Sorry for having deferred the description of the transmitter. The recent days I have been concerned with a new frequency layout for the transceiver. I found that the 17m-band could be an interesting topic because when tuning on internet based SDR pages the last days I saw many strong signals appearing. This might be due to the fact that sun is higher now in the northern hemisphere and conditions will even be better with solar cycle #25 now about being to commence.
Based on these considerations I changed the band plan for the 5-band radio: 10m band has been removed, instead 17m has been added.
The new band layout now is 80m-40m-20m-17m-15m.
Here are the respective values for coils installed into the band pass filters (BPF) and the layout for the final low pass filter (LPF).
Hint: Inductance for the BPF coils have been measured with (probably) excess error ratio. Thus calculations are resulting in a different resonant frequency for the LCs when using Thompson’s formula!
Currently some additional tests with the the transmitter are pending, but full description will follow the next days. So, stay tuned! 😉
This is some sort like “Copy & Paste”, a useful mean if you want to create a doctorate, like the former German Minister of Defense Mr Guttenberg once did. 😉 I don’t want to achieve a doctorate but the receiver of this radio is more or less the same I have constructed for the Midi6-transcevier. So I just copied the schematics and put down the changes in this paper.
To see a full sized picture of the RECEIVER, please click here!
Starting the tour on the left you can see the band switch unit, beginning with a BCD decoder that converts a 3-bit pattern created by the MCU into a 5 line decimal output. The ULN2003 then is a driver designed for motor controls but it is very useful as a relay driver as well. Integrated clamp diodes and open collector circuit make it practical as a driver circuit for this unit.
Next is the band pass filter section. I still use relays for switching the respective filter because I found that it is the best way to keep unwanted signals low from passing the filter, provided you use relays that can serve this purpose., Here signal relays TQ2-12V by Panasonic have been applied. Coils are small TOKO style coil formers with 5.1 mm (2×2.54mm i. e. 2×0.1″) pin spacing.
RF preamp is equipped with a dual gate MOSFET like the BF900 or so. The “AGC” this time is to be manually, just connect the AGC input (which now is an “MGC” to say it correctly!) of the stage to a 10kOhm variable resistor allowing a voltage swing between 0 and 12 V and this will lead to a preamp stage with gain control in the range of 25dB. This variable resistor is to mounted into the front panel, just to be concise.
The receiver’s mixer is an SL6440 which has great IMD3 performance (about 30dB) and has been used instead of diode ring mixer. Some dBs of gain are achieved as well but not the amount you can expect from an SA602.
In practical terms the ic really proves what the manufacturer promises. On 40m e. g. with a large doublet antenna no IMD products are audible even when strong broadcast station are next to the amateur radio band. A really worthy trial with this receiver!
Due to the fact that the following SSB filter is used for the transmitter also, another signal relay switches the filter between the receiver and the transmitter branch.
Next the MC1350 video amp is installed to do the major amplification with the interfrequency signal. It is gain controlled by the AGC circuit on the right side of the schematic. Gain is minimum if AGC input is around 7V or higher.
The product detector is a dual gate MOSFET which is only there because this one has a slight amount of gain and does not consume much space on the tiny boards.
The audio preamp stage is also very simple, just a bipolar transistor with negative feedback applied via a large resistor (390k) also biassing the unit to an appropriate value.
The audio main amp here is not an ic (like the inevitable LM386 e. g.) but it is a push-pull arrangement using 3 bipolar transistors. The stage that enhances the voltage is designed with a BC547, the stage that is bound for current amplification uses a pair of complementary transistors (BD137 -NPN- and BD 138 -PNP-). Audio power is about 1 Watt which is suffice for a small radio.
AGC uses an operational amplifier, any type like the LM358 will work great. The LM358 contains two identical amplifier stages. The first is used to bring the audio signal to a certain level, then rectifying this voltage and subsequently bringing it into a time constant consisting of a charged capacity (2.2uF) and a discharging resistor (3.3M), The circuit has very fast response, so there is no annoying “plopp” when a strong signal breaks in) and the decay is very soft.
The second stage just works as an instrumentation amplifier putting out up to 12V to control the input of the MC1350 at PIN5.
To end this article let’s have a look at the practical setup of the receiver:
As mentioned in the introductory article to this radio the digital components in this transceiver are pre-manufactured modules that have only been put together in a more or less sensible way. ;-). These modules are:
AD9850 as variable frequency oscillator (VFO), China made no-name board,
Si5351 as local oscillator (LO) produced by “Adafruit”,
Arduino Pro Mini w. ATMgea328p as Microcontroller Unit (uC, MCU), no-name
ST7735 colored LCD, no-name,
MCP4725 as digital-analog-converter to preset transmitter gain via MCU, “Sparkfun” clone from China, no-name
All units are able to run on 5V which made it easy to layout the schematic because only one 5V/1A voltage regulator had to bu used.
