CW DC TRX Type 002
Direct Conversion Transceiver for Amateur Radio.
DL6ZB, Rolf Heine


1. Receiver

DC Receiver Type 002
Picture of the DC receiver showing the diode ring mixer, diplexer, AF filter (0,5-1,5 kHz) and AF amplifier.

This is my Type 002 DC receiver of the 002 transceiver project. This receiver uses a L/C Tschebycheff filter optimized for CW. The muting of this DC receiver design is full-bk suitable.

Someone would have choosen a 4066 cmos switch mixer, although I prefer those diode ring mixers because of its simple broadband design. Just connect the VFO with a +7 dBm output power to the ring mixer. A receiver like that one certainly needs a bandpass input filter or pre-selector. A preamplifier helps isolate the VFO signal from the antenna, although it is attenuated at least 35 dB by the diode ring mixer.

The L/C Tschebycheff bandpass filter operates from 0.5-1.5 kHz with a flat insertion loss. It has 50 ohms input/output impedances. This design allows the use of readily available 10mH inductors. After various tests I found out that a -3dB AF bandwidth of 1 kHz is ok for most circumstances on 40m. Maybe I'll add an active narrow bandwidth filter behind the last AF stage in the next step of design, although I'm satisfied with the actual design.

The noise figure of the first AF stage is about 2.3 dB which is sufficient to detect even weak signals. A 2N4124 would offer some lower noise figure, although in my opinion it isn't really necessary at lower HF bands. This receivers "hears" really everything! Nothing more necessary. The "capacitor-multiplier" T6 helps cleaning the supply current for the first AF stage. The capacitor C15 (22µF) is multiplied by the factor of current amplification of transistor T4. This easy trick of decoupling the power supply from the first AF stage, which was used first by W7EL, is very helpful to prevent humming.



Circuit Diagram Type 002

10 mH: RS Components No. 675-5333
33 mH: RS Components No. 715-7276

Toroids for diode ring mixer: trifilar 3x50µH wound on Amidon FT37-77,  50µH=8 wdg.


2. QRP Transmitter for 7 MHz

Mounted into transceiver


The QRP transmitter offers approx. 1 watt RF output. Alignment is as simple as the circuit is. There are two points of adjustment:

a) Adjust C2 for maximum output power.
b) Adjust R1 for about 1 watt output.

What can you expect from this transmitter? In short: distances to a maximum of about 500 km during daylight, 1500 km during night when operated at wire antennas. Signal reports are 539 to 579. Do not expect DX, although it could be possible with good antennas and at proper condx.

CW QRP TRANSMITTER





Transceiver performance:

Simply one word: Amazing!

It's hard to believe that such a simple receiver design clearly detect signals from all over the world while the transmitter catched applause from most qso partners.

AM breakthrough can be prevented with a simple bandpass filter or even a pre-selector (see my pre-selector page) between the antenna and the receiver RF input. When this receiver is used without a pre-selector or bandpass-filter AM breakthrough might be observed. This is because all possible signals reach the antenna input which is a very extreme condition for all receivers - even the best!

A word about humming: the builder must take care of humming with DC receivers. Proper shielding of the VFO in a metal housing and good grounding techniques are essential. Avoid the use of mains power supplies whenever possible. Batteries are definitly the better choice. I use a sealed 12V lead-acid accumulator with 4.5Ah capacity which I bought from a local dealer (Pollin Electronic, www.pollin.de). It is enough for a couple of evenings operation.

If low impedance headphones are used humming may be observed due to higher currents in the unavoidable ground loops. I usually use ancient high impedance headphones from Sennheiser. After I tested one of these readily available low impedance headphones against the high impedance Sennheiser model, I observed very little humming as expected when the antenna was disonnected. After reconnecting the antenna no humming was observed because the shortwave noise simply covered the hum.

Even small signals are always clear with the typical HiFi-sound of those tiny DC receivers.




Below: pictures of my homebrewed 40m full-bk DC transceiver.
RF output: 1 Watt.

It's a prototype version in a modest housing made of copper-clad boards. The transceiver will be transferred into a new housing after an "evaluation period".
 
Latest improvements:

Click on picture for better resolution.

FrontOpen

The described transceiver was inspired by W7EL's "Optimized QRP Transceiver' which I had built before, although I made significant changes in the frontend and 50 ohms post-mixer amplifiers / filters.


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