40m DC RX Continued

So i finished of wiring the receiver up and putting power and an antenna on it and having a tune about. Well it works, I was receiving the Manly Warringah radio club doing an open night for the cubs, where the cubs were sending their names back and forth using the phonetic alphabet. The kids were having fun and enjoying the radio. Also, a number of Japan and Indonesian stations were copied as well as some VK3 and 5 stations on CW. All in all, I am happy with the radio other than the finicky tuning system.


This is how i had the receiver setup on the bench for its initial test.


Did not plan the internal layout all that smartly, but it did fit in there eventually.


And it looks all pretty with its nobs on.

40m DC Rx


40m DC Rx

So today we start on the next project in the series, a 40m direct conversion receiver to mate up with the 40m cw transmitter i built previously. I will update this post as I make progress.


What we are starting with.


Diodes in.


Electrolytic capacitors in, kind of an upside down way of kit building, but i am looking to add in some landmarks because of the lack of silk screen on this kit.


Without a silk screen it is a good idea to lay the components out on the board layout I think. HIHIimg_20161010_165811

Capacitors all in.


There was some resistance, but i overcome it in the end.


Most things now on the board. Next job is to wind the toriods and add wires where the power and speaker sockets go.


40m DC Receiver


Among the many projects that i have neglected lately, the direct conversion receiver has probably seen the least amount of love. So feeling energetic today I gave the board layout a little love, moved somethings about and added in a few others. The main change has been the addition of some Rx Tx switching. I have a single pole single throw relay in my box of tricks, i do not know where it came from, but i thought i would use it to switch the incoming signal on and off. Not really eloquent, but it will do the job and I wont have to round RF from the back panel to the front. Other than that, reverse polarity protection diode in the 12v input like and a little tweaking of the band pass filter. After i give this all the twice over, it will be export gerbers, fire up the CNC and route a new board for this and get it boxed up.


40m Band pass Filter

So i came across this schematic on Ebay for a 40m dc receiver kit, the front end looked nice so i copied it for my own project.


I am assuming that the band pass is transforming impedance also 50 ohm in and 1500 ohm out into the 612 mixer. I built mine for 50 in and out, by replacing the 270 and 18pf caps with 220 and 56pf.


Using my scope and signal gen i tuned the filter for max amplitude on 7.100mhz, and then did a sweep of the filter from 5 to 9 mhz and plotted its response on the graph above. Looked better than I expected.


Here is the filter as built on the board. Followed by a mess of failures where I was trying to add in a preamp that just kept loading up and not doing anything. Such is life.


Direct Conversion Receiver with DDS VFO

So not to be outdone with just one project on the go, I also get it in my head to rebuild the 2-IC DC Receiver from page 1.8 of EMRFD. I have built this simple receiver before, it was one of my first projects ever and it worked first time out of the box, well kind of.

My original was built Manhattan style, but at the time, being rather newbie, i was not able to make the LC oscillator circuit with sufficient Q to make the thing work. It was yanked out and initially replaced with an xtal, making it rock bound and of limited use, then the xtal was removed and replaced with an Arduino DDS VFO and my love of DDS was born LOL.

My first project that gave excellent results, I was receiving stations from all across Australia, New Zealand, Japan and the USA. It was thrilling. I was now hooked on homebrew.


Above is the schematic of the circuit, i have changed out the local oscillator for a DDS VFO, and i have made a couple of minor changed to the audio amp, mainly double the gain it will now put out maximum gain, the only issue no gain control on the audio amp.



Board was laid out in Sprint Layout, with extra holes drilled for mounting the DDS board onto it.


Its a nice simple project, wont take long to solder it up and get it on the air and tuning the 40m band.



So it is all together and it works ok. I did have to change a few things, mainly making the audio single ended rather than differential, this gave much more audio at the speaker. Also the tank, while it functions, does not seem to do all that much, tuning it by the trimmer, yields no change what so ever. Its something i will need to look at.

While its working, it could do with some improvements, especially to the front end. Here is a video showing it working.


CW Filter Board

So the plan was to add some kind of filtering to the DC Receiver to give some selectivity. Easiest way to achieve that is to use audio filters, of which there are plenty of types that can be used, passive, scaff, active, etc. I settled initially on an fixed width active opamp filter. Further down the track i will change that to scaf capacitance filtering but, i wanted to have this down pat first..


The filter parameters are shown above. 8th order Chebyshev, 500hz wide, 600hz centre and -40db down at 2k. A fairly aggressive filter which should take out a lot of the adjacent signals and noise.


The circuit was optimised for low noise, and one of the nice features of doing that makes all your cap values 100nf and a bulk of 5% 100nf is cheap as chips off ebay, leaving only many odd values of resistors to locate.


Circuit is laid out with R13/14 making a virtual ground for the OP Amp negative voltage rail, a jumper runs from V- to V- to supply the VCC/2 to the negative voltage rail.


Gerbers were created and converted to G-Code and the circuit mirrored and routed, drilled, cut out and given a coat of board lacquer ready for populating with parts. Board came up rather nice. .


40m Direct Conversion Receiver


The above picture is a block diagram of the receiver section of the 40m direct conversion transceiver i have been building for some time. There is nothing ground breaking in the design, it is fairly typical fair for a direct conversion receiver, taking elements from many different designs and bringing them all together into, hopefully a cohesive whole.