To watch a high resolution version (4.2MB!) of the wiring scheme, please click here!
a) The lines for ISP (MOSI, MISO, SCK, RESET and GND) have not been drawn but the location to the respective ports is mentioned in the table sited in the right top corner. Reset rquires 10kOhms to +5V and a 0.1uF cap to GND.
b) Certain clones of the MCP4725 DAC module will produce conflicts with the I²C/TWI-address of the Si5351 LO module. Original “Sparkfun” boards come with I²C/TWI-address 0x60, 0x61 or 0x62 (depending on literature/web resource you get this information from).
This address is set by the manufacturer AD inside the hardware on customer’s demand. On the other hand the Chinese made modules I am using have basic address 0xC0 which is the address of the Si5351 also. Thus this leads to conflicts on the I2C/TWI-Bus. One solution is to close a solder bridge to +VDD on the very tiny DAC-board which will set address to 0xC2.
c) For the 4(!) user switches (not 3 like in the photo above!) the pull-up resistor on PORT PC0 is set on. There are is a resistor (in the range between 560 Ohms and 2.2 kOhms) with each switch, that pulls voltage to GND when the respective key is pressed. This leads to a voltage drop at the analog input that will be detected by the ADC channel.
This voltage drop depends on the pull-up resistor and on some other factors so it must be determined for every controller setup individually. To solve this, in the respective functionthat returns the numeric value for the key pressed there is a small commented code that you have to de-comment temporarily:
Restart the software, press every key, put the indicated key value into the code (line 4) and re-comment the orange lines when fnished. Next re-upload the software to the controller.
d) Source code in C is available on my Github repository. Please note that even if an Arduino Pro Mini MCU board is used, the code is not designed for the Arduino “world”. It does not use functions of the Arduino environment and may not function with the Arduino bootloader.
To compile the C source and generate the HEX-File you need the GNU C Compiler either for Windows or Linux.
The well-known mixer NE612 (NXP) will be compared to an AN612 (Matsushita/Panasonic) mixer that has been unsoldered from an old CB-SSB-radio. Comparison will include output voltage level and spectroscopic analysis of a 9MHz SSB signal.
When we talk about about integrated double balanced mixers (DBM) and say the number “612” we usually talk about the NE612 (aka SA/NE/602/612 in free combination of letters and digits). This IC uses a so called “Gilbert Cell” and has been developed by Dutch manufacturer Philips (nowadays NXP) some 30 years ago.
The IC has been intended to be used in cellphone applications, is a low voltage device (6 to 7V VDD approx., 8V DC max.) and has low power consumption . Frequency range is up to 500MHz (input signal) and gain is around 12 to 15dB. It has an integrated oscillator circuit that can be used with crystals connected to PIN6.
The IC has been widely adopted by amateur radio constructors and is still available today mainly in SMD package. When we examine homemade QRP radios published on the internet e. g., in 90% of cases one or more NE602 mixers will be found in the transceivers. One real advantage of the NE612 family is that only a few external components are required for building up a relatively acceptable working rf mixer.
In my radios I usually use the NE602 and its equivalents therefore for the DSB generator circuit and the transmit mixer. For receiving purposes it can be used for the higher bands (f >= 14MHz), on the lower bands the relatively low IMD performance (IMD3 about 15dB) shows severe shortcomings particularly on the 40 meter band where strong off-band broadcaster generate high signal levels and therefore overdriving the mixer’s input stage.
Due to the low IMD performance the IC also has weaknesses when being used as a DSB generator. The following findings occured when I analyzed the spectrum of a simple DSB/SSB generator equipped with an NE602.
NE612 DSB generator circuit under test
The NE612 here has been equipped with an additional resistor network (2x56k and a var. resistor with 10k) to get better carrier suppression features. To enhance output a transformer has been added to use PINs 4 and 5 which are the output stages of the circuit.
When driven with an dual tone audio signal (the 2 frequencies not harmonically related) we get an output voltage of about 50mV pp. and the spectrum shown below:
We can observe some IMD 3 and 5 products about 30dB below peak voltage. This is an outcome a little away from what can be expected from an SSB generator.
AN612 also is a very simple mixer that has been developed by Matsushita (Japan, now Panasonic) and has been used in various types of SSB radios for the 11m-Band (CB). In contrast to NE612 it does not contain an internal oscillator.
The IC comes in a 7 lead IC case (SIP7), please refer to datasheet. The IC is manufactured still today and available from various vendors on the internet. I ordered a package of ten from a Chinese ebay seller and found the ICs worked the same way like an original one from a PRESIDENT CB radio. They actually were no fakes.
