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 Post subject: Receiver sensitivity versus ambient noise.
PostPosted: Dec Sun 24, 2017 11:48 pm 
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Location: Champaign IL 61822
I notice many boatanchor lovers (myself included) obsess over radio sensitivity,
to the extent of trying all sorts of different front end tubes. So today
I did a bunch of tests to see once and for all, well, at today's noise level, which
my Airspy says is absolutely normal,its never lower, if it matters.

Bottom line, out to 30 MHz: even the lowly Heathkit AR-3, with 12BE6
first tube, has quite adequate noise figure to get the background noise
well above the tube noise.

I tried this with all my boatanchors as well. I tried two antennas, a 20 foot one
strung out the window, line cord as ground. and a 50 foot antenna
and 15x5 foot lattice as ground, going into a low noise transistorized
emitter follower preamp (0.6 voltage gain into 75 ohms).
At every frequency on every radio with either antenna the noise from the antenna
swamped the front end noise.

The AR-3 is a bit low on gain above 20 MHz, but even without using headphones swapping
a 75 ohm resistor for the antenna made a very clear noise level difference,
as did detuning the RF coil trimmer. Its true that if the RF was not tweeked to tune to
the correct spot the AR-3 would fail to be able to pick up
antenna noise.

Above 22 MHz the AR-3 has inadequate performance for a 5 foot wire antenna. The high end
boatanchors remain fully adequate.

So as I suspected ... worrying about you boatanchor's 6BA6 (or 6K7 or even 38) front end
tube is silly unless you have a really really short antenna, an improper feedline, bad
RF-oscillator tracking, or perhaps a QTH on an island running off of solar cells.

Its also clear that somewhere between 30 and 88 MHz the ambient random (galactic plus
atmospheric) noise drops well below that of a pentagrid convertor.

Relax and have a Merry Christmas with your boatanchors.


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Sun 24, 2017 11:58 pm 
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Great information. Thanks for posting it!


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 3:44 am 
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The old quick and dirty test for receivers with an antenna "trimmer" was if a noise peak could be heard when adjusting the trimmer then sensitivity was adequate for that band and atmospheric and/or local noise was the limiting factor.

Headphones can make a big difference with receivers with sufficient sensitivity but insufficient overall gain. Having to run the audio amp wide open to help overcome ambient audio noise in your radio room can add enough hiss and hum from the audio sectuib to make weak but otherwise copyable signals fall below the audio noise floor. Sometimes an audio filter makes a world of difference in copying a signal that is close to the noise level, CW in particular, since it will help you to easily focus upon the signal. I was amazed how useful an Autek QF-1A is for pulling weak CW signals out of band noise on 160 and 80 meters.

One trick I used when heavily contesting was adding one of the old DBX audio processors in line to compress the dynamic range when operating CW. Otherwise the tendency is to crank the audio too high when trying to pull out weak signals interspersed with strong signals. After operating a couple of contests you realize you are running too much receiver volume by the end of the contest when next time you turn the rig on the audio blasts you out of the room. It is the same process many people fall into in turning their car audio systems up too high while driving and then are shocked at the volume level when the car is started the next morning.

dtvmcdonald, thanks for posting your findings.

Rodger WQ9E


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 12:06 pm 
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Many years ago, I wrote one article about noise. You can read on http://www.filtrando.com (only on Spanish).
That are valid also for HF.


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 3:33 pm 
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Location: Santa Rosa, CA
Thanks for starting this thread. Its an interesting subject and one I've
been exploring for quite a while. Noise in a receiver *system* is quite
complex and involves several things; environmental noise (natural
and man made), antenna and feed line noise, as well as noise
generated in the receiver itself. Bandwidth is a key item as well.

Of course there is little we can do about environmental noise so the
best we can do is tackle things we have control of. Keep
in mind that all antenna/feed line/receiver noise *adds* to environmental
noise. So minimizing this does make a difference but certainly has
diminishing returns which was the point of the op.

Good sensitivity at HF frequencies is relatively easy but getting it
with good noise perfomance is not. So far the receiver with the best
sensitivity AND lowest noise I've come across is the RACAL RA-17D.
Its an unusual design in many respects with a very good preselector
and front end.

Ray O at ER magazine has written several articles about this subject,
most related to the R-390A that are very well done. Another ER
article (not by Ray) talks about the contribution of phase noise
generated in the VFO's of receivers using a PLL. Its an eye opener.

