Cobra 148- Much Higher Performance Receive Setup Than Shottky Diode Receive Mod

[cbphreaker]

@loosecannon

You don't sound like a novice!!! Yes, that is exactly what I was trying to say.

Thanks for taking the time to write that out.
Best
CBphreaker

 

[loosecannon]

well CBP, when you are self taught you end up with a lot of holes in your knowledge.

im well versed in some things, but completely oblivious in others.

the one thing i have is tenacity, and a willingness to read things that i cant understand, then re-read them until i start to get it.
do that long enough and you start to pick up on things.

i will be fixing my detector tonight, so i will post back in a couple of days about the results.
LC

 

[711]

I got side tracked and I'm still waiting on my 082 to come in. Most opamps are design to work with dual polarity power supplies. If you look at most data sheets, they have a minimum voltage needed for proper operation of the opamp. With that in mind, we can also trick the opamp into thinking we're using a dual polarity supply by using a single supply and creating a virtual ground by halving the supplies voltage through a voltage divider on the Vref input. Normally you'll see two 10k's or higher in series to ground. There's a bit more to the virtual ground topic but that's where Google steps in. Another thing to add is the feedback resistors helps to set the gain of the opamp. The bigger the feedback in contrast to the input resistor , the more the gain will be. In our detector circuit, one thing to pay attention to is what input you are using. The inverting input will flip the signal 180 out. So by connecting the output to the input with no resistor, we have no gain or unity gain but a way of isolating each stage per se(buffer) we can use the other stage to increase or flip our signal before the next audio stage. I'm planning on using an LM386 as an audio driver. It's what a lot of DIY headphone guys are using as an small speaker driver. I'm using a circuit with a little bass boost.

 

[cbphreaker]

@711

You've proved a nice write up as well. Thanks for taking the time on this post.

One may want to consider reading the manufactures data sheet when selecting opamps as some may become unstable in unity gain.

The input impedance of the op amp's inverting pin is quite low and relies primarily on it's input circuit to provide the proper matching impedance to the preceding circuit. Sometimes it's no big deal and sometimes it's a total burn, it depends on your application.

 

[loosecannon]

well ive been using my new detector circuit for a few days now, and i do like it.

I will say that there is no way to use the ANL feature when this is installed, as it cuts the audio way down.

my radio does seem more sensitive to weak signals, and i am now able to hear weak voices when before they would just be blips on the meter. or at least it seems that way.

the only caution i would give to others building this circuit, is that if you have gotten used to how your radio sounds with the ANL on, it will be hard to get used to listening to the static again.

im starting to get used to it, but when no stations are present, i must say that i liked the way the "empty" channel sounded with the ANL on.

im going to keep it in the radio for a while to get a real read on its performance vs. the stock detector.

thanks for everything CBP!
LC

 

[cbphreaker]

Great news!!

The voice you hear aren't in your head. I thought the same thing too!! But it's the extra .2 -.3 volts that the feedback returns as signal.

I go back and forth between the PDD and PLL det. and for dx and hearing, it the PDD as a clear winner.

I have a little PCB with the active ANL circuit, I just need to install it and verify it's operation and I will do that tomorrow so I'll get that to you within a couple days and post it on the thread. I want to fit the PCB in the original holes where the stock ANL was.

I abhor the CB type ANL and strive to bring justice to widebanded/heavy modulating stations via this circuit. I hope (fairly sure) adjustable ANL will give a good balance to noise vs what I want to hear!

Congrats on the successful build!

CBphreaker

 

[StrangeBrew]

.... it will be hard to get used to listening to the static again. ....

If your radio has an RF gain knob try turning it all the way down, then turn your volume all the way up until you hear the noise floor of your receiver and back off until it goes quiet again. Now bring your RF gain up slowly and use it as your volume control.

Not sure if this will do what you want but it's something to try, just play around a little.

 

[loosecannon]

Strangebrew, it's an interesting idea, and i can see it's merits, but alas i just can't run my radio that way.
I have this thing where i absolutely have to be getting all the receiver sensitivity i can possibly get at all times, and because of this, my RF gain stays turned full up all the time.
it's a sickness, but i don't want to be cured. LOL
LC

 

[Ace_King_21]

i built this per cb phreakers direction and it works great. the mod for the anl is around if you like shorting all your signal to ground.

