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1-10 of 22 messages
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7360 noise
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by G3RZP on April 8, 2009
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Does anybody know the ENR of a 7360 mixer at HF? Or how many microvolts are needed at the grid for a given SNR in an SSB bandwidth?
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RE: 7360 noise
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by WA2FNS on April 10, 2009
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I recall seeing some comparisons between mixers in an article, probably revolving around the 75A4.Found much reading material but not the specific info you requested,will look further in my stash.Can we post webpages or sites here ?...........John
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RE: 7360 noise
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by G3RZP on April 10, 2009
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Some sources suggest an enr of 1500 ohms. This would tie in with calculation of it as a pentode amplifier, but the conversion transconductance is lower than for an amplifier. The SS1R claimed 10dB S/N for 0.5 microvolts in 5kHz: this works out as about a 14dB noise figure. However, the step up in the preselector is unknown. The transconductance of each beam plate to each anode is listed as 800 microamps/volt so this should have some relationship to enr, while the gain figures published by RCA in mixer and balanced modulator service suggest a very low conversion conductance.
It does appear almost as if the 7360 doesn't offer much over the 12AU7: similar input levels for -40dB IMD, but possibly similar noise levels - assume a voltage step up from the preselector of around 15 for the 12AU7 and you'll get about the same sensitivity as the SS1R.
But I have a hard time believing an enr of 1500 ohms, particularly as that's what one calculates using the standard noise equations for a pentode.
I only have one: I suppose I should make a test rig to check if it works before wasting time trying to make measurements!
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RE: 7360 noise
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by N2EY on April 10, 2009
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There was an article in QST about 1962 by the folks who designed the Squires-Sanders SS-1R. In it, they say the 7360 has a noise figure of about 5 dB at 10 meters. Article title was "New Trends In Receiver Front End Design" or something like that.
There's a ham who has built an interesting 40 meter receiver using the 6JH8 beam-deflection tube as the mixer, again with no RF stage. Google should bring it up.
73 de Jim, N2EY
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RE: 7360 noise
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by N2EY on April 10, 2009
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Here's a receiver using a 6JH8 mixer that includes a lot of theory and practical info:
http://g4oep.atspace.com/retro/retro.htm
73 de Jim, N2EY
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RE: 7360 noise
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by G3RZP on April 11, 2009
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The G4OEP receiver has about the same sensitivity as the SS1R, which indicates 12 to 14dB NF. Putting the signal on the control grid leads us to the 1500 ohm enr as an amplifier, and with a gc of 0.5gm, this suggests an enr somewhat higher. Again, we don't know the actual step up in the rpeselector.
Thanks for the input gentlemen.
73
Peter G3RZP
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RE: 7360 noise
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by N2EY on April 13, 2009
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I took a look at the QST article by W2PUL from September, 1963 that shows the 7360 as a low-noise receiving mixer.
According to that article, the ENR of the 7360 as a mixer is 1500 ohms and the conversion gain is 20 dB. (No impedance levels are mentioned, though).
The article also says the *measured* noise figures are as follows:
29.5 MHz: 5.5 dB
21.25 MHz: 4.4 dB
14.25 MHz: 4.3 dB
Which sounds pretty low, but that's what the article claims.
The voltage step-up of the preselector isn't mentioned specifically, but the front-end coils are described as having Q of 150 to 200, which means the step-up is probably substantial.
In 42 years of amateur radio I have only seen one SS-1R, and it wasn't hooked up at the time, so how one really performs I can't say.
One thing that is mentioned in the QST Product Review is that the SS-1R wasn't very tolerant of antenna mismatch. Its input circuit was designed for 50 ohms resistive, and severe departures from that caused all sorts of odd behaviors.
Seems to me that unless someone had some 7360s sitting around to try, the 6JH8 as used by G4OEP might be a better choice (since it's relatively common and inexpensive).
73 de Jim, N2EY
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RE: 7360 noise
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by G3RZP on April 13, 2009
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Jim,
I have a real problem with a 1500 ohm enr. If you use the equations for noise for a pentode, you get 1500 ohms, true. But mixers have a lower conversion transconductance, so it seems more than unlikely. Now I agree that a high step up (and Q of 100 or more suggests that) will give an overall low noise figure for the receiver. A 6J6 is one of the lower noise triodes, and that's up at 1800 ohms. The 7360 must have a partition noise component, so the 1500 just does not sound plausible to my mind. Because of the switching, (cf the transistor tree mixer, often called the Gilbert cell, although it wasn't invented by Barrie Gilbert), I would expect around 6000 ohms, and that would jibe reasonably well with the 14dB NF of the G4OEP and SS1R receivers with a reasonable step up.
I understand the SS1R used only one tuned circuit before the mixer, and that had a carefully designed arrangement that produced a notch at the image frequency. Such systems have a tendency to be very critical on antenna feed impedance, as you might expect, especially as the antenna is most unlikely to look like 50 ohms at the image frequency.
Incidentally, a peculiarly British (and to my mind stupid) thing is to refer to image as being 'second channel'. Fortunately, it's a term that's falling into disuse.