Agree with Rodger that a good set of headphones and a QF-1 or -1A
is the way to go when receiving CW. The mistake many people make
with the QF-1/A is using low Z 'phones.
Steve

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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 6:21 pm 
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Location: Champaign IL 61822
zarco wrote:
Good sensitivity at HF frequencies is relatively easy but getting it
with good noise perfomance is not. So far the receiver with the best
sensitivity AND lowest noise I've come across is the RACAL RA-17D.
Steve


I own an RA-17C12. Yes, it is excellent. BUT ... except above 20 MHz or so,
it makes my point! That point is ... low noise performance is generally
simply unnecessary below 20 MHz. I normally run my Racal with AT LEAST
10 dB on the attenuator between 10 and 20 MHz and often much more than that below
13 MHz.

An emphasis on low noise performance is simply silly below somewhere
in the 10-20 MHz range.

More important, as you express, for CW (and digital modes), VASTLY more
important, is frequency stability! It not a coincidence that many high end
50s military boatanchors emphasize allowing crystal control! If you have adequate stability
all the way down to audio, you can get great sensitivity by going to very low
bandwidths. I can see WWV's carriers on all frequencies at (almost) all times
by performing an FFT at audio with 0.05 Hz bin width ... using my high end
synthesizer as the LO.


Last edited by dtvmcdonald on Dec Mon 25, 2017 7:02 pm, edited 1 time in total.

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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 6:53 pm 
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In the last several years having restored over forty communication receivers of all makes and models, I was struck by something I had not expected, but which should have been obvious. Some individual radios are just better than others--regardless of the make or model. I have a Hallicrafters SX 110 that out performs many radios when it comes to sensitivity, drift, and noise. The next SX 110 I got was a complete disappointment and is probably representative of the model. The biggest surprise was when I had completed the re-capping of my Hammarlund SB 400 and power supply. I rotated the on switch and . . . . nothing. I turned up the volume . . . still nothing. I decided I had probably not hooked the speaker up properly or that it was loose and, investigating the back of the radio, I accidentally touched the antenna terminal with my finger. A broadcast band station blasted out at me. That is the quietest radio I have when it comes to radio generated noise.

When I look at the insides of those early radios--especially the military models, there is a lot of steel and aluminum shielding--especially around the tuning capacitor and RF sections. Band switches are enclosed. The more complex a radio is the more opportunity for noise to enter or be created. Very simple radios do well because they are . . simple. I have long thought that the best radios made are car radios. They have challenging noise environments, small antennas, little room for components in a small enclosure, and passenger compartments were not always as quiet as they are now. When I finish work on a radio and want to know how the broadcast band is working, I try to tune in a station that is weak on my car radio. Very rarely does the table model with a good antenna up in the trees outperform my car radio. The advantage car radio manufacturers have is they know that hundreds of thousands of radios will be sold and the engineering budget is worth it.

Norm

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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 7:11 pm 
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I agree Norm and the better receivers hear nothing until an antenna is connected, the SP-400 is definitely in that category.

Try going from a 1980s set with multiple PLL synthesizers to a well built direct conversion set with a clean LO and good audio section and it is quite an eye opener. The later DDS sets can be very good and a well designed PLL/VCO synthesized set can play pretty well but most of them don't. A lot of the sets have pretty poor VCO construction which greatly increases phase noise as the loop is busy just countering shift due to excessive drift and/or from mechanical shock. Providing fine, near analog, tuning steps with fast lockup time is also a challenge for good PLL/VCO design and some of the better sets of the era used a PLL/VCO synthesizer for coarse frequency selection and a very stable analog VFO for finer selection. This works very well but it doesn't allow easy implementation of memories which seemed to have become as much of a selling point for receivers as having a S meter.

On average, a PLL/VCO synthesizer is always on frequency but it is constantly adjusting the VCO to stay there thus creating phase noise.

The first time I used a Kenwood R-599A I was very surprised by the lack of noise and the way signal popped out on 10 meters. Had I spent some time testing it I never would have purchased the new Yaesu FT-980 I ordered back in 1983. It had horrible phase noise and a poorly designed power supply that required the noisy cooling fan to run almost constantly even while purely in receive mode. When I went to order it the store had just taken in a beautiful fully optioned Drake C line and I think I paid $350 for the setup. I should have stopped with it because the R-4C was far superior to the receive section of the FT-980.

Rodger WQ9E


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 7:20 pm 
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Location: Champaign IL 61822
Norm Johnson wrote:
The biggest surprise was when I had completed the re-capping of my Hammarlund SB 400 and power supply. I rotated the on switch and . . . . nothing. I turned up the volume . . . still nothing. I decided I had probably not hooked the speaker up properly or that it was loose and, investigating the back of the radio, I accidentally touched the antenna terminal with my finger. A broadcast band station blasted out at me.

