Thursday, May 5, 2016

EMRFD Direct conversion 40m receiver in a tea caddy

I woke up one day eager to build something simple (at least I thought is was simple) and opened up the first chapter in EMRFD and decided to build the 40m direct conversion receiver. I already had most of the components in my junk box. By the way, Experimental Method in RF Design (EMRFD) is the best book about homebrewing amateur radio gear. You should get it if you don`t have it.

I know you are sitting on the edge of your chair, eager to see the end result, so I will give it to you straight away.  Here it is: My 40m Direct Conversion Tea Caddy Receiver.

Then, lets rewind back to the build process. I started off with the schematics from EMRFD.

The first chapter of EMRFD is available online, so I guess I will not go to jail by showing the circuit diagram here. 
The receiver is based on a NE602 gilbert cell mixer and the famous and old LM386 audio amplifier. The rest of the components are a few capacitors, including three tunable, two T37-6 toroids, and some resistors. 

Prototype of the mixer circuit almost done.

Prototype of the receiver is now finished. In the first version I followed the circuit diagram from EMRFD 100% and used two 75pF air tuning caps (from Russia). The receiver was very difficult to tune with those caps, and I got some oscillations. It was a very fine Theremin, however.

The above video show the performance of the receiver. It is not very sensitive, but it is working. In this version I had some problems with the preselector filter. It seemed like all the RF went straight to ground. Trying to debug this problem did not help much, so in the above video, the antenna is connected straight to the NE602.

For the second version I used just one tuning capacitor of 80pF. It was even more difficult to tune, but I got rid of the oscillations. In addition, I went ahead and used a different preselector circuit from Sudden receiver.

On the final version, I used a 60pF polyvaricon with a reduction drive as the tuning cap and adjusted the oscillator circuit to enable the receiver to tune from 6.9-7.3MHz. It was build using Qrpme MePads in sort of Manhattan style. I soldered the component on two PCB boards which was soldered together to fit in a oriental Chinese tea caddy.

Finally, I added a switch and a 5mm LED. The receiver is powered by 6xAA batteries. It could probably run on anything between 6-9 volts. In the datasheet of the NE602, it says that the maximum supply voltage is 9V, so I am probably pushing it a bit. I used a protective diode and a series resistor for safety reasons and to keep the voltage down a bit.

I used an SMA-connector on the back.

The final receiver. Eager to sniff RF from the ether...

The schematics for the receiver (using Fieldnotes schematics software). The series diode is not in the schematics, but I put it in for good measures. I used somewhat different component values in the colpitts oscillator than those in EMRFD.

The verdict

It was a very fun build. It is a simple circuit on the paper, but it sure teaches you a lot regarding radio functionality. Alright, the receiver works, but it does not seem very sensitive. I should be noted, firstly: that I do not have any 40m capable receiver to compare with. Secondly, my antenna is not very good on 40m. Thirdly, I have no idea what I am doing.

That being said: CW pops in very nicely, while SSB is a bit difficult to tune in. The oscillator seem suprisingly stable given that it is a VFO rather than a VXO, and the fact that I used random capacitors and no fancy NP0-ones. At least it was stable enough to receive RTTY with fldigi during a brief experiment.

In a future version I would like to add some sort of audio filtering before the LM386. Some sort of audio gain control would also be nice, since it is a bit loud on my iPhone headphones on strong signals. 

Further reading: