{"id":269,"date":"2015-07-30T15:59:08","date_gmt":"2015-07-30T13:59:08","guid":{"rendered":"https:\/\/pa0rwe.wordpress.com\/?page_id=269"},"modified":"2020-10-16T15:45:08","modified_gmt":"2020-10-16T13:45:08","slug":"dc-transceiver","status":"publish","type":"page","link":"https:\/\/pa0rwe.nl\/?page_id=269","title":{"rendered":"DC HF Transceiver"},"content":{"rendered":"

\"Front<\/a><\/p>\n

http:\/\/www.saunalahti.fi\/hohtola\/ham\/DC-TRCVR\/DCradio.htm<\/a><\/i>
\nhttp:\/\/midnightdesignsolutions.com\/dds60\/ <\/a><\/i>
\nhttps:\/\/www.aa0zz.com\/<\/a><\/i>
\nhttp:\/\/wb6dhw.com\/For_Sale.html#Si570 <\/a>
\n<\/u><\/i><\/p>\n

\n

Modifications to the DC-TRx (August 2010)<\/span><\/u>
\n<\/b>Some disadvantages of my DC-TRx where:
\n–\u00a0 above the 14 MHz band the VFO signal was not stable and clean
\n–\u00a0 updating the firmware was not easy to do
\n–\u00a0 the use of an ‘outboard’ S-meter<\/p>\n

The bad VFO signal was caused by the DDS-60 and the divider train (74AC86\/74AC74) behind it. I decided to replace the DDS-60 by the Si570 which has already a square wave output signal available. Also the 74AC86 and 74AC74 where placed near the Si570 and no longer in dil sockets.<\/p>\n

I had to replace the original PIC 16F628 by the 18F2550 to make USB available and for controlling (by I2C) the Si570 as well. Now updating the firmware by USB is a new feature. On the picture below you see at the top the Si570 on a WB6DHW pcb and the 74AC86\/74AC74 (on their back) just below it.<\/p>\n

\"VFO<\/a>

VFO modification with Si570<\/p><\/div>\n

After implementation of above modifications the results where very good. Much better VFO signal above 14 MHz and also an almost perfect HF output signal. The second harmonic suppression was on all bands more than 50 dBm. Look at the (updated) pictures at the end of this page. The S-meter information is now on the display and shows (after pressing the Band button)\u00a0 for about 10 seconds a graphic s-meter. See picture below showing S9 signal strength.<\/p>\n

\"Digital<\/a>

Digital S-meter modification<\/p><\/div>\n

How it all started in 2005….<\/span><\/u>
\n<\/b>After several experiments with DC receivers (e.g. the QRP2001<\/i>), I found a very interesting DC-transceiver design of Matti, OH7SV and Juha, OH2NLT on the Internet.\u00a0\u00a0 <\/span>This design addressed me directly and I decided to reconstruct it.<\/p>\n

The Transceiver
\n<\/b><\/u>The transceiver is based on the Tayloe detector concept. In the receiver, after the Tayloe detector, 4 op-amps amplify the LF signal, followed by a Polyphase network and a 300-2400 Hz Band Pass Filter. Below you see the block-diagram of the transceiver (with thanks to OH7SV and OH2NLT).<\/p>\n

\"Block<\/a>

Block schematic<\/p><\/div>\n

In the design only ultra low noise op-amps (LT1028 and LT1113) are used to get an overall high, noise free dynamic range of 124 dB! See and read the articles on the site of OH7SV and OH2NLT.<\/p>\n

The signal in the transmitter is going the opposite direction, from the microphone into the BPFilter and the Polyphase network to the Tayloe detector.<\/p>\n

About the Poly phase network you can read a very nice article from Pim, PA2PIM.\u00a0 Take a look at Pim’s site:\u00a0 https:\/\/www.antennoloog.nl\/polyphase-networks\/<\/a><\/em><\/span>
\nI also did some measurements to the network; I connected the two 90\u02da phase outputs to the X- and Y input of my scope, and see the result:<\/p>\n

\"Poly-phase<\/a>

Polyphase network output<\/p><\/div>\n

The modules
\n<\/b><\/u>Just as OH7SV and OH2NLT did, I built a motherboard with separate modules for the Band Pass Filters and the Polyphase network. That is very practical for doing measurements on a particular module or to change it by another design. On the picture below you can see the modules below the mother print. The green module is the modified DDS60 VFO. In the other picture you can see it all build together in the enclosure. The linear amplifier (5 Watt) is described in a separate chapter.<\/p>\n

\"Modules<\/a>

Modules and main circuit board<\/p><\/div>\n

 <\/p>\n

\"Insight<\/a>

Insight view<\/p><\/div>\n

VFO
\n<\/b><\/u>The VFO that I use in the transceiver is the DDS-60 kit, available from the American QRP Club. This VFO is built on a very small pcb (5,5 x 2,5 cm) with smd components and covers dc-60 MHz. Control is possible by PC or PIC. To get a nice square wave output directly into the 74AC86 and 74AC74 ports \/ flip-flop to obtain the I and Q signals for the detector, I modified the DDS60 as follows:<\/p>\n