Friday, November 29, 2013

14.318 Mhz CW Direct Conversion Receiver

This receiver is made for the testing the CW xtal transmitter (14.318 Mhz )as shown in othe previous posts.
Ckt is similar to the 4.000 Mhz, 75meter rx. but, this time i have put the much needed front end gain block the RF pre-amp taken from PA2OHH.

First main goal of this receiver was to be extremely portable, small antenna, powered  by 9v pp3 dry batery, less tuning. With the diode dbm  it was not possible to meet all these criteria, to find an alternative went for this design. It lakes the audio filter, but for occasional and portable operations it is tolerable.

IC ta7358ap is used as front end. It saved me lots of trouble of making seperate  vfo, buffer and product detector. After all I am using the single frequency.

One stage of audio pre-amplifier (bc547 or bc108) and standard LM386 is used to drive the speaker. It gives loud signal. You can use headphone also.
LM7805 is used, instead of zener diode. It looks little overkill, but  gives good voltage regulation and once installed i dont have to worry about it.

Rf preamp is taken from PA2OHH website, i chose this to find out alterntive of the  cascode rf pre-amp of rm96 .... its performance and end result will take some time, please wait.....

whole ckt and details will follow. de vu3inj

NB as of 7 Dec 2013  Receiver Front End is okey and working. It needs to be peaked/tweaked for max signal, i will run a small vfo on portable battery , take it to some distance(3km) and see how well (DCR 14318 ) resolves the microvolt level signals.

11 Dec 2013  I was  queried by a friend regarding the design. It has been not declared finished as some components values and parts are being changed to get more performance and improvements please wait.....

17 Dec 2013
The friend once again ringed me up. The gist of our convesation......

At low freq may  be a single tuned ckt will work but higher we go, we needs to put more tuned ckt in the front end as the DC receives are very wide band and there is no selectivity except the filtes. gain of receiver is good but there is AM radio signal coming into the dc receiver. which is the problem of every high gain receiver poorly designed and poorly constructed. ..... if you make the input band pass filter with the ferrite slug cores it will be easy to tune. use 30 swg, 30 turns with 33 pf on alladin former (or use a piece of sketch pen with siug ). Couple the top of these tuned ckt with 2.2 pf capacitors.

Please don't reach to the conclusion that ckt does not perform as i have not finished it fully. it needs few tweaking / experimentatations. as front end is not giving performance that i wanted.

I am using this rx to monitor my tx signal. i am also making another similar ckt for second 20 meter tx within this week my result for this rx will be told and uploaded. i have to upload the write-ups for it. dont worry i will not let u down.

I made a two three changes in the ckt to reduce motor boating, hum etc but still there is  AM breakthrough when coupled with big dipole. antenna With short 1 meter antenna rx works very good for monitoring my own local cw signals ! Actually this was the purpose it was designed for...... It never was a stand alone complete receiver. 
At  this time it stays as it is and will further improvise with some good front end filter and tell the result later on as now working on the transmitter projects .....

I am still working on this thing. If you find this interesting and tempted to build please look also

(I have opened the box for some improvements..... Jan2015)