The IC has a higher VDD so that it can be connected directly to the 12V rail of a standard battery operated radio. In contrast to the NE612 there is no need for a voltage regulator. Also the whole circuit only needs 7 external components:
Performance is quite interesting. When comparing this circuit to the NE612 DSB generator, we find that the output voltage is 4 times higher than that of its namesake. It equals to 200mV pp. The output spectrum also has slightly improved concerning IMD performance:
We see a little fewer IMD products with slightly decreased signal strength.
The AN612 is a not very well known but so much the better interesting mixer IC for the ambitious radio designer who wants to build hardware defined radios. The main locations in a radio will be the DSB generator and the transmit mixer. The IC is cheap, very well available and reveals a slightly higher performance than the other “612”, the NE612. And, overall, the circuit is very simple.
After having built this respective board with two NE612 ICs (one for DSB generator, one for the TX mixer) I was not satisfied with carrier suppression of the DSB generator. It turned out as only 40dB. Afterwards I constructed a new board with an old SIEMENS Mixer IC (S 042 P) that is still available NOS from various sources. With this one I gained carrier suppression rates of around 55dB. I think this is OK for a homemade transceiver.
The board looks as follows, set up on a 6x4cm 0.1″ veroboard:
The circuit starts with an AF amplifier equipped with a bipolar transistor where also a power supply for Electret microphones has been added. The radio now can handle dynamic and Electret microphones adequately.
Afterwards we see the S042P mixer IC where I have changed the circuit slighty to the one used in my 40-meter-QRO TRX. Audio input signal is now to PIN8 of the IC, Lo input on the rf side of the IC to PIN11 and PIN13. To reduce carrier level and enhance carrier suppression a 5.6pF cap is in series because the relatively high level of signal coming from the LO amp would deteriorate the performance of the DSB generator without countermeasures.
Output from this DSB generator is also symmetric and fairly high. Thus a low valued capacitor has been inserted prior to the SSB filter, sited on the RX board.
After that we see an amplifier with limited gain due to high emitter degeneration and the NE612 as TX mixer. The latter one also with an symmetric output to get more gain from it by using the two inherent output transistors.
TX-power amplifier stages
As I have described in the article of my “Give me 5“-Transceiver some years ago, building a broadband power amplifier is challenging due to one special problem related with the wide range of frequencies that this amplifier must be able to cope with. an extra gain of 5 to 6 dB is commen, when the frequency is divided by the factor of 2. Usually the necessary compensation is done by adding adequate capacitors and inductances using their frequency depending reactance.
With this radio I tried something new. I added an amplifier that is gain controlled by an adjustable voltage. Here a dual-gate MOSFET with gain control to gate 2 sets up the initial stage of the whole amplifier strip. The stage’s gain is set by a simple bipolar driver transistor controlled by a digital-analog-converter (DAC). A numeric value for each individual band is stored with in the EEPROM of the MUC. This numeric value is calculated during adjustment, then stored in the MUC and recalled whenever the radio is switched to a certain band. The DAC is an MCP4725 breakout board, containing a 12-bit device.
After that we see an amplifier that is common solid state technology. Preamp stage and predriver stage are set to A mode which requires a heat sink for the predriver stage. Here a 2N3866 is used as amplifying element.
Driver stage is single ended, operates in AB-mode and also is protected by a heat sink.
After that a somehow uncommon technique has been applied. Instead of using a broadband transformer to reduce the stages output impedance to the some ohms input impedance of the final stage, a set of 6 switchable low-pass-filters is used.
This filter section has been optimized to an output impedance of 50 ohms for each band thus enabling me to test and optimize the transmitter to a maximum with a defined output impedance (remember, this is an experimental radio! 😉 ).
After this filter section the final amplifier stage follows which is able to drive the output power up to 15 to 20 watts on all bands but depending on the DC voltage used for transmitting. The max. power gained during tests was 22 watts pep at 15V DC with two NTE236 transistors. Unfortunately the turned out not to be so rugged and blew in the tests. The eleflow 2SC1969 inserted later showed no problems at all. Thank God! When running on 12.0 V DC the amplifier puts out 12 watts at all bands.
The final part of the transmitter section is the last low-pass filter that is positioned next to antenna relay in the same compartment:
The whole transmitter looks like this:
The various units are:
1: DSB-Generator and TX mixer
2: Amplifier stages 1 to 4
3: MCP4725 transmitter gain controller
4: Intermediate LPF board
5: Power amplifier
6: Final LPF section
7: TX/RX switch board
Here a little bit of analysis to end with the article. First is the output of the SSB-Generator/TX-mixer board with maximum output (Around 500mV pp) set to the 40m band.