Of course there is a bit more noise because there is more receive - an easy fix is just to turn the volume down and that works great too.

 

[loosecannon]

congratulations Ace king.

yes, using the ANL in stock form with this mod shunts too much signal.

try paralleling a couple of 1n4148 diodes with the ANL diode and see how that works for you.
you can try anywhere from one to about four diodes in parallel to achieve the level you want.
LC

 

[Ace_King_21]

Just put in the diodes, works great

I used 2 diodes and that was just perfect.

Thanks very much for the tip

 

[711]

Finally moved and getting back to this project. Built the board in Eagle, waiting for it to be panelized. Hope I set up the second half of the opamp for audio correctly. The precision rectifier portion is already biased but for the second half I'm not certain if I need to bias the the Vref input due to being a single supply setup. We'll have to see, will update when I get more into it. I was going to use an 084 and use the other 2 opamps for gain just in case this wasn't enough to drive the speaker but got lazy and will see where this one lands at. Like CBphreaker, i want to avoid going anywhere near the restrictive stock audio amp. Did the board late last night and didn't align all the lettering perfectly. Guess its the test batch, far from perfect.

 

[loosecannon]

great stuff 711.
any chance you'd be willing to add a couple more boards to your run?
I would pay you for them of course.
just PM me to let me know.

as for the Vref thing, here is a quote from CBphreaker to me, and i believe he is talking about your question:

Short pins 6 and 7 - Build a "Vref voltage" which is 1/2 Vcc and apply that voltage to pin 5. Don't for get to by pass pin 5 to ground with a .1 poly cap.

Vref is nothing more than a voltage divider using the same values connected to Vcc- so like 10k and 10k or 47k and 47k. The voltage at junction of the two resistors is fed into pin 5 and by passed to ground via a cap.

if you are referring to terminating the second half of the op amp, this is what i did and it works well.
I used 10K resistors for the voltage divider.
LC

 

[rabbiporkchop]

When it comes to Cobra 148/2000GTL receive mods there seems to be only a few. The most notable able is the (1) 1st rf amp and (2)dioide swap and finally the (3)"Goldfinger" cap mod.

I purchased all the parts to do the 3 mods. As I stared at the parts I thought, "Wow, lots of parts...I hope I don't mess it up and have to put the covers back on and say I'm sorry..." At any rate, I proceeded to start replacing and after a short amount of time it was complete.

The performance was dismal at best. Kinda like a jedi mind trick..."Your radio receives better.." , not quite.

Almost the exact same feeling of putting louder exhaust pipes on a car makes it feel faster. I really did feel as if I had received a "Goldfinger", and not the radio.

Anyway, I sough out a "more better" approach and here is what I have built, installed and now currently run:

1. Crystal IF Filter (7.8Mhz)
2. Low Distortion/High Dynamic Range Detector

Link to Schematic of the Detector:
http://www.amwindow.org/tech/htm/lowdisdet.htm


The audio that pours from that radio is FANTASTIC. The IF filter really cuts the noise down and puts a leash on bleedover, it also adds a scoshi gain right in the 7.8mhz bandpass. Nice...

The new detector is really nice. When listening the high dynamic range is very apparent as the sound is full, warm and more natural sounding. This setup can demodulate up to 200% modulation and it really shows.

I recommend this setup up as a much higher performance alternative to the RX mods out there. The only "mod" I did was taking the output of the detector and running the audio through a second opamp, boosting to line-level then out to a much better audio amp and speakers. I don't want my precious recovered audio molested by half rate coupling/bandwidth limiting circuits/caps and the worst high-level audio amp (TA7222 ..blah..) Cobra could have used, ever.

I've also included an audio clip of the detector in action. My personal taste is he needs a bit more highs but over all I like it.



View attachment 17668 View attachment 17669

How many microvolts of signal from the generator did it take to produce an audible tone after the modification?

 

[711]

LC, I need to verify its working . Once it is I'll let you know for sure. It's easier for me to build a board then to perf board it . I was going to use an TL084 for future expansion but will see how this one goes first. I rushed to get the order in but will revise it once I optest it. the issue I had was if the audio portion of the opamp required the virtual ground to operate on single supply? I've seen other audio circuits where the first opamp amplified the signal and they used the second half as a buffer at unity gain. We'll see in a couple weeks. Next rev I'd like to add a tone knob. This is what I used for the audio.