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Steve - WB3HUZ
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« Reply #25 on: June 18, 2009, 10:27:01 PM »

Recent receiver specs.

Icom 756

Sensitivity (typical):
Frequency range (MHz)   SSB, CW, RTTY (at 2.4 kHz BW)   AM (at 6 kHz BW)   FM (at 15 kHz BW)
0.50-1.799   --   13 µV   --
1.80-27.99   0.16 µV *1   2 µV *1   --
28.0-29.99   0.16 µV *1   2 µV *1   0.5 µV *1
50.0-54.0   0.13 µV *2   1 µV   0.32 µV *2

10 db S/N for SSB, CW, RTTY and AM, 12 dB SINAD for FM
*1=pre-amp 1 is ON, *2=pre-amp 2 is ON




Yaesu FT-2000

Sensitivity:

SSB/CW (2.4 kHz, 10 dB S+N/N)
2 uV (0.2 - 1.8 MHz)
0.2 uV (1.8 - 30 MHz)
0.125 uV (50-54 MHz)

AM (6 kHz, 10 dB S+N/N, 30% modulation@400 Hz)
3.2 uV (0.2 - 1.8 MHz)
2 uV (1.8 - 30 MHz)
1uV (50-54 MHz)

FM (BW:  15 kHZ, 12 dB SINAD)
0.5 uV (28-30 MHz)
0.35 uV (50-54 MHz)
(with RF2 amp on)



Older Receivers

Collins 51J-4

Band 1 - Less than 15uV gives 10 dB s/n
Bands 2 to 30 - Less than 5 uV gives 10 dB s/n



SP-600

The m-c-w sensitivity of the receiver to any signal within its tuning range is two microvolts or better, for a signal-plus-noise to noise power ratio of 10 to one at the receiver audio output terminals.
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w3jn
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« Reply #26 on: June 19, 2009, 09:51:32 PM »

Are you talking during alignment, or rated sensitivity?

WHen comparing receivers, some sort of standard comparison needs to be established.  10 dB S/N, or S+N/N, or SINAD (S+N+distortion)/N are generally used.  Like Steve I've never, ever seen 20 dB S/N as a standard comparison (or rating) figure.

10 dB is more than adequate to recover decent audio.  I did a lot of comparisons of receivers using my Agilent E6380, which will measure automagically any of the three (S/N, S+N/N, or SINAD, or even distortion) using the 10 dB standard.
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« Reply #27 on: June 20, 2009, 01:27:30 PM »


I'm talking about injecting a signal to the antenna input and looking at the last IF stage's output before detection using a spectrum analyzer. You want 20dB above the noise floor at that point all the way down to 1uV for a tube receiver and less for SS receivers. If you are only 10dB above the noise floor it will often get covered up when a noise source is injected and that floor rises.

For what purpose?  Comparing receiver sensitivity?  Alignment?  Some other reason?

Not sure what the purpose of looking at the last IF stage with a spectrum analyzer might be.   If you do this for the purpose of measuring ultimate S/N, you're overlooking at least three whole stages of the receiver - the detector, first audio, and audio output - any or all of which can contribute significantly to the noise level and thus the ultimate S/N ratio.  Every test method I've ever seen, or done, uses the audio output from the receiver.  Anything else, in my view, is neglecting many variables and is tantamount to comparing apples to locomotives. 

Moreover, it's relatively hard to measure noise power in a spectrum analyzer but relatively easy using an audio power meter.  Also using a spectrum analyzer with a 50 ohm input impedance will load down the stage considerably unless you're using active high impedance probes (which still could skew results due to introducing extra capacitance in teh circuit).

A 10 dB S/N  is as good as 20 dB *for the purpose of comparing receiver sensitivity*.

Quote

Everyone whom I have ever worked with has used 20dB as the standard.

I've been doing this professionally (repairing, using, and spec'ing/purchasing high performance receiving equipment for my employer) for well over 25 years and very seldom have heard of anyone using a 20 dB standard except for VHF/UHF gear.  In any event, for HF receivers, NF is relatively unimportant as atmospheric noise generally overcomes any internally-generated noise (unless you have a receiver with a crappy synth).

