On Ebay

http://cgi.ebay.com/ws/eBayISAPI.dll...98597 70&rd=1

Lucky

Although the RA17 is a VERY good receiver, this person must have had a bad
6790/GM. Also the 6790 does not have Sync detection, so I am not sure where
this eBay seller got that misinformation from. I have a
6790/GM.......typically, the 6790s have crystal filters, although LSB/USB
mechanical filters were an option. I do have this filter set, but haven't
bothered to install them. The crystal filters work fine. Also, the 45MHz
roofing filter in the 6790 is at least an 8-pole unit and a quad JFET ring
mixer is used. This receiver has an IP3 of +30dBm, so it isn't a slouch.
More eBay hype....................................it would be nice if this
person could at least get its facts straight. Another thing.....how did this
person measure the S/N ratio, with a Noise Figure meter? With your typical
15db excess environmental noise, any differences in sensitivity would be
masked. It could be that this person was talking about audio quality.
Those old tube units do sound very good.

Pete

"Lucky" wrote in message
...
On Ebay

http://cgi.ebay.com/ws/eBayISAPI.dll...98597 70&rd=1

Lucky

"Pete KE9OA" wrote in message
...
Although the RA17 is a VERY good receiver, this person must have had a bad
6790/GM. Also the 6790 does not have Sync detection, so I am not sure
where this eBay seller got that misinformation from. I have a
6790/GM.......typically, the 6790s have crystal filters, although LSB/USB
mechanical filters were an option. I do have this filter set, but haven't
bothered to install them. The crystal filters work fine. Also, the 45MHz
roofing filter in the 6790 is at least an 8-pole unit and a quad JFET ring
mixer is used. This receiver has an IP3 of +30dBm, so it isn't a slouch.
More eBay hype....................................it would be nice if this
person could at least get its facts straight. Another thing.....how did
this person measure the S/N ratio, with a Noise Figure meter? With your
typical 15db excess environmental noise, any differences in sensitivity
would be masked. It could be that this person was talking about audio
quality.
Those old tube units do sound very good.

Pete

"Lucky" wrote in message
...
On Ebay

http://cgi.ebay.com/ws/eBayISAPI.dll...98597 70&rd=1

Lucky




I figured someone on here might be interested in one. Yes, I've seen
numerous cases where descriptions were just plain out wrong.
I even wrote a couple people to correct them but I was told to mind my own
business. Nice...

Lucky

I know what you mean. I have a friend that always says that good deeds don't
go unpunished! Sometimes, when you try to help somebody out, the don't
appreciate it.

Pete

"Lucky" wrote in message
...

"Pete KE9OA" wrote in message
...
Although the RA17 is a VERY good receiver, this person must have had a
bad 6790/GM. Also the 6790 does not have Sync detection, so I am not sure
where this eBay seller got that misinformation from. I have a
6790/GM.......typically, the 6790s have crystal filters, although LSB/USB
mechanical filters were an option. I do have this filter set, but haven't
bothered to install them. The crystal filters work fine. Also, the 45MHz
roofing filter in the 6790 is at least an 8-pole unit and a quad JFET
ring mixer is used. This receiver has an IP3 of +30dBm, so it isn't a
slouch.
More eBay hype....................................it would be nice if
this person could at least get its facts straight. Another thing.....how
did this person measure the S/N ratio, with a Noise Figure meter? With
your typical 15db excess environmental noise, any differences in
sensitivity would be masked. It could be that this person was talking
about audio quality.
Those old tube units do sound very good.

Pete

"Lucky" wrote in message
...
On Ebay

http://cgi.ebay.com/ws/eBayISAPI.dll...98597 70&rd=1

Lucky




I figured someone on here might be interested in one. Yes, I've seen
numerous cases where descriptions were just plain out wrong.
I even wrote a couple people to correct them but I was told to mind my own
business. Nice...

Lucky

I'm really kind of baffled what the guy means my 18 db signal to noise.
It's kind of meaningless without knowing how it is measured. Noise is
propertional to the square root of the bandwidth, so you need to know
what filter is being used.

I'm not sure you would use a noise figure meter. Seems to me you would
want the SNR measured from RF input to speaker output because that is
how you use the radio.

I may do this measure some day for yucks, but it will be a bit of work.
I'd have to fire up the old HP334A just to measure the THD of my RF
generator, not that I ever used the box in that mode.

http://www.conknet.com/~b_mobile/NoiseStuff.html
This link should clarify things.