Nest we see the carrier suppression when dual tone audio in has been suspended. Carrier is about 55db under the signal peak.
And here an output signal with max. power at 3.5 and 7 MHz:
So, that’s all for today, thanks for watching and 73!
The receiver had to match a lot of requirements that should be described first:
Particularly on the lower bands and with effective long wire antennas the receiver front end will see high signal levels that it has to cope with. IMD always is a serious topic in this case.
Sensitivity particularly on the higher bands, where noise level is ow and signals are weak, is also an issue.
Dynamic range and extensive AGC gain compensation should be as high as possible.
This lead to a circuit that has proven its stability in lots of my radios:
Band filtering for each band with a double and loosely coupled LC circuits
Dual-Gate MOSFET (part of the AGC chain) as the first amplifier
Diode ring mixer (with Schottky diodes)
Post mixer amplifier with Dual-Gate MOSFET (part of the AGC chain)
SSB Filter (now 10.7 MHz) also used for transmitter (relay switched)
Main IF amplifier with MC1350 (part of the AGC chain)
Audio preamp with bipolar transistor
Audio final amp: (once again! 😉 ) LM386
Before describing the receiver itself we will have look at the band pass filter unit, that is shared between receiver and transmitter:
To minimize stray energy traveling from the input to the output of the filter, two SMD relays have been used on each side of the filter per band. And to reduce feedback fromt the transmitter (when the BPF is used to filter the TX signal after the TX mixer) the filter has been placed far away from the TX amplifier section.With an overwhelming result: The transmitter is nearly unconditionally stable now (compared to the TX section used in the “Give me 5”-Transceiver that had severe shortcoming in this aspect.
Control leads for the relays follow a designated coding scheme:
The receiver’s circuit
VFO signal is coupled into the DBM via a 10nF capacitor. The same is valid for the amplified RF signal from the output of the first amplifier stage using a Dual-Gate MOSFET (40676, BF900 or equ.).
Another Dual-Gate MOSFET is used as the post-mixer amplifier. All Dual-Gate MOSFETs so far are part of the AGC-Chain. This maximizes the possible gain swing to about 40 to 50 db. and enhances the receiver’s capability to handle even the strongest signal levels without distorting the output signal and the end of the audio chain.
Next is the SSB-Filter. Due to this is an “experimental” transceiver, the filter has not been soldered to the circuit board. Instead it is fixed with an aluminum clamp into two parts of header strips. Thus I can compare numerous SSB-Filters (9-, 10.695-, 10.7-MHz commercial ones, various home made ladder filters etc.). Here the different performance is very interesting to be explored.
The filter is accompanied by a special rf relay (manufacturer “Teledyne” with excellent performance concerning separation for the two channels) so that it can be used as the SSB filter for the transmitter section.
After the filter section the IF amplifier follows. This one uses an MC1350 video amp (old but good and still available, even in SMD!) and this IC also is controlled by AGC. The input is unbalanced (PIN6 to GND) the output is balanced and terminated with a tuned circuit.
Demodulator is an SA602 mixer IC.
After that the signal is handed over to the audio chain. But before the signal is processed in the next stage the frequency range is limited by a low-pass filter to reduce hiss. This filter also has two switched capacitors (controlled by MCU via NPN-driver stages) to adapt the sound to the preferred settings of the user. The software contains a respective function.
The audio amplifier consists of two sections: A preamp with a bipolar transistor and the inevitable and well-know LM386.
The full circuit on a 6×8 cm veroboard:
Starting from left top corner there is a 1:4 input transformer (not in the schematic), the preamp, the DBM, post mixer amp, SSB filter, relay, MC1350 as IF amp, demodulator and 2 stages of audio amp.
Performance is excellent. The circuit has no problem with high signal levels (in-band and out-of-band) especially on 40 meters. No IMD problems are noticeable even when used with high gain antennas like a 2×25 meter doublet with a tuner. On the higher bands noise figure is pretty OK what I think is based on the usage of Dual-Gate MOSFETs in 2 of the 3 amplifier stages. The MC1350 deteriorates this to a certain degree but is still very much acceptable for a shortwave radio.
This short article will describe the adapter board that is connected to analog data sources and that is converting the respective voltage data into suitable voltage levels for the ADC inputs PA0:PA4 at the microcontroller:
The following data will be converted and later shown on the display:
User keys (Key1:Key3)
TX power measurement
PA temperature (Sensor: KTY81-210 switched against GND)
AGC output (DC) from receiver => S-Meter
This article covers the remaining digital (or “analog to digital”) stuff, next on the agenda will be the receiver.