EMRFD Message Archive 4012

Message Date From Subject
4012 2010-01-18 19:01:55 Tim Buzzing noise in DC receiver with no antenna... weird
I put my Mini-R2 back together tonight for a first listen in a while. 40M CW sounds really sweet, lotsa European and African DX booming in.

Before I put it on the antenna I powered it up with no antenna... and heard a terrible horrible buzzing noise. I was worried something was wrong with it. Then I plugged it into my antenna and the noise disappeared, replaced by the sweet sounds of 40M CW. It's reproducible, I can do this over and over again: no antenna = awful noise, antenna = nice sounds. What is it about my setup that makes it work this way?

Is the buzzing noise just "tunable hum" to the max because of unterminated mixer RF ports? Or is it more complicated?

Tim N3QE
4017 2010-01-18 21:52:30 KK7B Re: Buzzing noise in DC receiver with no antenna... weird
Hi Tim,

Interesting--I haven't encountered that. Try terminating the RF input with a 50 ohm resistor and see if the noise goes away.

Best Regards,

Rick KK7B

4021 2010-01-18 22:49:27 victor Re: Buzzing noise in DC receiver with no antenna... weird
Try to touch the outer part of the antenna connector (The coax shield) to the outer part of the Rx connector (Ground) and see how it affects the noise. May be that when the Rx is disconnected from the antenna its ground connecti
4024 2010-01-19 05:06:37 ajparent1 Re: Buzzing noise in DC receiver with no antenna... weird
You might get that if one of the mixers is not working due to wiring error.

I ran into that with my miniR2 during reciever deveopment, I forget
to hook up the LO quadrature and was only driving one mixer. I had classic tunable hum from the haywire bench layout (testing).


4028 2010-01-19 06:54:47 Garey Barrell Re: Buzzing noise in DC receiver with no antenna... weird
Tim -

You have received several good suggestions.

One thing that has happened to me more than once, (ok, I'm not very
bright...) . Do you have a fluorescent lamp or one of these terrible
"green" CFLs over your bench? They have a nasty habit of generating
noise, and since they're right over what you're working
4030 2010-01-19 07:31:14 Tim Re: Buzzing noise in DC receiver with no antenna... weird
Thank you victor, in fact I am now experimentally satisfied that it has to do with a lack of ground AND the fluorescent lights.

What's weird, is that even with battery power and a 50 ohm resistor like KK7B suggested, I was getting the buzzsaw noise.

I had cannibalized the mini-R2 for the mixers a while back and am in the process of getting it set back up. I was so inspired by the pictures of KK7B's "square" VFO box in QST last year that my new 40M VFO looks like a carbon copy :-).

Maybe a lesson for me as I set it back up, is to make sure that I have a good ground setup when doing any bench testing even without antenna.

I so so enjoyed listening to 40M last night with the mini-R2. I feel bad that I cannibalized it a while ago. The wide dynamic range and the "wide" sound is remarkable gives a clarity that I was happy to find again. Within minutes of hooking it back up I started hearing how different transmitter's TX's had uniquely distinguishable characteristics, I had forgotten how clear and direct the sound of this receiver was!

Tim N3QE

4033 2010-01-19 08:42:04 Garey Barrell Re: Buzzing noise in DC receiver with no antenna... weird
Tim wrote:
4037 2010-01-19 14:23:35 ajparent1 Re: Buzzing noise in DC receiver with no antenna... weird
4044 2010-01-20 06:00:56 Tim Found it! Re: Buzzing noise in DC receiver with no antenna... weird
Actually after doing some investigation last night I'm not sure that it was the fluorescent lights causing the worst of the noise. Yes, there was some buzz from the fluorescents and I could turn it on and off but it wasn't the worst buzz, the one I originally complained about. I found that out after turning on the computer and hearing that the worst buzz, the one I originally complained about, must be from the CRT computer monitor.

After lots of searches for galvanic introduction of this noise, I finally found it: the inductors I used just after the mixer act as very good magnetic pick-up coils. I do not have a really good idea why they were so much more effective with no ground than with ground; my gut feeling is that magnetic pickups ought to be completely independent of any galvanic isolation. The PCB box that my R2 is built in cannot offer much if any magnetic shielding - or can it?

