Tapping the 455kHz IF CB receiver to utilize a rig's features? Why not???

[ThaDood]

Another, sort-of done, project??? Well... What if you have some cheap-ass digital, or analog, SW portable radios, or even a decent one, but most portables are missing some really likable features, like a (NB) Noise Blanker, (ANL), Automatic Noise Limiter, RF Gain, or even SSB? Now, what if you have some unused, but working, old-ass CB's, that have all, or even some of these features? Doesn't matter if they are 23CH, or 40CH, rigs. Most CB's use a final IF FREQ of 455kHz, and most SW portables are using that as an IF FREQ as well. So, how about making an IF tap, to go from an SW cheap-ass portable, to the RX IF 455kHz tap of a decent CB rig? And, use the neat features of the CB to enhance SW listen? Actually, the idea has been used ass-backwards. Remember those add-on car CB receivers, that down-convert to an AM radio FREQ, to be de-MOD and heard on your car AM radio? Try to find one today, but they were sold at Western Auto, Sears, JC Pennys, Montgomery Wards, Lafayette, and even Radio Shack. Well, it should be pretty easy to do the reverse, Shortwave radio portable, or even a cheap desktop, to CB IF. The one disadvantage to this??? Audio-wise, you'd get just communications-like audio, and nothing robust. However, for tight, sensitive, SW listening, it would be like listening to my Icom IC-745 rig. COMM-type audio only comes from that. Now, (To take it a step further.), if you have one of those RTL, or KIWI, SDR, dongles, the +60dB of many CB's selectivity could help take-out IMD from like a close-by AM broadcast transmitter, i.e. feed 455 kHz IF from a decent CB, to  the SDR. It would be like a triple jump, cheap SW portable, to CB, to SDR. Hey... Could be done???

[brandon7861]

First post here, brandon from northern MN. 

This is a neat idea, but I think there are a couple issues with it. 

It is common for the AGC in CB radios to control the gain of multiple stages (rather than just one stage) to achieve the desired dynamic range, it's not just controlling the last IF, so the AGC won't work right.  Consider what a radio has to do to make a S9+30 signal sound about the same at the speaker as a S1 signal, there is a huge difference in signal level there and automatically controlling the gain of just one stage isn't enough. Going straight to the last IF bypasses most of that gain control.

The front RF gain control knob typically attenuates using diodes at the front end, well before the last 455kHz IF stage, so you'll get no RF gain control that way either. 

Noise blankers are usually implemented before the last IF stage as well, much closer to the front end, so that wouldn't work either.  Some noise blankers sample at 23MHz, which might not have the same noise profile as SW signals, so I'm not sure the NB would work for SW, even if the signal went through the whole radio. 

The selectivity would be bad too since only a small part of the CB's frequency-selective components are being used.  That brings me to SSB, its crystal filter is also before the 455 IF stage, so you can't use this for SSB.  The crystal filter's steep edges are what selects the desired sideband (upper or lower, filtering out the carrier and the other sideband) and controls bandwidth. Bypassing it removes that function.

It might be tempting to broadband the front end of a CB and use a VFO in place of the first LO, but that too comes with challenges depending on the radio's design.  Take a cobra 148 for example, the first three cans are tuned to 27MHz and two of them are lightly coupled with a capacitor, an arrangement that depends on the transformers being tuned (high Z coupling).  And you cannot just yank them out or broadband them because you need that preselection to prevent all the undesired signals from entering the mixer with the desired signal. 

A better approach would be to build an upconverter with a VFO dial on it that converts the SW station up to 27MHz and leave the radio the way it is. Admittedly, I've never built an upconverter, but I assume it would take some amplification to get the SW signal above the mixers noise floor and some gain to cover conversion losses.  And as mentioned above, the mixer input should have some degree of filtering, not only reduce IMD products, but also to prevent the new LO from reaching the antenna and radiating out. 

Some of the export CB radios had varactors to tune the front end cans so the pass band tracks what the radio wants allowing a narrower pass band with greater selectivity while still covering a wide frequency range.  You wouldn't need anything that extreme since the radio can further filter it once you generate 27MHz, but at least have a SW bandpass filter on the upconverter's RF input.  There are schematics online for basic upconverters, you could probably find enough parts in a junk CB to build one.

Well, there's my first post, I hope I didn't ramble too much.

[ThaDood]

Hello 'brandon7861'!!! 1st off, I appreciate the RSVP'ed post. And, a good insight from your end on what I was contemplating. I have thought about Upconverting to 27MHz, but then figured dumping a 455kHz IF would be easier. However, as you have pointed out, this could also bypass many of a rig's viable features. Still, it would be an interesting experiment to try, for those of us strapped for cash, yet have bunches of leftover crap in our junk boxes. THANKS, again!!!!