).
Other options were also tested, but naturally gave only unsatisfactory results.
This caused the greatest difficulties in the mechanical assembly.
If tuning using Varicap is selected, D801, D802 and corresponding components do not need to be fitted (Fig.12).
L802 is formed onto R811.
The frequency range of the VFO lies between 28.0 MHz and 30.4 MHz which, when divided by 8, gives the range 3.5 to 3.8 MHz.
The frequency drift lies at about 500 Hz/h at 28 MHz, depending on the quality of the capacitors used.
3.3. High-Frequency Phase Shifter Module
This assembly takes the form of a plug-in module. This route was chosen in preparation for optional expansion to the multi-band version.
However, the module can also be connected to the mixer assembly directly, using short wires.
IC 901 should be soldered in without a base, to avoid any problems which might arise from base capacities (Fig.13).
This assembly includes the two mixers, the high-frequency pre-amplifier and the two pre-selector stages.
The mounting sequence is as described for the low-frequency board.
The components plan (Fig.14) shows the basic version of the mixer, with the options as per Section 2.1.
We should start by wiring up both mixers on the top face - e.g. with bridges instead of the balun transformers and potentiometers in the vicinity of R002.
The SMD component under C409 should not be forgotten.
When the assemblies have been successfully tested, the coils and radio-frequency transformers must subsequently be secured using "radio-frequency wax".
During assembly, make sure you obtain perfect symmetry for the mixers. To this end, the repeaters must have exactly the same number of windings, and the material used must be identical.
SMD coils are fitted in the vicinity of the radio-frequency pre-amplifier (Fig.15).
3.5. The Radio-Frequency Filter Module
Pay attention to the direction of winding of the repeater during assembly.
The ferrite bead on T 501 shoud not be forgotten.
Following a successful test, C 508 can be replaced by a bridge (Fig.16).
3.6. The transmission amplifier
You must naturally think about a special mechanical solution for cooling the transistors.
One very simple, but effective solution, is to mount the printed circuit board directly onto the cooling body.
This requires a flat bottom face - i.e. all components, including the SMD components, are soldered on the top face (Fig.17).
The printed circuit board was designed for SMD components, which naturally also brings other advantages:
Thus, for example, the low-inductance feedback resistances, with 2-W R 612 and R 613, which are very difficult to obtain, can each be replaced by 12 normal 22-W SMD resistances.
Another advantage is the small space requirement, so that the PA assembly can now turn out to be considerably smaller.
Now for the assembly:-
First, all SMD components are firmly soldered on (Fig.18).
During wiring, the connections should be continuously tested, using an ohmmeter, to avoid short circuits, which could have serious consequences.
The printed board assembly is directly screwed to the corresponding prepared rear wall.
The rear wall itself is made of 4mm thick aluminium and acts both as a cooling plate and as a mounting plate for the PA.
The board is drilled and provided with M3 threads - seven screws for the printed circuit board and one M3 x 5 screw with insulating sections for each PA transistor.
When the 7805 voltage regulator and all the transistors have been soldered on, check the resistances between collector connections and earth, and between basic connections and earth.
The D 601/601 diodes must be in thermal contact with the rear wall (heat conducting paste). Two breaches must be provided in the printed circuit board for this purpose.
Finally, the radio-frequency repeater is incorporated. The capacitors C 604, C 609 and C 615 are soldered into the assembly directly. C 614 is soldered to D 602.
Up to 14 type BD 135 transistors can be mounted on the printed circuit board.
The origin or manufacturer of the transistors is important in connection with use in the KW-PA. I have experienced the best results using BD 135-10's from SGS-Thomson.
To keep specimen scatter values as low as possible, the transistors should all be from the same production series / batch.
It certainly makes sense to test transistors from various manufacturers.
3.7. Low-pass filter
The printed circuit board is prepared for two different types of relay (Fig.19).
Should other relays be selected, the pin configuration should be suited to the printed circuit board.
3.8. Transmit / receive switching
The PTT option can be fitted if desired.
Wires with different colours should be used for L 1101. The direction of winding at the output of L 1101 is important.
The value for C 1104 can be increased up to 4,700 mF if the TRX is mainly intended for battery operation.
Fig.20 shows the components plan.
(To be continued)