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Old 13th Oct 2020, 1:41 am   #21
Synchrodyne
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Default Re: Eddystone Model 850/2

A comparison with other Eddystone valved receivers that were contemporaries of the 850:


830/7 – 300 kHz to 30 MHz:

Better than 3 µV for 15 dB SNR, across whole range, 3 kHz bandwidth.


880/2 – 500 kHz to 30.5 MHz:

Above 1.5 MHz: 3 µV for 15 dB SNR, 3 kHz bandwidth.

Below 1.5 MHz: 5 µV for 15 dB SNR, 3 kHz bandwidth.


940 – 480 kHz to 30 MHz:

3 µV for 15 dB SNR across whole range, at 30% modulation and 50 mW output. Bandwidth unspecified, but likely to be 4 kHz, i.e., the “narrow” position.

In all of the above, keeping the noise low for adequate performance above say 15 kHz, where receiver noise rather than atmospheric noise was the limiting factor, was likely to have been of prime concern. All had ECC189 cascode (1st) RF amplifiers, presumably for that reason.


Against those numbers, the 850/2 performance does not look to be too unreasonable, and anyway, clearly better than needed as determined by the prevailing atmospheric noise.


The earlier 680X, 480 kHz to 30 MHz, had a 6BA6 (1st) RF amplifier, so may also be compared:

Better than 5 µV for 15 dB SNR, across the range, 50 mW output. Bandwidth not specified, but could have been the 2.4 kHz minimum.

A set of charts (attached) was also provided for the 680X, showing more detail, including variations with frequency.

The apparent low gain of the RF amplifier in the 850 may well have been deliberate, in order to obtain good signal handling. Having sufficient pre-mixer gain to diminish the effect of mixer noise would have been but a minor issue in a receiver whose frequency coverage did not go beyond 600 kHz. Rather the RF stage served to provide additional pre-mixer selectivity, improve the AGC range, and under the effect of AGC, provide pre-mixer attenuation when required for large signals. By way of a rough comparison, my experience with domestic-type MW receivers is that those with a 3-gang front end with RF stage can handle significantly bigger signals than those with the 2-gang, no RF amplifier arrangement.


Cheers,
Attached Files
File Type: pdf Eddystone 680X Data Sheet p.04.pdf (440.9 KB, 43 views)
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Old 13th Oct 2020, 8:43 am   #22
trh01uk
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Default Re: Eddystone Model 850/2

Quote:
Originally Posted by Radio Wrangler View Post
Don't worry too much about the receive path's antenna factor because it'll be the same for the signal and for the noise, so you still get a manky signal to noise ratio before the receiver factors in.

And as atmospheric noise at 10kHz is about 40dB higher than it is at 1MHz, you'd need a very nasty receiver noise figure to become the limiting factor. It's normally phase noise that does the deed.

David
David,

yes, I take your point about the antenna factor being the same for noise or wanted signals. I was really just a bit intrigued by the apparent direction of the noise floor in that plot. It appears to fall as the frequency falls, rather than rising as we are saying according to atmospheric noise physics.

Not sure whether its relevant but I figured out that G3PIJ was using an Andrus SDR - and I am not up to date enough on such devices to know how your comments on phase noise would relate to the way a typical SDR processes signals.

Another thing that occurred to me is that a simple 150' long wire is probably going to have an output which falls as frequency falls, which might explain the dropping of signal and noise as frequency goes down.


Richard
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Old 13th Oct 2020, 9:36 am   #23
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Default Re: Eddystone Model 850/2

The plot attached to post 17?

The scaling is in dB with respect to 1uV per metre E-field strength, and it shows the atmospheric noise increasing at lower frequencies and decreasing at higher frequencies.

If you look of a plot of the noise figure of a receiver, it could go any way, there are all sorts of factors. Flicker noise, 1/f noise and 1/fsquared noise contributions tend to make receivers noisier at low frequencies. The reciprocal-mixing effect of phase noise joins in in the same direction as well.

There's a lot that isn't intuitive in the noise business. One good example is Gallium Arsenide semiconductors. They are fast, and they are low noise. Everyone and their granny has heard these things. They get pole position in advertising.

But GaAs semiconductors are noisier in the area below a few hundred MHz than plain old silicon transistors can do.

So if a radio boasts a GaAs front end, it's an advantage on the 70cms band, but a goof of a disadvantage on 2m and lower frequency bands.

The Agilent Noise figure analyser has both silicon and GaAs preamps, and uses the silicon at lower frequencies. I believe Duncan put the switchover point at 500MHz.

