Some People...

[Steve - WB3HUZ]

This doesn't square with your previous statement. If you don't need it when the level is less than 20 dB, then why you now say you do need it. Which is it?

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.

There is in fact a noise floor which increases when you connect an antenna to the receiver. If the IF signal is too low the noise floor will cover the signal up..

This had nothing to do with the sensitivity of the receiver. Noise external to the receiver will always be present. But it will not be the same when considering location and the antenna in use. The reality is that if you hear an increase in the noise on your receiver when you connect the antenna, then your receiver is sesnsitive enough. So, you are correct in that a receiver may not hear signals as small as the stated sensitivity when connected to an antenna. So what? That's a given. But that's because over most of HF, the SNR of a received signal is defined by noise external to the receiver. It matters little what the level is at the output of the final IF. That level could be huge, but if the SNR is negative, guess what, the SNR out the output of the RX is negative. Injecting noise into a receiver front-end when making sensitvity measurement tells you nothing about the receiver's performance. You can arbitrarily change the so called sensitivity be changing the level of noise.

So, how would injecting noise make it easier?

It would not make things any easier, but if the output level is too low it will help you see if it’s still at least higher than the noise floor when noise is injected. If it’s at least a few dB higher with a certain uV level injected into the antenna input then the detector should be able to detect it at that uV level when the antenna is connected, etc. I like to visually see what’s happing on the analyzer. You could probably just listen to the tone test with noise injected, but to me having a visual reference is better.

One of the receivers brands I have never gotten into are the HQ’s. I always wanted to try a couple, but I’ve been so happy with my HRO60 and now my NC303 that I just don’t care anymore. Those two are good enough for me and I doubt if I will ever buy another receiver again unless it’s for parts.

[w3jn]

The cathode resistors in the HRO-60 aren't there for bias (since bias is applied from the AVC circuit) so much as they are for degeneration purposes.  This will necessarily reduce gain (not a big deal, since there's a ton of gain available to begin with).  The real culprit in both receivers are the 6BE6 mixers.  Noisy, and prone to IMD.  Nevertheless, either receiver in my experience works satsifactorily.

The reason I "picked apart" your posts, as you put it, is that I repeatedly asked you questions which you were unwilling or unable to address.  I still don't know why one would *inject* noise when checking S/N.  Calculating noise figure, yes.  Figuring S/N - can't think of a reason.

[Ralph W3GL]

  Okay you two,   paint balls at 20  paces!

[Tom WA3KLR]

I visited my dad’s QTH again today and did some more measurements on his NC-183D.  I did some proper AM sensitivity measurements with an ac voltmeter connected to the speaker for AM 30 % 1000 Hertz modulation 10 dB S+N/N measurements on 160, 80, 10 and 6 meters.   An image rejection measurement is added for 160 meters.  Any image rejection measurement over 80 dB I now put as >80 dB since the generator level is probably exceeding the r.f. dynamic range of the receiver for those ratios.

Not stated in my previous posts, the r.f. generator 50 Ohm output was run via an RG-8 cable directly to the receiver input terminals, one antenna balanced input terminal was tied to chassis ground, the receiver selectivity was set to maximum bandwidth and the tone control was set to highest frequency response.

NC-183D Updated Sensitivity and Image Rejection Table 6/24/09

Frequency   Audible    AM sensitivity      image rejection
                  Signal      10 dB S+N/N
1900                            1.5 uV          >80 dB, single conversion   
3850           <0.1 uV       1.0 uV           75 dB, single conversion
7200             0.2 uV                         >80 dB, double conversion     
14200         0.35 uV                         >80 dB, double conversion     
29000         0.35 uV      0.95 uV           58 dB, double conversion     
51000           0.5 uV       1.8 uV           40 dB, double conversion

Image Rejection Specifications from the 1958 ARRL Handbook catalog page 35 ad:
“Image Rejection (At high end of band)”
“Band    Image ratio”
  “A          40 dB”      (This would be 55 MHz.)
  “B          65 dB”      (This would be 31 MHz.)
  “C          80 dB”      (This would be 12 MHz.)
  “D          80 dB”      (This would be 4.4 MHz.)
  “E          80 dB”.     (This would be 1.55 MHz.)

Some of my measurement equal or exceed these specs and some don’t quite meet them.

The 1955 ARRL Handbook catalog page 33 ad merely states:
“Image rejection
 Signal/image ratio better than 55 db at 30 Mhz.”

This is met. 

Brian,

The sensitivity and image rejection numbers I have measured are reasonable for the design.  You stated particularly bad image rejection on the lower bands which is odd since the lower frequencies should fundamentally have the best image rejection performance.  Your particular NC-183D must have not been properly aligned or working properly.

Did you truly do a full alignment on that NC-183D or not?

[Tom WA3KLR]

By the way, here is a photo I took today of the NC-183D I have been testing.  It spent the first 10 years of its life sitting new in the box in a warehouse as a contingency for a Philco military contract.  The sets were then sold to Philco employees in the late 1960's.  This is when my father acquired the unit.

(Available light/tri-pod photo.)

[w3jn]

Actually, I take back my last statement as I notice both cathodes in the HRO-60 are bypassed.  If they were not, their purpose would be to provide some NFB.