Quote
Forget what most of the comm service monitors and amateur gear specs tell you because you can’t see the whole picture unless you’re using one of the really high-dollar modern monitors which has a built-in spectrum analyzer, tracking gen, etc. You also need a noise source.


Pretty sure the ARRL lab and manufacturers use professional grade test equipment to run their figures.  For my own purposes I use a late model Agilent 8935/E6380A which has all of that, and more (except the noise source), as well as other sundry test equipment including a HP8640B, HP 3577A network analyzer, Tek 2460 scope, and a Tek 495P spectrum analyzer.

You need a noise source to measure noise figure, but not to measure S/N, S+N/N, or SINAD.
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Tom WA3KLR
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« Reply #28 on: June 20, 2009, 02:36:00 PM »

I did a study today on my Hammarlund HQ-120X receiver tuned to 3900 kHz.  It’s i.f. bandwidth is comparable to the National NC-183D.  I measured sensitivities at 6 dB and 10 dB audio S+N/N plus a subjective level of “just audible but comfortable audio tone“ that I used in one of my previous postings on my dad’s NC-183D "sensitivity" and image rejection measurements.  The idea here is to provide a comparison to my previous test and a feel for real world minimum readability performance compared to the 6 or 10 dB S/N sensitivity numbers.

I used a HP8640B r.f. generator and the ac voltmeter function of a HP331A distortion analyzer.  The speaker was set to a comfortable listening level.  The ac voltmeter was connected to the speaker terminals.

I could hear the 1000 Hertz tone as low as 0.13 uV at the just perceivable level.  I found that the 0.2 to 0.3 uV level as probably like what I recorded as “just comfortable audio tone” in my sensitivity levels at my dad’s QTH.

The levels for classic AM 30 % mod. sensitivity were:
6 dB S/N = 0.69 uV
10 dB S/N = 1.1 uV.

Next I played a tape recording of a QSO into the r.f. generator; modulation level adjusted with a scope first.  The QSO was of 2 stations received here with excellent signals.  Both stations were just at a 5 readability with 0.4 to 0.5 uV. (a few dB less than the 6 dB S/N sensitivity level).

For a more comfortable listening level though I picked 2 uV.  The S/N at 2 uV measured 17 dB.

When I test HF receivers, this is the kind of results I usually get; I can hear the 1000 Hertz tone in the noise floor around 0.14 uV.  If I get that, I don’t bother actually finding the 6 dB or 10 dB S/N level.  We are used to knowing that we can hear a signal less than 0.1 uV on SSB mode but the same noise figure receiver on AM mode boils down to a 1 uV 10 dB S/N sensitivity number.  So this is not poor, this is a good AM sensitivity; you have to get used to the correlation.

We are also used to the sensitivity numbers being specified in a 50 Ohm system.  That is what I have been quoting.  For a receiver with a 300 Ohm input impedance like the NC-183D, to put the same r.f. power to the receiver input, you need to apply more voltage by the ratio of the square root of 300 Ohms/50 Ohms which is 2.45:1.  So for the NC-183D, the comparable sensitivity performance would be 2.7 uV (1.1 uV x 2.45) for 10 dB S/N.  Agree?
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Pete, WA2CWA
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« Reply #29 on: June 20, 2009, 02:52:43 PM »


Pretty sure the ARRL lab and manufacturers use professional grade test equipment to run their figures.  For my own purposes I use a late model Agilent 8935/E6380A which has all of that, and more (except the noise source), as well as other sundry test equipment including a HP8640B, HP 3577A network analyzer, Tek 2460 scope, and a Tek 495P spectrum analyzer.

You need a noise source to measure noise figure, but not to measure S/N, S+N/N, or SINAD.


