On my radiotransmitter-blog I’ve postet an article about my latest project: A compact handheld QRP transceiver for SSB with self-containing battery pack for portable use:
In my recent project, the mini QRP SSB transceiver for 20 Meters (14 MHz) I encountered another unexpected problem: accurate sideband switching with the NE 612 as SSB generator can be difficult. Very difficult.
First my circuit looked like this as pointed out in the schematic:
Two crystals are used, one slightly higher than the passband of your ssb filter, one about 1.5 kcs lower.
When switching I found that the oscillator was not operating correctly and the frequency after the switch was put to the other position was some kcs off the crystal frequency. That time the switch was mounted in the front panel and the leads to the circuit were made of flexible cable with a length of about 8 cm. I speculated that this might come from unwanted capacities or inductances caused by the leads.
To avoid these unwanted influence I experimented with diode and transistor switches. No improvement could be found. Then I mounted a small relay instead the semiconductor switches close to the NE612. Again, no improvement. When analyzing the circuit I found out, that in case I remove the second crystal the remaining one operated perfectly. Oha!
So, I deduced that the internal oscillator was oscillating with two crystals in parallel. A lot of resultless experiments to have both crystals in the circuit but minimizing the influences followed. No cure was in sight.
Later I finally made the decision to use only one crystal. Due to the fact that I wanted to be able to operate on both sidesbands (don’t ask for a reason why! ;-)) I made up my mind to use only one crystal and to pull it to the desired frequency. So it ended up simply with one 9-MHz-crystal, a variable capacitor, a coil and the sideband switch:
Needless to say that this worked from the scratch. Coil data is simple: 12 turns on a TOKO coil carrier adjustable with ferrite core. The switch is again put into the front panel, no unwanted influences are observable.
Bye for now and 73!
Peter Rachow (DK7IH)
Here are some pictures of my recent project, a very small SSB QRP transceiver (schematics can be found here) that covers the 20 meter band. First let’s have a look to the front panel:
And here we go for an inside view:
Starting on the left you can see the AD 9835 DDS chip on the very compact breakout board by artekit.
The breakout board is mounted on a 2 x 8 pin socket row to have the possibility to change the IC ver quickly in case of damage (I set all ICs into sockets without any exception). Above the DDS there is the ATMega 8 micro. Right from the rf-shield made of brass you can see the 9-MHz local oscillator with the ssb modulator and product detector IC NE 612.
For more information on board layout and part distribution have a cursory glance on this commented picture:
By the way: Had some nice contacts today with stations from all over Europe with 5 watts out. The rig is really fun! 🙂
73 de Peter (DK7IH)
After having finished my last ssb transceiver project I was to a certain degree dissatisfied with the mechanical size of the rig. Even though it performed very well and I did some nice DX with it (including cracking a pile up for a Jordanian station) I wanted to find out, if I could shrink the mechanical dimensions of the device to another significant degree. A pocket-size transceiver with somewhat around 5 watts PEP output was the goal that I wanted to achieve.
First I searched the web for attractive circuitry. I quickly came across the „Kajman Transceiver“ made by SQ7JHM, OM Jurka (Link to schematic). At the first glance the circuit seemed extraordinary simple. But the problem was: The device has been designed for operation on the 80-meter-band. So some extensions and improvements had to be made to make it perform well on 20.
First, the original recveiver seemed to be a problem. The layout is very simple. Above all, there is no rf preamp. This might be OK for the high noise levels on 80 and the relatively high field strength that signals have got there. But for 14 MHz this performed, as I had expected, very poorly as I could easily find out when breadboarding the receiver strip and having it run with a simple VFO. And, in addition, the receiver did not lack only sensitivity, the noise figure was also very poor.
So there were two possibilities to cure this problem: add an rf preamp to the receivers’s front end (to improve noise figure) and maybe add an if amp stage between the two NE612-mixers to improve overall gain.
After having put a dual-gate mosfet rf preamp stage to the receiver it suddenly opened up its ears. Performence now was great compared to that before the modification had been made. With a simple dipole I could listen to VKs ans ZLs in the morning and to state side (even West Coast) in the evening. Being very satisfied with the outcome of this simple measure and strictly sticking to simplicity of cicuitry and limited space I went away from adding an if amplifier. Maybe I’ll give it a try later.
The transmitter had also been modified after a short intensive thinking. I re-constructed my proven 3 stage design with single-ended preamp and driver and two 2SC1306s in push-pull mode as the final. That should deliver 5 watts easily. And, not to drive your tension too high, it finally did.
As Collpits, Hartley and Co. VFO-times are over, I took another AD9835 DDS chip out of my shelf, grabbed into the microcontroller box and an AT Mega 8 microcontroller found its way into the circuit. To have the TTSOP-16-package of the AD9835 on a veroboard I bought a very compact breakout-board for the AD 9835 made by artekit in Italy. Very neat, very compact, very nice. And cheap.
So, enough the talking, here’s the hard stuff, folks! The circuit (not yet finished, but OK as a preview):
The crucial thing is the arrangement of the 2 NE612-mixers involving 2 relays close to them and the 9-MHz-SSB-filter. The relays reverse the signal way between receiving and transmitting. For this purpose I used small print relays made by Finder company.
That’s all so far, the next days I will put in some photos of this neat little 20-Meter-Band-SSB-Transceiver to give more details to the interested reader.
73 the Peter (DK7IH)
Today it’s time to give a presentation about the receiver’s front end. When testing this one with an interfrequency of 9785 kHz I suddenly encountered that I had a strange demodulation phenomenon due to a very strong broadcasting station inside the if channel. After some days of listening and trying to decipher the words in English language I recognized that the „jammer“ was Radio Romania International transmitting on 9783 kHz. OK, folks, right beside the if-channel. Front end selectivity was the problem.
After conceiving about this problem, the conclusion that I drew was that with a receiver front end of only 2 tuned circuits the problem could not be solved to satisfy my demands on receiver purity. But a circuit of 3 LCs should do. So this solved the problem finally. So, here is the circuit of front end, rf preamp and first mixer:
The circuit on the transceiver’s veroboard:
C u soon! 73! De Peter (DK7IH)
Today I’d like to present my new homemade 14-MHZ-SSB-Transceiver to my fellow radio amateurs. Some basic information should be given first before details and circuits are to be discussed:
Frequency range: 14,000 kHz to 14,350 kHz
Transmit power: 15 to 20 Watts max.
Mode: SSB only.
The main units of this transceiver are:
- SSB-Generator: LM358 operational amplifier as microfone amplifier and MC1496 Gilbert-cell mixer for signal production.
- TX-Mixer: MC1496
- Transmitter consisting of 4 stages: 1 preamplifier, 1 predriver, 1 driver, 1 final (push-pull), all equipped with bipolar transistors
- Receiver: single conversion, interfrequency 9.785 MHz, 1 rf preamplifier (dual gate MOSFET) followd by mixer (dual gate MOSFET), SSB-filter, if-pre and if-main-amplifier (MC1350), product detector (CA3028A), audio preamp, audio final amp (LM 386)
- Frequency generation: DDS with ATMega32 driving an AD 9835
- S-Meter and AGC-circuitry: ATMega8 driving led-chain for S and RF presentation an r2r-network with op for generating agc voltage.
To be continued…
(C) 2014 by Peter Rachow, DK7IH