Norm


My Racal RA17 is probably my best set for that. Many otherwise fine boatanchors
have a rather silly Achille's heel: they couple a calibrator to the input itself
through a say 5 pF cap .. AND ... the wire from that cap to the calibration
oscillator is unshielded. I've tried some with that wire disconnected, moved away, and
grounded, to vast improvement in pickup. The Racal uses a totally different
calibration scheme and truly extreme shielding (it has too ... there are internal
signals at every MHz at all times! ) That said, all my boatanchors are well shielded
enough that my local killer FM station that despoils reception on unshielded
sets above 10 MHz is either totally gone or barely audible at the highest
frequency setting that converts it down using a LO harmonic.


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 7:38 pm 
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As to LO quality: I have a PTS-40 synthesizer that has very low phase noise. Its even better than the more common PTS160s which our custom built NMR setups use (milliHz
resolution at 100 MHz). I've used it as LO on sets that have external LO inputs
or allow one to use a synthesizer connected to where they expect a crystal. It recently had
its electrolytics replaced and was set to within 0.05 Hz of a Rubidium standard at 10 MHz.
After 8 months its still within that of WWV. Of course, the crystal has had 30 years of
continuous running to age. These are great synthesizers, and are available cheap on
ebay. You do pay more for the PTS40 over the PTS160 and more for ones with
a front panel (most are computer control only). Mine has the front panel. (Do recap the power supply if its not already done.)

These gizmos are the multiply/divide/add/subtract synthesis type and don't
use VCOs.


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Mon 25, 2017 8:57 pm 
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Thanks for the info on the PTS family of synthesizers! One of these would be great for a NC-400 which has some very good qualities coupled with less than stellar HFO performance.

Some of the noisiest rigs of the early synthesized era were the Yaesu FT-980, Yaesu FT-One, and Collins KWM-380. I own all of these (well the HF-380 version of the KWM-380) and the phase noise is sufficient to greatly reduce their usable sensitivity. Fortunately things soon got MUCH better.

I bought a Yaesu FT-1000MP in the early 1990s and although not as phase quiet as the current generation of DDS synthesized rigs its synthesizer is quiet enough that it isn't a problem. It does suffer from some noise generation and I found while contesting in the mid-1990s that for 160 and 80 CW my Drake C line with its 250 hz filter and external Autek QF-1A audio filter was far better at pulling the weakest signals out of the noise compared to the FT-1000MP with a full complement of Inrad filters, the noise reduction mod, and its internal DSP setup. Later I bought a beautiful looking but non-working FT-1000D on ebay for $175 shipped. The current owner had grown furious with the problems he had experienced getting it repaired after he had sent it out to add the only IF filter that wasn't stock (the CW filter for the sub-receiver). When it came back sensitivity was horrible and after trying three different repair shops that couldn't fix it he put it on ebay. I bought it and left it in its box for several months and then put it on the bench. As advertised sensitivity was almost non-existent but the repair turned out to be fast with no parts required because the only problem was the AGC bus line to the front end was left unplugged and once put back in place the rig worked beautifully. According to test data it isn't as quiet as the FT-1000MP but I really prefer its receiver for both CW and SSB but I do miss the very fine/slow tuning rates available with the 1000MP.

Side note, for those who try to use their FT-1000D for AM transmission there is a mistake in the manual. The FT-1000D has two controls that affect output power for AM, the RF output control and the drive control. The RF output control acts like a very fast response ALC and approaches the function of a hard limiter. Yaesu instructs you to set the AM carrier output to 50 watts on AM using the RF output control but if you do so then it doesn't allow positive going modulation and sounds horrible. The correct setup is to turn the RF power to maximum and then use the drive level control to set the proper carrier level for AM (not over 50 watts and preferably less).

Also be careful setting up this rig for CW using the manual because they overuse the ALC function on CW and if set up per the manual you will create clicks which will be a problem if you also use a high power amplifier. Like on AM Yaesu wants you to use the RF power control which operates as a limiter and if you do this then you are removing most of the shaping that is needed to provide the proper rise time to avoid clicks. Like AM set the RF power output to prevent exceeding your desired power but use the drive level to set the actual power output so that it is below the limited level. It will make the other operators much happier :)

Rodger WQ9E


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Dec Wed 27, 2017 2:26 am 
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QST Feb. 1966. Page 22. Improving Your Receiver with a Frame Grid R.F. Pentode. Interesting article for those of you that want to experiment with boatanchor's front ends. Probably mostly an improvement above 14 MHz. Specs out alot of these tubes and talks about what works and why.


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Jan Sat 13, 2018 8:29 pm 
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I read this post with great interest because I'm starting to do some DXing with a couple of boat anchors, and have read several other posts attempting to reduce the noise generation.