If one is interested in how ARRL conducts its testing of equipment, the measurements that are used, etc., the ARRL Test Procedures Manual is here for all membership to review:
www.arrl.org/members-only/prodrev/testproc.pdf
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Tom WA3KLR
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« Reply #30 on: June 20, 2009, 06:09:10 PM »

Brian,

I see what you are doing.  Somewhat of a bizarre test, but it seems like a test to spot i.f. deficiencies, AVC noise problems. 

I used to work on an ADF receiver that had an extreme amount of reserve i.f. gain and AGC action.  When there was a bad crystal in the i.f. filter, the i.f. amps were able to completely counteract this loss of gain.  Old radio designs would have had low output pointing to the i.f., but this receiver had normal output but it appeared to have low sensitivity – poor S/N for the test signal in, pointing to the front end.  The problem was that the signal was greatly reduced in the i.f. due to a fault.

I stand that my dad’s NC-183D has good sensitivity and good image rejection. It is still unclear to me if the one you sold had an undiagnosed problem in it.  The NC-183D also has the capability of high overall gain and has an AVC amplifier in it and the AVC bus is applied to 5 remote cut-off tubes, all 6BA6s, in the 2 r.f. stages and 3 i.f. stages, an unusual amount of AVC control range capability.  This scheme could do the same thing as the ADF receiver I used to work on; a fault of increased loss in the i.f. somewhere gets hidden.  Again I am unclear if what you are seeing is a component fault or a design problem.  Your test seems to be for determining if a stubbornly “poor sensitivity” receiver‘s problem is due to poor front end noise figure or is due to loss in the i.f. which degrades a good S/N after the front end. 

Do you still have another NC-183D or are you talking about the one you sold?

What is the level of the noise source you inject (microVolts for what BW.)?

What is the tube type you propose to use in the i.f. to fix the problem?  My opinion is from what I have to go on, that instead of modifying the whole receiver, find the hidden fault.

You have quoted a 20 dB S/N ratio for a 0.1 uV signal (-127 dBm) in your work application.  This implies a floor of -147 dBm.  This implies a BW of 500 Hz with a 0 dB NF or a BW of 316 Hz with a 2 dB NF. Is this the work application bandwidth and NF?
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w3jn
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« Reply #31 on: June 20, 2009, 07:22:54 PM »

I guess I still don't understand the purpose of this test, and what it tells you.

Quote
It’s not really a bizarre test because you are looking right at what the AM detector or product detector is going to be seeing minus the audio. It’s the way I was trained years ago because that level is very important. I worked in R&D helping to design receivers, etc.

Why is adding noise important?  Adding noise really tells you nothing about the S/N performance of a receiver.  The wideband noise will be detected by the AVC detector, and reduce the gain over and above what an unmodulated carrier would produce.

Quote
I don’t have a noise source at the moment, but what I always used was just a simple random noise generator. You basically just increase the input level until the noise floor peaks. There is a point where it shouldn’t go any higher.

So you're measuring AVC action?
Quote
As you can see when the noise floor is increased (Pic B) via a noise generator the signal is almost eliminated and in some cases the floor will be higher than the actual signal and it will eliminate and override the signal completely. This is why the IF output signal should always be 20dB above the noise floor before the noise is applied. As long as the signal is a least 10dB above the noise floor after the noise is applied everything should be ok. That is what everyone shoots for.
Quote
Right now on my NC303 I'm just using an inexpensive spectrum analyzer to see the IF output level. I'm using a Heathkit IG-42 lab generator for the signal. As long as the IF output is around 20dB higher than the noise floor on the bench without a noise source it should be ok.

OK for what?  What is this telling you?  How is it that you're not loading down the last IF stage with the SA?

Quote
Also, when you hook up an antenna the noise floor does in fact rise.

