So I assume we want 27MHz as noise blanker. What is simplest method to determine this?
IF you're asking about the Noise Blanker coils' own frequency - it should use a NON-resonate spectrum in which to detect noise, otherwise the peaking of the section before and the section after - the birdie generated at specific frequencies will generate a "on" signal that will kill all RF passing the output of the Noise blanker.
The Process of using the Noise blanker is to obtain a noise from a range of spectrum close if not in the range of frequencies you're trying to quench the noise from (if the non-resonance can be obtained)
You can try the 16MHz range - but the effort may not work for it SHARES the same the PLL will put out - hence a birdie, the On signal and killed RF Strip.
Cobra and Uniden did a little trick to a peaking coil by changing a cap in the 16MHz Peaking coil used in their FET 1st IF front end (LA181) - and renumbered it(LA180). They used a larger value cap LOWERING the Resonance frequency to about 13MHz to 16MHz (tune range) in the NB strip. (You can use the OPPOSITE IMAGE - for the detection - just don't go tuning for carrier - the cap value used is keeping you away from that)
In using a Heterodyne principle, you have a mixing product in two regions - ABOVE and BELOW - the NB peaking coil is designed to use the OPPOSITE image of the Heterodyne that is passed into the 1ST IF..
That's the problem - too far out of band, you don't get much outside impulse noise to work with that passes into the strip except the internal birdie and whatever is in the image.
Then you have the bandwidth product of trying to locate and shunt the impulse and yet discern that from the Audio embedded in with it.
Many radio manufacturers have abandoned the older "amplify and shunt" and go with more of the ANL Clip and limit/hold the filter used in it - provides.
They call it DSP - I call it junk.
I do have the Cobra 29 NW LTD Classic, 2020 edition.
Ok, understand that you are several decades into Schottky and revisions to BOTH ANL and NB sections in both Detection and Semiconductors used - you don't have the luxury of the discrete 380/495's and 1815/733's of older generations.
- These older unit had an Hfe gain factor and bandwidth product that makes the higher frequency roll off - and the subsequent frequency response to that upper limit - more of a challenge so they worked with values to force the clip and limit/shunting - they HAVE NOT changed those values since - so that means the faster response of the newer dies of the parts subbed into the strip they use now - is way higher and those values have not changed to follow their new performance curve.
I also have a supply of NOS ITT 1N40/1N34 1N60 germaniums
plus many pounds of NOS Ti 1N4148s from early 1980s.
I think these were the last of the Ti Made devices for when
they were doing down hole oil and gas drilling electronics.
I Did not know that - I learn something new every day.
One thing though, and you may find yourself in a spot having to abandon the project due to it's effects.
- - because you cannot find a Diode in Schottky that will LESSEN noise - it may do the opposite - RAISE the noise - but not by what you think.
- The Diodes of old were "slower" and were more intrinsically exhibited a filter-dependent characteristic. They were not of pure Germanium - more like it's ore Galena in a crystalline Salt form. (Cats whisker) so they exhibited a reactive element...
Some feel it's similar to this...
But, were dealing with Frequency Response.
This puts it as more of a pass filter network
The above is If you were to examine a Diode as it's Electrical Characteristic ...
Physically they are different too...
I'm currently evaluating performance and want to enhance my CB.
I'm going through researching how to make my radio better.
One of the few things I don't have is FFT rig that goes into the RF range.
I only have AF range to about 100kHz.
So is there a simple method to scale 27 MHz to the 100kHz range and look
at it through simple Audio FFT?
To see the 27MHz effect, - er, no - not readily - but thru the use of Heterodyne (Image) you can place audio into a carrier and see the effects of bandwidth performance thru the NB on and NB off and review the results by simply producing a carrier and inject audio either as white noise or tones you pick - to see their output on a monitor radio or thru the receiver of your DUT. (Device Under Test)
Understand too, that as you work into this, your best friend is your DVM.
At least, in using it's "Diode" setting - for as your work with Schottky and Germanium versus Silicon or even Selenium - you will find that their performance in specific areas are different (Audio reproduction is one effect) and that forces you into a redesign of the output network filter to obtain the best results from the OEM versus Revised.
So when you use your DVM, note the "Diode Drop" values - you'll find some of the best Audio Detection is thru Germanium using a 0.23 to as high as 0.47 (Diode Forward Voltage Drop) but Schottky will be more in line with Silicon as 0.45 to as high as 0.82 - rivaling Si with Selenium being more like 1.40 or even higher.
Germanium Diodes, as they age, give higher DFVD (Diode Forward Voltage Drop) as their junction bond degrades from vibration, ages and power spikes. You may encounter this but too, note the "sound quality" from the detector and audio reproduction - as you make these changes you may find restoring the OEM parts may be a better fit for the radio then to try and force upon it any sort of upgrade.
Same of Selenium types of rectifiers - worse of the Contenders.
Si and Schottky are similar with Schottky having the lower end of the drop but remember too - the "recombinant" region (P to N - Intrinsic) is non-existent - so you cannot "filter" the noise present from the rush of electrons trying to cross the barrier (roaring of a wave so to speak) and some recombine with the Holes in the Positive layer - you lose some of that information during the recombine as the older PN barrier designs do. Due to that Schottky thinner layer, the Electrons take the energy around them present at the time - which may or may not contain the audio information - just the energy level from the "Bounce" - they pass this information thru (over) the threshold) barrier and out the conductor - it doesn't mean it's the "Right" information - just information.
So in my experiences with PN devices, the Holes the Electrons tend to carry into - work as part of this intrinsic audio filter effects (With Schottky you can experience or hear a snapping or crispy/near crunchy sound effect compared to Germanium) many experience when they swap out the different diodes and observe the effects in the noise level and in how the AGC will work in regards to the newer changes.
Why the discrepancy? Do your search finding EQUVALENT performance parts as you post your results - it is best if they can be duplicated by others.