Brian, I don't recall you stating until now that your injecting noise was *not* for the purposes of measuring S/N.  I've asked you that several times and until now never got a clear answer.

[Tom WA3KLR]

Brian,

The starting level for the image rejection was probably the point at which I found the 10 dB S/N sensitivity or close to it.  I use the s-meter reading for reference and I remember the S-meter read S-7 in one case.  On the NC-183D S-7 is low level, not the 13 uV S-7 is supposed to be today.

Well it seems you did not realign the one you just sold and now it sounds like you are actually describing IMD products rather than poor image rejection.  That would explain why we are going around in circles over the so-called poor image rejection rap. 

The image for the 40 meter broadcast band is in the 25 meter broadcast band.  I suppose there can be a cumulative problem of both IMD and signals at the image frequency but I believe now that the proper labeling of your performance complaint of the NC-183D would be due to the rf dynamic range being exceeded by the strong 40 meter broadcast signals. 

I have never noticed this with what listening I have done on our NC-183D.  But this is one of the possible down-sides of  a 2 r.f. stage receiver.  The way to overcome this IMD problem is to simply put a 20 dB pad at the antenna terminals or switch to a more minimal antenna under those conditions, rather than modify the whole receiver.

[WA1GFZ]

Maybe it is time for W1VD to use real science and measure the performance using accepted test methods

[w3jn]

Maybe it is time for W1VD to use real science and measure the performance using accepted test methods

Indeed.  I am getting more and more confused.

Any receiver will exhibit unpredictable response when overloaded.  Thus, that isn't a valid test of of image rejection, and it's the reason why measurements are done at reasonable signal levels.

[Steve - WB3HUZ]

He could just never understand that good selectivity at the receiver front-end will produce more gain with better image rejection including improving the signal-to–noise ratio as well.

You are correct about the first two but not about the last. Over most of HF, the SNR of the signal (for any given bandwidth) is determined external to the receiver. A receiver cannot improve this SNR, whether it has a tuned front-end or not.

[W1VD]

I'm following the thread... although it's making my head hurt at times   

A 183D slipped through my fingers last fall because I wasn't able to make the trip to Boston before someone else got it. Still looking for one to add to the collection and to run measurements on it. If anyone knows of one available in good condition please let me know.



 





   

[Steve - WB3HUZ]

Please read what I wrote, especially the part where I wrote "for any given bandwidth." Of course, if you reduce the bandwidth, you can improve the SNR. But there is no amplifier used in amateur grade HF receivers that improve the SNR. If the signal is above the noise floor of the receiver (really just the RF stage), then the SNR is defined solely by noise external to the receiver. There is no way around this. This has been understood in the HF world for many decades.

From "HF Communications, A Systems Approach" by Nicholas Maslin:

5.1.7  Condition for External Noise Limitation

Consider an incoming signal of mean power S at the antenna and an incident noise power N_o in a 1 Hz bandwidth. Let the (omnidirectional) antenna have efficiency n at the frequency considered. After reception by the antenna the signal power is nS, the noise power is nN_o and the signal-to-noise ratio remains S/N_o. At the receiver input other losses such as those caused by imperfect matching reduce the signal strength to n'S and the noise power to n'N_o.

As the signal passes through the receiver, the receiver noise power N_r, per Hz contributes to the total noise power and the final signal-to-noise ratio becomes

n'S/(n'N_o + N_r)                    (5.4)

If conditions are such that

N_r << n'N_o                          (5.5)

then the signal-to-noise ratio at the receiver output is effectively the same as at the antenna. The inequality (5.5) can be expressed in logarithmic units as

n' + F_a >> F_r                      (5.6)

where n' dB is the loss in the receiving system, F_a is the effective antenna noise power factor (in dB) and F_r is the receiver noise figure (in dB). Inequality (5.6) is the condition that the receiving system performance is limited by external noise. Provided that (5.6) is valid the receiving antenna efficiency is unimportant. The condition in (5.6) is almost always true, except when using a poorly matched receiver at the lowest frequencies in the HF band under conditions of low external  noise.

Quote from: Steve - WB3HUZ on Today at 10:35:36
Over most of HF, the SNR of the signal (for any given bandwidth) is determined external to the receiver. A receiver cannot improve this SNR, whether it has a tuned front-end or not.

Unfortunately that is incorrect. Both the RF stages and the IF stages have their own separate signal-to-noise ratios. Good filtering that produces more narrow bandpasses increases the RF energy level being produced by the stages and better attenuates adjacent frequencies and noise outside of the designed bandpasses. One of the results is a better signal-to-noise ratio. That is one of the reasons for using analyzers such as the HP noise figure meter when designing RF and IF amplifiers. You do it both without and then with the noise source applied.

[Tom WA3KLR]

Now that I see the picture of Brian’s National receivers and speakers, I am reminded that when I worked on my dad’s NC-183D I also made sure that I took the speaker along to check it out.  Sure enough, the speaker was distorting!  So in the process of the receiver’s overhaul I selected a new speaker to replace the original one.

This should be a standard practice when working on someone’s receiver – to also ask for the separate speaker that is used with the receiver and give it a good listening test with an amplifier and program. 

[K3ZS]

I just checked my NC-183 stock speaker.   It is a "Jensen Alnico5 PM Speaker".  This is an original 1947 model.