The demodulator section of the transceiver’s receiver starts with the product detector, which is made of another SA602. To get more audio volume a preamplifier has been added before the LM386 follows.
The AGC section hast got 2 crucial components: One resistor (this case 100k) and an electrolytic capacitor (in this case 100uF): They determine the time ramp for the AGC regulation curve. This means they define the response and decay time for the AGC and thus should be made easily changable for example by putting them into socket strips.
Hint: In certain cases it can be useful to add a potentiometer to give you control on the audio input of the AGC preamplifier.
The if amplifier has been slightly revised. I added a preamplifier after the 1st mixer to enhance overall gain. Due to the fact that the veroboard is crowded with the components that had already been installed, the preamplifier has been worked out in SMD technology using the reminaing space on the reverse side of the board underneath the SSB filter were still some room has been available:
This new part of the circuit is not marked in the block diagram I’ve posted some days ago. As main amplifier of this stage, MC 1350 is used. Due to space saving reasons the tuned circuit to terminate the if amplifier IC has been made of a very smmall pig-nose core:
The parallel capacitor has been experimentally optimized by putting various capacitors into a 2 pin part of a socket strip and keeping the best valued. The MC1350 is gained controlled by an AGC amplifier and DC rectifier section to be described later. Please notice the correct termination of the SSB filter with 2 resistors 2.4 kOhms each.
For my compact 40 metres transceiver there was not plenty of space for complicated circuits. So I had to find a simple but effective solution for the singe stages. Everybody knows that the first stage in the receiver’s front with the 1st mixer, which is a crucial one, determins the overall performance of the whole receiver to a wide extent. So, which mixer should I use?
Among the “standard” mixers available on the market there is one, that uses only a few external components as a mixer stage that, aside from mixing two signal, delivers a recognizable amount of gain (around 18 dBs): The well-known NE612 (aka “SA602” and other derivates).
But there is one problem: The NE602 has been developed for VHF communications where excessive signal strenghtes are not the primarly issue. On 40 metres the situation is different. Very much different. OK, even if strong in-band signals are present they won’t push the NE602 to its limits as I could find out, the problem are the extremely strong signals from broadcasters at 7.200 khz and above.The NE602 reacts with lots of spurious signals if input levels are too high. Thus, developing a front end, that is able to cope with extremely loud signals only some Kilohertz away fron the operating frequency was a challenge. Intense filtering was the key to success. Here is my solution:
For extreme receiving situations with excessive out-of-band signals there is a 10dB attenuator switchable from the front panel. As I found out this is only required if you use an antenna that delivers high rf voltages in the evening from broadcast stations transmitting above 7.200 kHz (like my Deltaloop does). With my vertical antenna using the attenuator on the other hand is obsolete.
After the attenuator you can see a three-pole filter made of tuned circuits with a center frequency of about 7.100 kHz. The trick is the loose coupling between the single tuned circuits. This makes the filter extremely sharp but costs you some gain. To compensate the loss, the following stage equipped with an NPN-transistor is used. Noise figure enhancement is not the problem on 40 meters, so I did not use a FET. A bipolar transistor fills the needs.
After that another 2 tuned circuits, also extremely loosely coupled, follow. Next is the well-known SA602 mixer IC powered with the input signal from the 7MHz filtes and the DDS VFO.The input to PIN 1 and 2 of the mixer IC is symmetical which is preferably to the single ended unbalanced method seen in many other circuits.
The practical solution of the RF preamp is a flat package mounted to the side of the transceiver’s mainframe:
You can see the antenna input from the right, an on/off switch from top (not in schematic) and the output to the 1st mixer also on the right (connected to the reverse side of the PCB).
In my last article I talked about my ideas fo a new transceiver project beyond the QRP level. First pictures of cicuitry were also shown. In the meanwhile the transceiver has been finished, some minor changes had to be made and now it’s time to go to the details.
All construction objectives (compact size, sufficient output power to establish even DX contacts on 40 meters, good stability, good receive performance, rigidness for outdoor use) have been met so far as I can say. I had the rig with me, when I was on vacation on the Island of Jersey (GJ/DK7IH/P) from 12th to 19th of August this year. It was big fun operating the rig. Lots of stations were calling during the two days when I was on 40 meters. ODX was HL1AHS, OM Kun from Seoul. So, this was very nice for 50 watts and a vertical antenna made of a fishing rod.
First, to give an overview, let’s have a look on the completed transceiver. Cabinet size is 7.5 x 16 x 6 centimeters.
As you can see, the rig is very compact in size. The block diagram gives you an overview what is inside. Receiver section is on top, DDS can be found in the center and the transmitter is sited at the bottom of the diagram. As you can see, it’s again not rocket science and SDR-virus could not strike as well.😉
The next posts will describe the rig in details step by step. Proceed with the receiver’s front end.