David
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Old 13th Oct 2020, 7:08 pm   #24
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Default Re: Eddystone Model 850/2

David,

no, not the plot in post #17. The one I gave a link to in post #19, which shows a scan of the band using an Andrus SDR connected to a 150' long wire via a 9:1 unun:

Click image for larger version

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What's odd about this is the falling noise floor as frequency falls (range is 14kHz to 62kHz left to right). As I said above, this might just reflect the likely fall in output voltage from a long wire antenna as frequency falls - I haven't checked the theory of exceptionally short antennas to see.

Richard
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Old 13th Oct 2020, 9:38 pm   #25
James Duncan
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Default Re: Eddystone Model 850/2

Not much use to anyone as it only covers the LF band.
I put one that I had lying around for decades on Ebay and was very pleased and surprised to get £135 for it.It went to a collector in Germany
Rare as only a couple of hundred made mostly for submarines which used LF for comms.
leave it alone unless you want it in your collection.
MM0HDW
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Old 13th Oct 2020, 11:36 pm   #26
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Default Re: Eddystone Model 850/2

Quote:
Originally Posted by trh01uk View Post
The one I gave a link to in post #19, which shows a scan of the band using an Andrus SDR connected to a 150' long wire via a 9:1 unun:
I suspect it's the antenna factor rolling off at low frequencies. I've looked down there with EMC measuring setups with proper correction at an open air test site.... We had a reason to measure fields at VLF and not just switching to using a LISN, so I had a play.

The rise is exaggerated in the picture by having some man made sources at the right.

I also had a loot at the signals found by a VF loop with corrections that was used for susceptibility source calibration and I was in a quiet area, out in the plain of Kansas.... Flatter than Linconshire is on the day after they iron it!

David
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Old 14th Oct 2020, 12:01 am   #27
dave walsh
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Default Re: Eddystone Model 850/2

Snap James [post 12*]. I suppose performance is one thing but price is another. You are still the most northern place that I have ever shipped anything to from the South Coast

Dave W
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Old 14th Oct 2020, 6:32 pm   #28
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Default Re: Eddystone Model 850/2

Such informative posts, thank you. Many of the replies helped me gain a better understanding of LF comm.

If I can get this RX for a reasonable price I'm going to go for it. It'll make a nice addition to the collection and give me another radio to learn from and tinker with.

Tom
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Old 14th Oct 2020, 7:01 pm   #29
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Default Re: Eddystone Model 850/2

It'd be interesting to know the back-story to this radio- it's not as if the USA is short of home-grown expertise in thsi sort of thing! Even if the R389 is a bit of an outlier in terms of complexity and cost, surely something like the SP-600 VLF would have been a more obvious choice "over the pond". Eddystones were never very cheap sets, either and, as mentioned, a set like this would have had a pretty niche market.
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Old 14th Oct 2020, 8:14 pm   #30
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Default Re: Eddystone Model 850/2

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Originally Posted by turretslug View Post
It'd be interesting to know the back-story to this radio- it's not as if the USA is short of home-grown expertise in thsi sort of thing! Even if the R389 is a bit of an outlier in terms of complexity and cost, surely something like the SP-600 VLF would have been a more obvious choice "over the pond". Eddystones were never very cheap sets, either and, as mentioned, a set like this would have had a pretty niche market.
Sometimes companies make "odd" purchasing decisions for the kind of reasons you might not immediately think of. First, they may have a contract with a supplier to buy radios in quantity, and occasionally an unusual requirement comes up but they stick with that supplier because of the contract.

The above reason can be altered slightly so that even without a purchasing contract, a company might have a lot of radios from one manufacturers, and thus carry a stock of spares for that manufacturer's radios. And Eddystone really only made a zillion variants of one radio in each era - so at a glance this 850/2 is hardly distinguishable from all the other slide-rule sets they made. That means much of the mechanics and no doubt electronics is common to all of them.

And finally individuals in companies sometimes get away with totally quirky buying decisions if something takes their fancy, and the management aren't really watching what they are doing or can't think of a reason to object. Particularly if there is some lengthy technical (but spurious) purchasing justification made - they can be totally at sea and just go along with it to avoid looking dumb.

And of course the above assumes a company bought it - rather than some individual importing it into the USA - maybe long after its useful life ended, and merely as a collector's item.

Richard
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Old 14th Oct 2020, 9:41 pm   #31
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Default Re: Eddystone Model 850/2

It might have been imported as likely to be a large saving on the offerings of Watkins Johnson.

David
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