More Receiver Work

So with the VFO and Audio board working i turn my attention back to working on the receiver. Now the first version of this was an unmitigated disaster. Clever me, always throwing caution to the wind used MMIC gain blocks for IF amps and had nothing but trouble with them oscillating and going bonkers. Well, this time I threw caution to the wind again and used high speed op-amps for the RF and IF amps. Everyone has done a 2n3904 IF amp and i could have done the same, but, lets try something different. And i could not be more pleased. Actually using op-amps might just become my thing LOL they are actually surprisingly easy to work with once you know what you are doing.

 

I have posted about this before somewhere on the blog, but this is the circuit I used after reading some app notes from one of the big manufactures. Non inverting, 50 ohms in and out impedance and thats about it really. Unity gain bandwidth of the op-amps i used is 380MHz and they are a 20 cent Chinese part.

Doing the initial alignment of VFO and BFO frequencies I had more probes up its cackler than an abductee at a aliens convention.

 

The mighty RF Explorer spec-an for the win getting the VFO frequency mixing with the RF input to be right smack center in the passband of the 500hz wide IF filter. Yeah its a CW rig this one.

VFO A is the IF frequency and VFO B is the BFO frequency.

And this is the 600hz tone out the AF port. Which will eventually get routed the audio amp that I actually short circuited while fixing it until it was broken. Actually, the AGC and the Audio filter circuits work just fine, its the AF amp i killed. Its just a matter of de-soldering the amp module and replacing it with one that works. Every day I am getting closer and closer to something that actually works good enough to use more than once.

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Radio Bebek RF PreAmp

Radio Bebek RF Preamp
Powered Up and Testing
100mV in and 500mV out

So the RF preamp has about 5x gain. On 40m its probably not really needed unless there is a lot of loss in the filters. Which in this case might actually be the case as the RF path from the antenna goes through the LPF, into the Relay and then into the BPF. So a little gain ahead of the mixer might be a good thing.

One thing that I left out for now were the back to back clipping diodes. I am only building this as a receiver pretty much to begin with and I am not sure clipping diodes are really necessary. Next I will be working on the LPF and BPF. Need to do some LTSpice here to make sure my values are correct.

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Radio Bebek LO Buffer Amp

Ok, back to doing some work on the Bebek. In this block the only real issues are that there is an extra DC blocking cap on the board that is now shown on the schematic and that the Cap values on the schematic are not the same shown on the board. In this case I have defaulted to using the values on the board. 10nf and 100nf on the board are 1nf and 10nf on the schematic. The former values just seem more logical to use and the latter are most obviously an error on the part of the designer.

Bebek LO Buffer Amp
250mV in and 2.5V out

So the LO Buffer Amp has a 10x gain. I injected it with a 250mV signal to get 2.5V out. Now obviously this is not good if you are using a SI5351A as it puts out 1.5V. But this should work quite well with an AD9850 which puts out 200mV. It still might be a little to much drive for a -7dB mixer, but it should be ball park upper limit.

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