Which is why S/N measurements aren't done with an antenna (nor noise source) connected.   All of that is external to the receiver and unless I'm missing something here, is immaterial to a S/N measurement.
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Steve - WB3HUZ
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« Reply #32 on: June 20, 2009, 07:32:46 PM »

Yea, really. I put a signal into the receiver and get a certain SNR. I add a noise source and the SNR decreases. No kidding. The sun will come up tomorrow too. Tongue
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Steve - WB3HUZ
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« Reply #33 on: June 20, 2009, 07:36:51 PM »

Yea, really. I put a signal into the receiver and get a certain SNR. I add a noise source and the SNR decreases. No kidding. The sun will come up tomorrow too. Tongue

BTW, I've measured receiver sensitivities using 3 dB as the SNR on professional grade/military receivers that cost $250k. And yes, signals could be seen, heard and easily detected at such an SNR. The SNR at the last IF is no different than the SNR after the detector. Only in the case of a square law AM detector will the input and output SNRs differ.
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Tom WA3KLR
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« Reply #34 on: June 20, 2009, 07:43:29 PM »

Anyhow, to get a 20 dB S/N ratio at the last i.f. with the SA, in AM mode 6 kHz BW with 1 uV in, you need a 9 dB noise figure.  Changing i.f. tube shouldn't affect this unless there is a design fault to begin with.

So then with the 9 dB NF HF receiver you will get a 6 db S/N AM sensitivity of 0.39 uV instead of the 0.69 uV I got with the unmodified pre-war HQ-120X, so what?
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« Reply #35 on: June 20, 2009, 07:57:25 PM »


The trick is to have the same IF output level whether you have the generator input level set to 1uV or cranked up as high as it will go. The only thing you need to watch out for is compression because of possible overdriving some of the stages


This trick that you are talking about is the parameter of AVC loop performance, basically dependent on AVC loop gain and if maximum gain and control range.  It has nothing to do with sensitivity or noise figure.
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w3jn
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« Reply #36 on: June 20, 2009, 08:23:23 PM »

I guess I still don't understand the purpose of this test, and what it tells you.

Your right, you don't understand and its a no brainer Tongue Tongue Tongue

Then it ought to be easy to explain...?

Quote
No, what I'm saying is everything from the very first RF amp stage to the very last IF amp stage is part of the receivers overall sensitivity. If the IF output signal is nonexistent when a 1uV signal is applied to the first RF amp stage then the sensitivity is very low. Forget the AVC, bypass it during the test.

All true, but why all the machinations with the noise generator, and using a SA at the last IF?  The detector in the radio and measuring thru to the last audio will do a better job of giving S/N, as reading just a level on a SA doesn't tell the whole story, particularly if there's LO phase noise riding on the signal.
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Steve - WB3HUZ
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« Reply #37 on: June 20, 2009, 08:32:22 PM »

LOL. Noise figure of cascaded stages:


F = F1 + (F2-1)/G1 + (F3-1)/G1G2 + (Fn-1)/G1G2...Gn


Friis Equation
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Steve - WB3HUZ
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« Reply #38 on: June 20, 2009, 08:35:59 PM »

It's a super secret measurement technique unknown even to premiere receiver and spectrum analyzer manufacturers like Agilent, DRS, Watkins-Johnson, Racal, Rhode Schwartz, etc.


I guess I still don't understand the purpose of this test, and what it tells you.

Your right, you don't understand and its a no brainer Tongue Tongue Tongue

Then it ought to be easy to explain...?

Quote
No, what I'm saying is everything from the very first RF amp stage to the very last IF amp stage is part of the receivers overall sensitivity. If the IF output signal is nonexistent when a 1uV signal is applied to the first RF amp stage then the sensitivity is very low. Forget the AVC, bypass it during the test.

All true, but why all the machinations with the noise generator, and using a SA at the last IF?  The detector in the radio and measuring thru to the last audio will do a better job of giving S/N, as reading just a level on a SA doesn't tell the whole story, particularly if there's LO phase noise riding on the signal.
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w3jn
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« Reply #39 on: June 20, 2009, 09:32:02 PM »

I asked

Quote
Are you talking during alignment, or rated sensitivity?

You replied

Quote
I'm talking about injecting a signal to the antenna input and looking at the last IF stage's output before detection using a spectrum analyzer. You want 20dB above the noise floor at that point all the way down to 1uV for a tube receiver and less for SS receivers. If you are only 10dB above the noise floor it will often get covered up when a noise source is injected and that floor rises.

