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.
2 thoughts on “More Receiver Work”
Nice work, Rob and HAPPY EASTER to you too.
I’d like to play around with some opamps and could use some advise of what IC model numbers I should by. I did some googling around but nothing stood out. TI had a lot of info, etc. I usually can find things pretty fast on the internet but not this time. Thanks,
Jack in Fort Collins, CO W6VMG
Hi Jack, for using an op-amp at HF frequency you want to have a unity gain bandwidth of Freq * Gain, which for me was 30MHz * 10x gain == 300MHz. If you go on mouser or digikey you can search for an opamp with sufficient bandwidth. There are what are called current controlled op-amps that keep higher bandwidth at higher gains, but these are also more expensive than the more traditional voltage controlled op-amps.
The actual op-amp i used was this one: https://lcsc.com/product-detail/High-speed-WideBandOpAmps_Gainsil-GS8092-SR_C157725.html