The 6JH8 has slightly lower transconductance than the 7360 (4400 micromhos as against 5400) which should make it slightly noisier
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RE: 7360 noise
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by N2EY on April 14, 2009
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Peter,
It's not clear to me exactly how the 1500 ohm ENR was derived, nor the noise figures for the 7360.
IMHO, however, what really matters is whether or not a given receiver is sensitive enough for the application. For many combinations of HF band/location/antenna, a 14 dB NF is more than quiet enough, because the noise coming down the feedline will drown it out by many dB. In many other situations, 14 dB isn't adequate at all.
There's also the question of what the real-world conversion gM of a beam-deflection mixer is. I suspect it has a lot to do with particulars like load impedance, LO drive level, etc.
The receiver section of my current HF transceiver uses a 7360 mixer - and a 6EH7 RF amplifier. Maybe a better design could eliminate the RF amplifier, but I found that I needed it for image rejection, IF rejection, and good gain distribution.
The 4th edition of the RSGB Handbook (1968) has a single-conversion receiver by G3PDM with a dual-tuned preselector, 7360 mixer (with fixed bias!), noise blanker, and many other features. It even has a PLL LO VFO using mostly tubes. It's in the SSB chapter, not the receiver chapter, and there are no pictures of it. It seems to be relatively unknown compared to the popular G2DAF designs of the same era, which are well known even on this side of the pond.
There are better triode mixers than the 6J6. One that I hope to try someday is the 6ES8 in a Pullen circuit, which allegedly has ENR of less than 500 ohms, without the usual troubles of triode mixers. But not having tried it, I don't know for sure.
73 de Jim, N2EY
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RE: 7360 noise
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by G3RZP on April 15, 2009
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Jim
A lot of very interesting points there.
>It's not clear to me exactly how the 1500 ohm ENR was derived, nor the noise figures for the 7360<
That makes two of us! By using the usual equations for pentodes and tetrodes, I can get 1500 ohms for an amplifier. But this just can’t apply to a mixer.
> For many combinations of HF band/location/antenna, a 14 dB NF is more than quiet enough, because the noise coming down the feedline will drown it out by many dB.<
That’s very true. I live in an area where as far as I can tell (because the measurements are of necessity rather crude) the noise is a few dB below the ITU R recommendation P.372-8 level for a quiet rural area.
>. For many combinations of HF band/location/antenna, a 14 dB NF is more than quiet enough, because the noise coming down the feedline will drown it out by many dB.<
I’m not quite so sure about that. According the ITU-R rec, at 10 metres the cosmic noise ina quiet rural area dominates and is about +10dB microvolts/meter. That means that a dipole should give around 2.5 microvolts or so to a receiver. I find I get under todays usual 10metre conditions, about 0.8 of a microvolt off a 4 ele beam, depending on where it’s pointing – the noise comes up if I point at the town of 300,000 12 miles away.
It’s been said that some military intercept systems with very poor antennas can use a 10dB NF at HF. Additionally, I’m told that some measurements done by the Royal navy in the South Atlantic suggest HF noise levels some 6 to 10dB below ITU quiet rural – which you might expect. Seals and penguins aren’t noted for their use of electronic equipment!
>The receiver section of my current HF transceiver uses a 7360 mixer - and a 6EH7 RF amplifier. Maybe a better design could eliminate the RF amplifier, but I found that I needed it for image rejection, IF rejection, and good gain distribution.<
Most people run into trouble with the 6EH7 because they run it at full gain, so the mixer overloads A low plate load resistance and a 27ohm unbypassed cathode resistor help linearity without sacrificing too much noise figure – which you can afford to do at HF, as you say.
But have you looked at using two tuned circuits in an image rejection arrangement? It’s no good for wideband use, but should cover a ham band without difficulty.
The G3PDM receiver is an interesting beast, which isn’t widely known over here either. Last I heard, Peter was living in Taxachusetts. With only 6 crystals in the IF filter, I have some doubts as to how good the selectivity is. I still haven’t quite worked out how the sweep circuit works for the synthesizer, as the emitter of the UJT is always between 4 and 17 volts more positive than the base. I’m also not quite sure why the relatively expensive E88CC/6DJ8 was chosen for the oscillator – I would have expected a 12AT7 to do the job.
The first version of the G2DAF wasn’t so good on IMD, but the later versions worked well.
>There are better triode mixers than the 6J6. One that I hope to try someday is the 6ES8 in a Pullen circuit, which allegedly has ENR of less than 500 ohms, without the usual troubles of triode mixers<
I intend to try an E88CC/6DJ8 in the HRO rebuild I have planned. An absolute basket case that I bought cheaply and not even mechanically complete, the intention is to try a 6EH7 RF amp, with two tuned circuits in image reject arrangement ahead of it and a 6DJ8 mixer with a 6C4 oscillator, followed by 1.4 MHz crystal filters ( of which I just happen to have a large selection in the junk box).
The Pullen circuit is not one I’m familiar with.
Tnx agn Jim
73
Peter G3RZP
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