Automatic RF noise cancellation and audio noise measurement

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THE BIG QUESTION: How do you measure the audio noise level in a
receiver? Does anyone here know of any circuits that show the overall
audio noise level?

Let me tell you what this is all about.

SIDE NOTE: Thanks again to those of you who helped me with the
SWR/wattmeter project I worked on last semester. This project taught
me more about ferrite cores, op amps, and diodes than ANY class
possibly could. I learned about stray capacitance and what rails
mean. I also learned that not all 1N34A diodes are alike.

In this coming semester, I will be working on an even more ambitious
project - an automatic noise canceller for 160m to 10m. It will
definitely be a control system, and it may even use DSP. The device
will also require a motor for automatically adjusting the controls.
The idea is to design and build an automatic version of the Timewave
ANC-4 or the MFJ-1025/1026 RF noise cancellers.

The ANC-4 and MFJ-1025/1026 are manual devices. There are automatic
noise cancellation devices, but their adjustments are at the audio
level rather than the RF level.

The back panel of my device will contain:
1. Connector for the signal antenna
2. Connector for the transceiver
3. Connector for the noise antenna
4. DC power connector

The front panel of my device will contain:
1. Power switch and the obligatory idiot light to show that the power
is on
2. 2 audio connectors: one connects to the transceiver, and the other
connects to an external speaker
3. Frequency range control for choosing the inductance values, as the
proper inductance values in the RF noise cancelling circuit varies
with frequency
4. Phase range knob so that the phase shifter can cover all 360
degrees
5. Manual noise phase knob
6. Manual noise gain knob
7. LED display to show the noise level
8. Noise phase adjustment push-button: Pressing this button activates
the control system to adjust the phase shift of the noise.
9. Noise gain adjustment button: Pressing this button activates the
control system to adjust the gain of the noise.

Other features:
1. I need to design my automatic noise canceller so that parts won't
blow up if I transmit 100W through it. This probably requires some
type of automatic bypass circuitry.
2. Transmitted power must not reach the noise antenna (at least not
without a great deal of attenuation)
3. It also needs a low insertion loss.
4. A preamplifier would be desirable.

How it would work:
1. You manually turn the phase and gain controls to the minimum
settings. (Or I could put in a reset button to do that. No, that
would add too much complexity to the circuit.)
2. You select the band using the band inductance switch.
3. You move the phase polarity switch to the negative setting. If
the noise cancellation procedure does not work, it probably means the
opposite setting is required.
4. Press the noise gain adjustment button. The device adjusts the
noise gain until a change in the signal level (or the S meter) is just
noted. (Some type of derivative function would come into play.)
5. Press the noise phase adjustment button. The device adjusts the
noise phase shift so as to produce a null in the signal level (or the
S meter).
6. Steps 4 and 5 could be repeated to make further adjustments.
There might be away to activate the control system with just one
button.

Does anyone know of circuits that measure audio noise? Making the RF
noise canceller automatic requires an audio circuit to monitor the
noise level. This noise level needs to be converted into a large DC
voltage, which feeds the motors that turn the variable capacitor and
potentiometer.

Jason Hsu, AG4DG
[email protected]@@@jasonhsu.com

 

Allow me to infuse a different idea. Think about noise as a fairly
broadband phenomenon. The same atmospheric noise that you hear on 21.500
Mhz will also be heard on 21.510 Mhz. Consider the possibility of biulding
two identical receivers, the main receiver tuned to the desired reception
frequency, the other tuned just a few Khz away, but out of the received
signal's bandwidth. Then, you detect the noise on the second receiver,
invert it, and sum it into the first receiver's signal chain. This concept
is used in ultra-low noise, high sensitivity deep space receivers to almost
totally eliminate noise in the received signal.

Now, as to your concern to keep 100W of RF from blowing up your noise
canceller. That's what antenna changeover relays are for. They normally
connect the antenna to the receiver, but when the transmitter is keyed up,
the relay connects the antenna to the transmitter output, and disconnects it
from the receiver. Simple, but effective, and has been used for many many
years in amateur as well as commercial radio equipment.
--
Tweetldee
Tweetldee at att dot net (Just subsitute the appropriate characters in the
address)

Time is what keeps everything from happening all at once.

 

There are numerous sources of noise at the input to a receiver:

1) Background HF noise that exhibits a 1/f rolloff characteristic. This
has seasonal and diurnal variations.

2) Atmospheric noise caused by local or near local weather. Remember
there are 10,000 thunderstorms in the world at any time.

3) Solar flare noise. During an solar X-ray event the HF noise, #1, can
increase by over 60 dB for short periods of time.

4) Local line interference noise.

5) In the future, BPL noise.

6) I'm sure there are other noise sources such as defective insulators
on power lines. ;-)

The ANC-4 and MFJ 1025/1026 ONLY operate on #4.

So, your first task is to become familiar with the spectral
characteristics of each so you can design a basic filtering circuit for
the undesired noise source. My ANC-4 does not respond to the 1/f HF
solar noise sources, atmospheric induced noise or solar flare noise.

Weather induced noise contains frequency components that are in the
audio passband, you can hear lightning induced static crashes in your
SSB/AM radio. How will you discriminate between desired audio and static
audio?