As this didn't answer my question, I then asked

Quote
For what purpose?  Comparing receiver sensitivity?  Alignment?  Some other reason?

You replied

Quote
As you can see when the noise floor is increased (Pic B) via a noise generator the signal is almost eliminated and in some cases the floor will be higher than the actual signal and it will eliminate and override the signal completely. This is why the IF output signal should always be 20dB above the noise floor before the noise is applied. As long as the signal is a least 10dB above the noise floor after the noise is applied everything should be ok. That is what everyone shoots for.

As this still didn't answer my question, I asked

Quote
I guess I still don't understand the purpose of this test, and what it tells you.

Why is adding noise important?

You replied, with an ad-hominem attack

Quote
Your right, you don't understand and its a no brainer Tongue Tongue Tongue

Making one last attempt, I asked

Quote
All true, but why all the machinations with the noise generator, and using a SA at the last IF?

And you repllied, never answering any of my questions

Quote
I did explain it in great detail and it does not take the place of normal receiver alignment tests as specified per the receiver's manual. However, there are things you cant see unless you probe around and look at various points within the receiver and the noise floor and IF gain are very important.

You explained what you did, but never why, nor what it's measuring, nor why it's important.

 
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« Reply #40 on: June 21, 2009, 07:53:51 PM »

How do I apply for the super secret receiver test clearance.
Do I have to pea in the cup?
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Steve - WB3HUZ
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« Reply #41 on: June 22, 2009, 08:51:46 AM »

So, if your test is 20 dB SNR w/o the noise source, what is the purpose of later adding the noise source?
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The Slab Bacon
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« Reply #42 on: June 22, 2009, 09:16:14 AM »

Tests, schmests, the real "proof of the pudding is in the eating ! ! ! !"

no arguments to the accuracy of some of these tests and the gear used to make them, but...................... the best test equipment is somethain that we all come with as standard equipment! Our ears and eyes! Roll Eyes

I have seen many times that the published specifications dont tell the whole story.
If a receiver is deaf or lame, you will notice it the first time you set in front of it and use it. If you are a knowledgable operator, you will take notice of it immediatly!!
If a receiver is "HOT" you shoud also take notice of it as soon as you set in front of it. If a receiver is good or bad you will usually take notice of it rather quickly. If you dont, I question your prowess an an operator!!

I have a 183-D and have never noticed anything out of the ordinary about it OTHER than it's rather good sensitivity on the higher bands. (noticably better than the plain 183)

however 2 receicers come to mind for this post (I have 18 of them) An NC-300, and an SX-28. The NC-300 has good overall sensitivity,(even on the higher bands) but a piss-poor noise figure. The internal noise from the 6BA7 first mixer is absolutely horrible. A 40+ incoming signal never completely quiets it out.

The SX-28, with one small modification has the absolute best signal to noise ratio of any receiver I have ever owned!! that modification is just to swap out the 6SK7 first IF amp to a 6AB7, It is a little lacking on sensitivity on the highest band (what do you want for a 1939 receiver) But the incredible signal to noise ratio makes it my favorite receiver for the lower bands!! (And receiver of choice)

The bottom line in my guts is to use all of your best test equpment to align them, but use your eyes and ears to evaluate their performance afterwards!!

                                                                            The Slab Bacon  
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The Slab Bacon
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« Reply #43 on: June 22, 2009, 01:17:46 PM »

Hmmmm................ I guess I'm "the other guy" Grin

Comparing a 183-D to an HRO-60 is a pretty unfair comparison. Kinda like comapring it to a R-390. I still maintain that a 183-D is a pretty descent receiver for what it was. As far as sensitivity goes one never knows what to expect in the real world. A while ago I got the surprise of my life while repairing an HQ-140 on the bench: With nothing more than a clip lead hooked onto the antenna terminal, I was listening to the goof balls on 11m! I thought that was pretty good sensitivity for a radio of that era and quality level. (what it cost back then)

Many of the published specifications were published to keep up with the "pecker matching contest" race between equipment manufacturers. Many of them mean a lot less in the "real world". If you spent a little more time operating some properly maintaind versions of this stuff your opinions might change rather dramatically.

