Monday, December 21, 2009

HF radio (5)

Had some time to improve the radio a little bit and added a notch audio filter. Had to change the opamp since it is hard to find the RC4134 so made the lucky guess of replacing it by an TL074.

This notch is not bad considering I didn't matched the resistors nor the goes almost from 0 to 7Khz, will change that latter on, I only need to go to 3Khz.

After seeing the images on the computer I discovered the I have an audio bandwith of 7Khz.... as you can see in this photo (the notch is around 3.5Khz)
The signal you see at around 1.5Khz is from an transmission that was on 10.100Mhz and is were I make the air tests since it sits almos all day there and has a big signal.

what's is the cristal filter doing? The problem is not the cristal filter but noise induced by the MC1350 (I think). Probably will upgrade the schematic to look like the Elecraft K2 in wich they have a small cristall filter after the MC1350.... now I understand why they placed it there.

Here is an output of my TS-50 appliance...see the cut at about 2.5Khz?

On the radio, I placed a LSB/USB selector based on a diode switcher and changed the feedback capacitor on the oscilator because it was not starting all the time, the oscilator itself changed from a double 2n3904 to an 3904/06 pair similar to the minipig-10 transceiver bfo (the one at 8.064Mhz), just removed L20,L21 and C57. Didn't used for now the diode switching part after the vfo.
Here is how the board looks now, there is already in place the AGC IC support near the cristal filter... that will be made another day:

Top row from left to right: Notch filter, BFO oscillator, AGC IC support and bellow cristal filter, VFO buffer.
Bottom row from left to righ: Audio amp, audio pre-amp, BFO mixer (SBL-1), IF amp (MC1350), relay cristal filter switch, first mixer amp, first mixer (SBL-1).

Last minute update: I got one LTC5507 hopping to make a power meter that goes to Ghz....(I allready bough an AD8307 but waiting for it's arrival) problem is I had no idea how small the component is... anyhow will make the power meter if I can find a soldering needle.. the IC it self is the same size as the pen tip....the same size as an SMD resistor... and has 6 terminals!!!!

Season greetings....

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