In summary, the theoretical value of noise is: [E^2 - tRkTB] where :

E is the RMS value of noise,
R is in ohms
T is in absolute temperature,
B is the bandwidth in Hz
k is Boltzmans constant (k = 1.38 x 10^-23 joules/degree K)

At room temperature (290 degrees K) with a 50 ohm system with a 1kHz
bandwidth, this converts to a noise level
of -144dBm. 15dB excess noise from the environment converts to a level of
(-144 + 15) or -129dBm. This translates to a level of slightly less than
0.1uV. This is the noise level that you might find in a very quiet rural
area. In my area just north of Chicago, the excess noise is around 4uV at
30MHz, increasing to higher levels on the lower frequencies. At frequencies
below 100MHz, the predominant noise is man-made noise and atmospheric noise.
Above 100MHz, the noise density decreases sharply with galactic noise being
the dominating factor. This is the reason that a receiver with a 12dB NF is
acceptable for reception below 30MHz. When you listen to receivers that seem
to be quieter than others on the same signal, this can be because of noise
in the audio amplifier or because of excess phase noise in the LO
(synthesizer). Excess phase noise manifests itself in a couple of ways;
first of all, it can cause that "white noise" that is sometimes experiences
on signals, but more often it is because of signal feedaround in the I.F.
filter chain. The second way that excess phase noise manifests itself is by
the rising of the system noise floor when tuning very close in frequency to
a very strong adjacent signal.
Two ways that I know of to clean up this problem a use a selective
crystal filter (500Hz BW) at the Phase Detector's reference oscillator input
in the synthesizer. The second way (mainly to improve close-in IP3) is to
use a multi-pole filter with good shape factor as the roofing filter. The
only problem here is when you use a multi-loop synthesizer to tune the minor
loop at the 2nd LO frequency. The Racal 6790/GM addresses this problem by
using a single loop Fractional N synthesizer. That is why they can get away
with using that (8 or 10-poles?) filter at the 45MHz 1st I.F.
As far as receiving higher frequencies, the benefit of a system NF of 2 to
3dB becomes apparent. There are GaAsFET devices that have a NF of a fraction
of a dB,but those are more useful for EME communications, etc.
Anyway, I hope this clarifies things.

Pete

..
..message oups.com...
I'm really kind of baffled what the guy means my 18 db signal to noise.
It's kind of meaningless without knowing how it is measured. Noise is
propertional to the square root of the bandwidth, so you need to know
what filter is being used.

I'm not sure you would use a noise figure meter. Seems to me you would
want the SNR measured from RF input to speaker output because that is
how you use the radio.

I may do this measure some day for yucks, but it will be a bit of work.
I'd have to fire up the old HP334A just to measure the THD of my RF
generator, not that I ever used the box in that mode.

In article ,
"Pete KE9OA" wrote:

http://www.conknet.com/~b_mobile/NoiseStuff.html This link should
clarify things.

In summary, the theoretical value of noise is: [E^2 - tRkTB] where :

E is the RMS value of noise, R is in ohms T is in absolute
temperature, B is the bandwidth in Hz k is Boltzmans constant (k =
1.38 x 10^-23 joules/degree K)

At room temperature (290 degrees K) with a 50 ohm system with a 1kHz
bandwidth, this converts to a noise level of -144dBm. 15dB excess
noise from the environment converts to a level of (-144 + 15) or
-129dBm. This translates to a level of slightly less than 0.1uV. This
is the noise level that you might find in a very quiet rural area. In
my area just north of Chicago, the excess noise is around 4uV at
30MHz, increasing to higher levels on the lower frequencies. At
frequencies below 100MHz, the predominant noise is man-made noise and
atmospheric noise. Above 100MHz, the noise density decreases sharply
with galactic noise being the dominating factor. This is the reason
that a receiver with a 12dB NF is acceptable for reception below
30MHz. When you listen to receivers that seem to be quieter than
others on the same signal, this can be because of noise in the audio
amplifier or because of excess phase noise in the LO (synthesizer).
Excess phase noise manifests itself in a couple of ways; first of
all, it can cause that "white noise" that is sometimes experiences on
signals, but more often it is because of signal feedaround in the
I.F. filter chain. The second way that excess phase noise manifests
itself is by the rising of the system noise floor when tuning very
close in frequency to a very strong adjacent signal. Two ways that I
know of to clean up this problem a use a selective crystal filter
(500Hz BW) at the Phase Detector's reference oscillator input in the
synthesizer. The second way (mainly to improve close-in IP3) is to
use a multi-pole filter with good shape factor as the roofing filter.
The only problem here is when you use a multi-loop synthesizer to
tune the minor loop at the 2nd LO frequency. The Racal 6790/GM
addresses this problem by using a single loop Fractional N
synthesizer. That is why they can get away with using that (8 or
10-poles?) filter at the 45MHz 1st I.F. As far as receiving higher
frequencies, the benefit of a system NF of 2 to 3dB becomes apparent.
There are GaAsFET devices that have a NF of a fraction of a dB,but
those are more useful for EME communications, etc. Anyway, I hope
this clarifies things.