The 1.2mH coils that do the pickup that I used were Xicon 43lh212's from Mouser. Are these shielded, unshielded, nobody knows, ???. Next time I'm gonna use Murata coils.

Somehow the effectiveness of the pick-up coils is much less when an antenna is plugged in and also less when just a ground is hooked up. Maybe the complex impedance at the IF port of the mixer is a strong function of what's plugged into the RF port?

I put the mini-R2 on battery power and hunted down sources of this noise in my basement and the whole house. I could walk around to various possibilities, turn them on and off, tilt them, etc.

I can hold my quartz wristwatch about 6 inches from these Xicon coils and in the headphones on the miniR2 very clearly hear the tick-tick-tick as the second hand is moved by the watches electromagnetics.

The fluorescent lights with electronic ballast also cause some buzz, noticeable with no antenna but not huge, and actually drowned out by band noise when I have an antenna plugged in.

The UPS under the bench had a small amount of buzz when in standby, but when input power is shut off it's an enormous racket.

The real source of the buzzsaw had something to do with deflection or degaussing in my old CRT computer monitor. When I turn it on and the degausser comes
4045 2010-01-20 06:40:34 Tayloe Dan-P26412 Re: Found it! Re: Buzzing noise in DC receiver with no antenna... we
That is the main reason why I moved to R/C active filters in my DC
receiver designs. I too was getting more stray "hum" than I wanted from
things like nearby power supplies being magnetically coupled into the
rig. I could pick up the rig and turn it to maximize the null to the
magnetic pick up. R/C active filters don't have the problem and have
the additional advantage of gain rather than loss.

The inductors that I used were shielded. However, when the MDS is -136
dBm, the shielding on the inductors are only so good.

The only benefit to the L/C filters is that they did have fewer parts.

- Dan, N7VE


4047 2010-01-20 06:54:06 Tim Found it! Re: Buzzing noise in DC receiver with no antenna... weird
The inductor that seems to pick up the magnetic fields is the one in series in the diplexer right after the mixer and before the common-base low-noise amp.

Can this really be replaced by an active filter? Op amps are amazing things these days but I have not really progressed past the NE5532.

I was unable to cause any noticeable magnetic pickup at any of the later inductors. I'm not saying there wasn't any, I'm just saying that the magnetic pickup before the preamp seems to dominate :-).

I did just order some different little cylinder inductors and will see if I can determine which ones are shielded and which ones aren't.

4048 2010-01-20 07:02:43 Tayloe Dan-P26412 Re: Found it! Re: Buzzing noise in DC receiver with no antenna... we
The NC2030 used op-amps and had very high sensitivity (-135/-136 MDS),
very high receiver performance (145 dB BDR, 107 dB IP3DR), and a current
drain of only 12 ma at 12v. The performance was a bit constrained by
the fact that most of the receiver ran on 3v, so the large signal
dynamic range was more limited that it could have been.

You can find information on it out on the web.

- Dan, N7VE


4049 2010-01-20 07:33:13 Tim Found it! Re: Buzzing noise in DC receiver with no antenna... weird
The NC2030 uses quadrature H-mode mixers? All the KK7B designs put a lot of effort into the diplexers following the diode mixer to terminate the IF (baseband) port nicely and dump off the unneeded spectrum at the same time. In fact if I look at the evolutionary chain from the "Optimized QRP transceiver" to the R2pro I see increased effort to terminate the diode mixer well across the spectrum. Are H-mode mixers somehow less dependent on termination? Or do you terminate them completely differently without a diplexer? My gut feeling is that they'd be just as dependent on terminati
4050 2010-01-20 08:11:10 Tayloe Dan-P26412 Re: Found it! Re: Buzzing noise in DC receiver with no antenna... we
It is not an H-mode mixer, it is a "Tayloe detector" and has also been
called a QSD (quadrature sampling detector). The QSD label bugs me a
bit because the detector is actually a R/C low pass integrator, not a
"sample and hold" as the label QSD implies. It is a difference only
detector compared to a normal mixer that has both a sum and difference.
Thus it also has lower loss than a normal mixer. In a DC receiver
application there is not a "sum" mixing component that needs to be
terminated like you are seeing for a normal mixer.