Quite often the ones that put up the biggest arguments here are the ones who spend the least time at the mike.

                                                   the Slab Bacon

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« Reply #44 on: June 22, 2009, 01:23:40 PM »

Wow.  I don't know where to start, so I'll just offer the fact that multiple RF stages are NOT for increased gain but rather for image rejection, front end selectivity, and last (but not least) improved AVC action.
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Steve - WB3HUZ
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« Reply #45 on: June 22, 2009, 03:48:33 PM »

So, how would injecting noise make it easier?


So, if your test is 20 dB SNR w/o the noise source, what is the purpose of later adding the noise source?

Because its very hard to get 20dB above the noise floor without noise injected on the higher bands like 10 meters on many of the old tube receivers. The losses within the long wires, the big rotary switches and the limitations of the tubes themselves make it really difficult even with the better receivers. If you can get it close then it wont matter and you don't need any noise source.

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« Reply #46 on: June 22, 2009, 04:06:58 PM »

Hmmmm................ I guess I'm "the other guy" Grin

Comparing a 183-D to an HRO-60 is a pretty unfair comparison. Kinda like comapring it to a R-390. I still maintain that a 183-D is a pretty descent receiver for what it was. As far as sensitivity goes one never knows what to expect in the real world. A while ago I got the surprise of my life while repairing an HQ-140 on the bench: With nothing more than a clip lead hooked onto the antenna terminal, I was listening to the goof balls on 11m! I thought that was pretty good sensitivity for a radio of that era and quality level. (what it cost back then)

Many of the published specifications were published to keep up with the "pecker matching contest" race between equipment manufacturers. Many of them mean a lot less in the "real world". If you spent a little more time operating some properly maintaind versions of this stuff your opinions might change rather dramatically.

Quite often the ones that put up the biggest arguments here are the ones who spend the least time at the mike.

                                                   the Slab Bacon


The HQ-140 is a real sleeper.   It has great AM selectivity, better than NC-183 or HQ110 (I have all of them), and good sensitivity and relatively cheap on the used market.    It doesn't drift much and has no paper capacitors (as far as I could tell) to go bad.   But, it is single conversion and has poor image rejection above 20M.
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« Reply #47 on: June 22, 2009, 07:33:21 PM »

That is exactly my point, there has always been the impression that the NC183D is a great receiver and compares to other rcvrs like the HRO60, etc. The problem is when you finally get one you quickly find out that it does not, it’s not even close.

Strange, I have an early model 183D and 2 later model HRO 60s and all compare in the sensitivity and selectivity departments.  The 60 are better at rejection but not by a great amount.  There isn't enough sensitivity difference to notice for me.

I align/test by using an isoTee and hooking the antenna to the direct connection in the Tee, then the receiver to the isolated connection.  In this fashion you read the sensitivity as a result of local conditions at the time. 

My 183D will hear a 1 Kc signal down to ~ .5 uv on 75 and 160 to ~.7 on 10 meters.  I do have a quite location here and can discern signals much better than my brethren in a city/suburban environment.

This is the second 183D for me, the first was a much later model and a bit more sensitive.  Adjacent frequency rejection was/is near the same as the 60 and aligned corectly with the AGC adjusted right overload is not a problem for my ears.
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« Reply #48 on: June 22, 2009, 09:58:30 PM »

Gee, my Racal has no RF stage if must be a hunk of junk.
Same with my TCI/BR 8174
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Pete, WA2CWA
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« Reply #49 on: June 23, 2009, 12:33:24 AM »

Gee, my Racal has no RF stage if must be a hunk of junk.
Same with my TCI/BR 8174

Hey Frank, don't feel bad; my Squires Sanders SS-1R receiver doesn't have a RF stage either.
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