Snip

-144dBm NF

Well, that's the way it is, even for expensive spectrum analyzers and
other test equipment.

I see the main problem as reducing the nearby man made noise since you
can't do much about propagation other than using directional antenna.
The directional antenna is difficult for most people to achieve with
full size antennas. The best thing to do is use an antenna that is not
sensitive to the local man made noise. Lower frequencies will always be
a bigger problem for the reasons you noted.

--
Telamon
Ventura, California

I understand noise computations. [In fact, my comment on it being
proportional to the square root of bandwidth is because you need to
multiply the input referred noise in v/root hertz by the square root of
the bandwidth, assuming the noise is uniform. Conversely, take the
square root of your equation.] You have mixed in way too many side
issues. So are you saying by 18db SNR, he meant a noise figure of 18db?

I've never seen the measurement done, but I assume when a radio specs a
sensitivity at a given SINAD, there is a distortion analyser on the
audio output. The filtered signal is the "S", and the "NAD" is the
residual of the signal.

Oh, I don't think you are being a jerk at all, and I don't take offense!
This newsgroup is all about sharing, and I appreciate your contributions,
along with everybody elses contributions. This is what it is all about!
Anyway, I didn't see that original spec that the person gave, but looking
at the ad again, I do see what you mean. The whole point I was bringing up
was the statement that the seller made about the RA17 being able to hear
weak signals better than the 6790.............I believe that there was some
sensationalism in that statement. What I was also saying was that if two
receivers have a low enough noise floor such that antenna noise overrides
the internal noise of the system, both of the receivers should be able to
hear the same signal. One of the units may have better audio quality and be
more pleasant to listen to, but I think that this is about as far as things
go.
Now, it could really be that in Racal's specification that they are actually
specifying a sensitivity of so many uV for an 18dB S+N/N ratio. I just don't
know.
I do understand what you are saying about distortion analyzers...........I
have an HP 339A of my own, but most of the time I use it for characterizing
audio equipment or for aligning the detectors of FM systems using the SINAD
method.
I remember the 334A also had an envelope detector so that you could measure
the distortion of RF sources up to 50MHz. I also do realize that one of the
best ways to check a radio is to measure the distortion of the RF, detector,
and audio chain by injecting a modulated RF signal at the antenna input at
around 1000uV and measuring the distortion at the audio output with a
distortion analyzer. This is one of the tests I do with my own homebrew
receivers............I shoot for under 0.5% distortion; if this occurs, I am
relatively happy.
On a final note............................feel free to shout me down
anytime, either in this NG or directly. I realize that I only know a small
portion of what there is to know, but I try!
I wish you the best!

Pete

wrote in message
oups.com...
Note that the advert said "Signal / noise 18 db. " Now of course the
seller doesn't know where you live, so the argument about excess noise,
though interesting, doesn't seem relevant to the ad. I'm not trying to
be a jerk, so please don't take any offense.

Most distortion analysers measure signal to noise and distortion. I
beleive the AOR 7030 has a similar spec, which would make sense given
the Lowe and AORUK history.

That would have been cool to see! I remember one of the days when I stopped
over at the Rockwell Inventory Disposal store. They had a bunch of these
frequency standard crystals (not to be confused with the crystal filters) in
vacuum tube types of envelopes. I bought a few of them and gave them away as
gifts to some of my friends. I've still got a couple of HC-49U crystals in
glass envelopes. One of them is 3.0000000000MHz. I'll find a use for it
someday.
I like those Racal receivers...........for the longest time I was looking
for a 6790, and Ken Ganinno, down in St. Louis had one available. I had a
KWM380 that was in pretty good condition so we made a trade, and I haven't
looked back. Last time I spoke with Ken, he was pretty happy with the '380.

Pete

wrote in message
ups.com...
That Racal had vacuum crystal filters too! ;-)