So I tried using headphones with my newly finished Hallicrafters SX-73 did nothing, didn't even cut off the speaker. I thought - "what gives." Turns out the headphone output comes right off of the final audios matching transformer :shock:
I guess I didn't spend a lot of time with the audio when I was fixing this. :oops:

So I'm thinking I could pick the audio up earlier and use a replacement jack to disable the last audio stage.

Quote:
QST Feb. 1966. Page 22. Improving Your Receiver with a Frame Grid R.F. Pentode. Interesting article for those of you that want to experiment with boatanchor's front ends. Probably mostly an improvement above 14 MHz.


thanks for the reference - I am definitely interested in trying this; I've read many posts here and other sites by Carl (Burnt Fingers) about improving signal strength at higher frequencies.

Again, great post - especially for the engineering Newbie.

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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Jan Sun 14, 2018 12:08 am 
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Smooth,

There are a few vintage receivers that use this same method for headphone operation although most aren't as advanced as your SX-73 which is a really nice mannered receiver and for some reason to me the Hallicrafters crystal plus LC multi-bandwidth IF is perfectly suited to that setup. It probably made sense for the main (military) market for the SX-73 but is a bit unusual for a civilian communications set. In military use the line output would have still been active with "local" headphones in use and most of the receivers of this era provided line output via a tap on the output transformer.

A set of headphones with adjustable attenuators is very useful because modern phones will have a hum issue with most older receivers. Using one of the headsets with a volume control allows dialing in a little attenuation so that the residual hum isn't a problem. This mismatch between modern phones and vintage receivers is another reason I like the external Autek QF-1A audio filter because it has a high input impedance and can be used to properly connect modern low impedance phones to an older receiver along with its primary benefit of tailoring the audio response.

Also be careful with the very old and very simple receivers that fed the DC for an audio stage directly through the phones instead of using an audio output transformer. I have a little Depression era shortwave set on the bench now that uses this method with the headphones connected directly between B+ and the plate of the audio output stage.

Rodger WQ9E


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Jan Sun 14, 2018 10:03 am 
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Greetings to the Forum:

Rodger writes:

Quote:
Also be careful with the very old and very simple receivers that fed the DC for an audio stage directly through the phones instead of using an audio output transformer. I have a little Depression era shortwave set on the bench now that uses this method with the headphones connected directly between B+ and the plate of the audio output stage.


I take it these receivers are mostly recommended for people suffering from clinical depression?

Regards,

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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Jan Sun 14, 2018 2:01 pm 
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Jim,

I expect there were a few cases of self-applied electroshock using old sets and degraded headphones.

The "Constructrad" brand 3 tube regen that I have just about rebuilt on the bench now is one of those types of sets. It is an AC/DC set along with the DC on the headphones method so it provides multiple opportunities for self-treatment with electroshock.

The original builder didn't help things any with his "floating component" building style that has several instances with two of the leads of series connected components tied to terminals and the connection point between components floating out in space near the chassis. The one sheet instruction diagram is of the old style with both schematic and pictorial diagrams and he did mount the included terminal strips but just neglected to use them.

The Aero kit regen I bought was beautifully constructed but this cheaper Constructrad was likely a gift to a child or to an adult who never should have been allowed near a soldering iron (or any sharp objects).

Rodger WQ9E


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 Post subject: Re: Receiver sensitivity versus ambient noise.
PostPosted: Jan Sun 14, 2018 5:41 pm 
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Quote:
Also be careful with the very old and very simple receivers that fed the DC for an audio stage directly through the phones instead of using an audio output transformer.


Thanks for the tip!

I've never considered using headphones, and so I'll put them on my want-list.

I do in fact have an Autek QF-1A; I bought it at a hamfest because the price was right and I had recently read about them. I have not yet used it, though I downloaded the manual and I've heard it takes some time to get the hang of it's use.

On my other list ("things to learn") is how to sensitivity, and measuring noise level attributable to each stage.

Am I assuming that since the designers (also SP600, 75a3) wired headphones at second audio did not see a benefit of improved signal copy using headphones earlier in the audio system (elimination of add'l audio noise)?

Quote:
I have a PTS-40 synthesizer that has very low phase noise. Its even better than the more common PTS160s


Interesting...I was building a VFO using AD9850 and and Arduino with an amplifier & filter. Half-way through the build I started reading about phase noise. You mention the PTS-40 as a LO; I'm wondering if I could use it as a VFO; the programming looks elemental and and some custom Arudiuno programming could control the PTS40 it easily.
[The VFO build is for my CE 20a; I can generate the 9mhz offset, while displaying the desired resultant frequency, and the programming handles whether the offest is +/-, band switching, etc]

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