On one hand sunspot cycle is on the decline. Conditions on the higher bands tend to deteriorate gradually. On the other hand I wanted a very compact transceiver for outdoor (particularly holiday) activities. Besides this and due to the fact that I prefer monoband operation when on tour, the potentially ideal band had to be found. Based on these prerequisites I made up my band to go to 40 meters for this project.
Some interesting concepts for QRP-transceivers for this band can be found on the web. The “Santerre” and the “ILER40” should be mentioned. These operate with power levels of about 5 watts which I thought should not be enough in most of the situations. My opinion is, that on 40 meters it’s not the best decision to operate on the standard QRP watt level, e. g. like the mentioned before and my other rigs have been designed for. With high probability you’re about to get lost in the naturally higher band noise and QRM that is around on 7 MHz. Thus, 50 to 70 watts possibly should be a more suitable power level to operate on.
From the electronic point of view this transceiver doesn’t involve very much new stuff. The 4-stage transmitter with push-pull driver and final from my multi-band rig with emitter degeneration circuitry in every stage to improve linearity but without negative feedback in this circuit. Some NE612/SA602-mixers in receiver and transmitter, MC1350 as the rx’s if-amplifier, LM 386 as audio amplifier, busines as usual. DDS with AD 9551 for frequency generation. No fuss, no SDR-stuff. “Old school” homebrewing. Not more, but not less.
The challenge this time was to increase package density once more to an achievable maximum. The maximum size I wanted to have was the size of a carton of cancer sticks, also known as “cigarettes”.
Another problem of the project was to get a high power rf amplifier with an output level very much beyond standard QRP and avoiding any unwanted coupling, parasetic oscillations etc. even when components are very densly packed. A clean signal is a must, not of the package size.
The center of the construction is made of the aluminium carrier of the 50 to 70 watts final that is in the center of a three layer arrangement consisting of
rf power amplifier
rf board with SSB generator, tx mixer, pre-amp, pre-driver and driver (push-pull).
The parts of this assembly:
The main frame carrying all the other stuff:
Dissambled mounting frame of QRP transceiver for 40 meters (2016 by DK7IH)
The aluminium plane centered will keep the PA 50 to 70 watts board. This one is equipped with two MRF455 rf power transistors made by Motorola.
Above and below this board the receiver and transmitter boards are mounted to a three layer package.
These 3 boards are stacked and plugged into the front unit that you can see underneath. The front subassembly is formed of
the front panel with user controls, microphone socket and LCD
the DDS system and
the relay switching board.
Both construction groups are joined in an angle of 90° by a set of plugs and sockets so that they can be put apart easily and fast for service or improvements. All boards are made of 5×7 cm “FR4” material Veroboards.
In the center of the right board package you can see the PA amplifer, capable of delivering 50 to 70 watts SSB signal, on top the rx board, at the bottem the transmitter board.
An important issue for a high power transceiver is the transmitter’s final amplifier. Particularly thermal conductivity should be kept in mind. In this case one problem, I thought, might occur because the power transistors are not mounted directly to the rear panel of the transceiver where heat can be lead to the outside easily. Here, the two MRF455s are sited in the center of the sandwich construction holding the 3 main rf boards. But as this transceiver is for voice operation only with its comparatively low duty cycle of 10 to 20%, this was not considered as an unsolvable problem. But thermal aspects must be kept in mind, anyway.
As you can see, the final transmitter stage is based on an aluminium sheet metal of 2 mm thickness. Under the board there is a second layer also of 2 millimiter thickness aluminium. By the rear end it is joined to a solid piece of square shaped aluminium rod that itself is connected to another two thick layers of alumium which are srewed to the rear panel holding a heat sink. For this heat sink I’m currently searching a more massive one.
The thermal test was a longer QSO with Dave, M5AFD . During this longer QSO with transmission times of up to 3 to 4 minutes each the center alu panel got a temperature of 60° to 65° centigrade. Thermal stress? Not worth mentioning yet!
So far I’ve done about 50 QSOs on 40 meters with this rig, gradually improving some things. Later I will publish the detailed scheme as soon as it is finished. By now prospects are good to bring this compact QRO transceiver with me on a holiday trip to Jersey Island planned this summer.
People frequently reading my blog know that I love to improve the things presented here. This is revision one of my semi-automatic antenna tuner for QRP. I found that the reading of the SWR value on the LCD is a little bit uncomfortable. So I decided to use an “old-style” analogue meter. This one has been pulled from a home stereo amplifier.
The problem with the installed directional coupler is that the amount of energy and so the induced voltage is a reverse function of coupled frequency. Thus an operational amplifier has been added to amplify the DC voltage from the directional coupler. A 50k stereo potentiometer regulates negative feedback an so reduces the op’s gain.
This afternoon I was sitting at my desk doing something in the computer. The QRP-transceiver was tuned to the 20 meter band (14249 kHz). Suddenly I heard ZL1SLO with a fair 58 report. The band was clear, no big pile-up, no “big guns”, so I gave him a call. And, beleive it or not, he answered to my call an gave me a 57/58 report. Not bad for 10 watts ouput power and a simple dipole.😉