This is all part of the “5 x 7” CW Transceiver project i started to outline here: “5 x 7 Tranceiver”

A few things have changed with this, especially so in the front end where the original circuit as been totally replaced with something a lot more simple.

Band Pass Filter: The band pass filter is a 7th order Chebyshev 50ohm in and out with 0.1db ripple.

Pre Amp / Mixer:  The pre amp and mixer both use Mini Circuit devices, MAR-6 MMIC for the pre amp and either TUF-1 or ADE-1 for the diode ring mixer.

Local Oscillator:  The LO is an Arduino microprocessor controlled AD9850 DDS VFO.

Buffer Amp:  Is a simple common emitter, common collector, gain buffer stage to take the LO amplitude to 10dbm.

Audio Diplexer:  This diplexer, out of EMRFD starts to narrow up the bandpass and increase gain of the AF.

Active AF Filter:  An active 500hz wide CW filter, 8th order design using a Quad OP Amp.

Audio Amp:  Finally, the audio amp using an op amp pre amp and LM386 driver.

The only things left to actually build are the 3 audio stages and then test it all together and see if or how it well it works. Failure is always an option as they say.


DDS Buffer Amp

So, i got some focus back into my home brewing with the goal to complete the receiver. The receiver is direct conversion, the DDS VFO is built, but as it outputs 270mV RMS, it is not powerful enough to drive a ring mixer that needs 7dbm or 1V RMS input. So a buffer amp was needed.

The buffer is pretty standard, common emitter gain stage with a common collector buffer. The DDS output is 200ohm, so a 4:1 transformer was used to drop the impedance down on the input. C1 is needed to block DC flowing into the transformer, as the bias voltage is set by the feedback resistor R4 and the transformer was loading it down so there was no bias voltage to the base of Q1.

Initially the Q1 emitter was to ground, giving about 10x gain, i found that this put me in the 3V RMS / 20dbm power range, much too high for a 7 or 10dbm diode mixer, so R2 was added to reduce the gain to about 3x. This gave me a nice 700mV RMS into a 50ohm load, 9dbm, close enough for now.


When running at close to 10x gain the waveform was full of harmonics, dropping the gain back knocked those on the head and the waveform is now nice and clean.


Looking at the output of the spectrum analyzer shows that first and second harmonics are way down compared to the desired frequency. So the dropping of gain certainly cleaned up the output a lot.


And this is where we are at currently, DDS VFO, Arduino micro controller and buffer amp all assembled and confirmed working correctly. Next job is to add the mixer in, then start work on the front end, JFET LNA (Low Noise Amplifier) to follow the Low Pass Filter.


CW Audio Filter

When it comes to direct conversion receivers, to get good selectivity you need to use good audio filters. There is no IF so you cannot use narrow crystal or collins type mechanical filters to narrow up the bandpass and filter out all those close by signals. For CW use this is important, because you might have 20 different signals in the typical 3K bandpass of most receivers, and listening to those sorts of crouded conditions, sounds like machine gun fire.

So for my direct conversion CW rig, I had decided to experiment with a number of different filter types and find the one that i find works the best. The first one to roll off the production line is an active filter using OP Amps.



Analog devices have a really nice online Filter Wizard that takes all the drudgery and though out of designing filters. As you can see in the above image, the filter is designed with a 600hz centre frequency, this is where I prefer to listen to CW and find it a pleasant listening experience. The filter width is ultra narrow. being only 200hz wide, so 100hz either side of the centrer frequency i should hear a fair bit of attenuation. This is a contest type filter, for very crowded bands. I will also build a 400hz filter for more casual listening or band searching and a 2K SSB filter for SWL’ing.


The schematic above has been optimized for low noise, one of the neat outcomes of doing this is all the capacitors become 100nf. And it becomes so much easier to buy a bulk of 100nf 5% caps than have to make sure you have a number of different values on hand. All resistors are 1% types of which some values might need to be rounded off. I am not sure yet on weather rounding up or down makes a big difference, but this is something i will have to determine experimentally at some point.


Something i did learn is that OP Amps need to have both positive and negative supply voltages for them to work right. After showing my work to VK2DDS , who pointed out I had things all wrong with power, I was sent on my way to learn about OP Amps and virtual ground. Now there are a number of ways to do this correctly, supply the OP Amp with negative volts, or use a Virtual Ground. By using another general purpose OP Amp  configured as above feeding the negative supply line of the OP Amp with 1/2 the voltage of the positive supply line, you and up with the situation of the voltage being able to swing 12v <-> 6v <-> 0v. This is the same as 6v <-> 0v <-> -6v because the voltage of ground is relative, it can be whatever you want it to be.



So now the final piece of the puzzle is the design a board, etch it and test it out to see how the thing actually performs. For this I am using a TL074 Quad OP Amp. The design work was performed initially in KiCad, a Linux circuit board design program. It has a funky way of working and one that I am not all that fond of, but, it is also extremely powerful and is software i will persist with as it will be handy when i design something a little more complex than this filter.

Now, that’s not to say i did it all in KiCad, in the end i swapped out to a windows program called Sprint Layout. This is more a drop and join visual sort of method i am more comfortable with and for small jobs very easy for complete beginners to use. So the board layout is all done and all that is left now is to etch the board, I use toner transfer for this and then build and test the filter and see if it worked up to expectations and design parameters.