This detector is a bit of an odd beast. It looks like an open on
frequency and a short far off frequency. Thus the detector has a
bandpass characteristic to it that is useful in improving receiver
performance by having the detector itself reject off frequency signals.
The roll off rate is that of a single R/C low pass filter, but I think I
was getting an extra 24 dB of rejection to signals 5 KHz away, which can
help overall receiver large signal performance a lot.

- Dan, N7VE


4054 2010-01-20 11:47:23 Tim Found it! Re: Buzzing noise in DC receiver with no antenna... weird
OK, now I think I'm getting it. In the physics instrumentati
4057 2010-01-20 13:57:14 Tayloe Dan-P26412 Re: Found it! Re: Buzzing noise in DC receiver with no antenna... we
There is no amplifier in the detector. Like I said, for DC receiver
work, a normal mixer splits the signal into two pieces, a sum and a
difference and the sum is thrown away. A Tayloe detector is different.

Think of an incoming RF sine wave that is on the detector frequency. The
detector segments each RF cycle into four different pieces. I like to
think of one of the four outputs centered on the positive peak, one on
the negative peak and the other two segments centered on the two zero
crossings. A stream of these RF pluses are sent into the four detector
caps. The system impedance (nominally 50 ohms) forms a series R, and
the detector cap forms the C creating a low pass R/C response. The
pulses centered on the positive peak, doing an integral of the incoming
RF pulse over its duration, which captures 0.9x the peak input voltage,
which is the same as a 0.9 dB conversion loss. Note that almost all the
signal gets used in charging up the detector capacitor as we are not
creating a sum and a difference but rather are simply integrating the RF
pulse onto the detection capacitor for this 1/4 cycle period.

Since each detector output sees a different portion of the RF sine wave,
we end up with a 0 degree, 90 degree, 180 degree, and 270 degree outputs
which can be thought of as I, Q, -I, and -Q. Since the input impedance
was 50 ohm, and since each of the four outputs see the source only 1/4
of the time, the output impedance of each is 50*4 or 200 ohms. Nothing
is being violated here. Input power (one input at V at 50 ohms) and
output power ( four outputs at 200 ohm each at voltage V) are the same
less the 0.9 dB conversion loss. I can be differentially combined with
-I and Q can be differentially combined with -Q to create a composite I
and Q. Conversely a differential input can be used and with the proper
selection of RF inputs (RF+, RF+, RF-, RF-) we can get by with just two
detector caps I and Q at a 100 ohm effective impedance on each output.

Now do some thought experiments with the input RF just slightly
different than exactly on frequency and instead of four DC values, you
get a slowly varying sine wave on all four detection caps, all four caps
having a different phase shift (0, 90, 180, 270) compared to each other.

Like I said, the R/C lowpass detection characteristic means that signals
either side of the center detection frequency will start to get
attenuated depending of the size of the detection cap and the RF input
impedance and to a small extent the series R of the analog switch
(typically 5 ohms for a bus switch). The bandpass characteristic can be
used to enhance receiver performance by rejecting large off frequency
signals and attenuating them before they get to the gain chain. No
normal mixer does this. Since this is not a normal mixer and there are
not high frequency "sum" products being generated, there is no need to
have a diplexer to terminate unwanted post detector high frequency
mixing products. Since no "sum" signal is being generated and then
thrown away, the conversion loss is much lower (0.9 dB vs. ~6 dB or
more). Since available 5v analog bus switches are available, 5v pk-pk
on frequency signals can readily be handled while off frequency signals
will tend to get shunted to ground via the detector caps.

It is a very simple, high performance quadrature detector that can be
built using commonly available commercial parts (FST3253 or 74CBT3253
dual 4:1 analog mux). Unfortunately these parts are not available in
DIP, but I have found the SOIC format (Not TSSOP!) easy to deadbug with
using # 32 gauge enamel wire for hookup wire.

- Dan