Solar flare noise sounds like a slowly rising rushing hiss in the
receiver. It has a slow rise time generally in the range of tens of
seconds to minutes. It is broadbanded and will easily over ride any low
level desired signal. The noise to signal, NOT signal to noise, ratio
can easily exceed 40 dB making detection almost impossible. These
characteristics are proportional to the intensity of the solar flux and
also the wavelength of the solar flux. Detecting desired signals in this
environment is extraordinarily difficult.

Manmade noise, these are processed by the ANC-4 and MFJ units, contains
basically repetitive waveforms from TVs, VCRs, Remotes, Routers,
Computers, etc.

A design approach would be to use digital filtering and a series of
Kalman filters [digital filter algorithms], assuming they are available
to the general public. The Kalman filters would be tailored to the
spectral content of the noise source you are attempting to cancel. A
second design approach would be a fast Fourier transform that
discriminates N/S from S/N. You want to keep the noise OUT of the FIRST
RF STAGE to avoid pumping the AGC and getting false signals. For HF
digital filtering this will require a very fast processor [possibly GHz
range]

How to measure the AF noise from a receiver? I'd start by looking into a
Bolometer circuit with AC coupling. This would give a pseudo RMS reading
that could be used as a reference for a series of servo based op-amps to
control your gain and phase circuits.

Depending on how far you want to pursue the science it could be a very
interesting project.

With a statement of principles it is YOUR Project, NOT the project for
the members of this list. So, go and have fun!

Deacon Dave, W1MCE

Among the many components of my background was 7 years as Project
Engineer designing special test equipment on contract for the USAF.

 

Hi Jason:

I think the "signal antenna" and "noise antenna" are both just
antennas. I can't see how you can make an antenna that only hears noise
and another that hears the same noise and signal. If that's the case
then by adjusting the relative phase and amplitude of the two "antennas"
what you are doing is changing the antenna pattern. This will improve
the overall s/n if there is a source of noise that's coming form some
direction more than from everywhere.

In "Beam and null switch step steerable antenna system" U.S. patent
<http://www.delphion.com/> 4,063,250
<http://www.delphion.com/details?&pn=US04063250__> Fenwick has an
antenna system where the main lobe is steered using time delay beam
steering (this is a frequency independent version of a phased array) and
in addition can steer a null independent of the main lobe. He used
mechanical relays and coax line, but a more modern version could use PIN
diodes and a computer to try all the possible main and null lobe
possibilities and use the best ones.

To measure the sensivity of an AM receiver you measure the RMS audio
output voltage with a signal present and without a signal present.
For a more detailed look at the noise you need a spectrum analyzer, like
the HP-Agilent 4395A that can make true RMS noise measurements, see:
http://www.pacificsites.com/~brooke/4395A.shtml

Have Fun,

Brooke Clarke, N6GCE
http://www.pacificsites.com/~brooke/electron.shtml#Antenna

 

Hi Jason


Theres a whole lot of unworkable ideas on this thread. I shouldnt rush
in without reading up well first.

I'll just address a couple of Dave's ideas.

Lot of work was done on this in the 30s, the cnoclusion was they
couldnt. However you can if you use the standard 405-line TV noise
reduction method. Basically detect brief signals that are greater in
amplitude than your wanted a.f., and zero or reduce the af output
during that time.

I dont see how you can manipulate signals that havent even reached the
first rf amp. You cant even read them as theyre microvolts. Putting
them thru any kind of digital filter - you work it out.


Jason your idea of mixing different noise sources is also a non
runner. Sorry, but youve got to consider _exactly_ what is in those
noise sources and how they differ. It will soon become apparent that
by the time you add them youve got 2 different things. The noise f
distribution might be the same, but the noise signal will be quite
different.


Regards, NT

 

Ideally you point the noise antenna at the noise, and the signal antenna at
the signal. In the real world the noise antenna is usually fixed, but might
be pointed at something like a power line. In any case, the noise antenna
is usually quite low, and you hope it won't pick up much of the desired
signal.

BTW, way back when, either Radio Shack or Lafayette sold a CB radio that
used a separate noise channel that was tuned to just outside the CB band. No
idea how well it worked.

Tam/WB2TT
Hi Jason:

I think the "signal antenna" and "noise antenna" are both just antennas. I
can't see how you can make an antenna that only hears noise and another that
hears the same noise and signal.

 

The line from the change-over relay to the receiver should be shorted
when the receiver is disconnected and either short or a half wave, or
else open when the receiver is disconnected and a quarter wave long.

Jerry

 

It even works for _on-channel_ jamming TX'ers off in another direction from the desired signal (cable co's use
the technique). Separate directional antennas are used for the desired and the undesired signals. The
undesired signal (jammer) is gain adjusted and phase shifted and then summed into the "main" receiver signal
path at 180 degrees (and equal amplitude) from the same jammer entering into the main antenna/rx'er path. The
obvious null for the undesired signal occurs. Nulling of the desired signal is highly improbable given the
directional aspects of the antennas. The sum can occur in the rx'er front end. Yes, this is signal
processing down at the microvolt level. One problem is tuning drift.