Does anyone have any idaes on how to lower the nf in a uhf preamp?

In article .com,
" wrote:

Does anyone have any idaes on how to lower the nf in a uhf preamp?


1. Lower then temprature of the device, by dumping it in Liquid Nitrogen.
2. Use a device with a lower Noise Figure, as the active element.

Daaa.....

3a. Tune (match) the input for lowest in-system noise figure.
This is in general not a match for maximum power transfer.
3b. Be sure the active device(s) is/are biased for optimum
noise figu collector/drain voltage and standing current.
4. Be sure there are not any unnecessary noise
contributions from components other than the
active devices in the amplifier.
If you want to consider the system as a whole,
4. Lower the loss between the antenna and the preamp
(eliminate feedline, for example).
5. Be sure that the preamp has enough gain that it
sensibly sets the system noise figure. (Be sure
that its gain gets the signal seen at the next stage
high enough that the next stage contributes
insignificant noise to the system.)
.... etc.

But beware of trading other things for noise figu beware of
poor intermod or desense performance, for example.

Cheers,
"Me" aka Tom


"You" wrote:
In article .com,
" wrote:

Does anyone have any idaes on how to lower the nf in a uhf preamp?


1. Lower then temprature of the device, by dumping it in Liquid Nitrogen.
2. Use a device with a lower Noise Figure, as the active element.

Daaa.....

As a point of reference, a typical NE3508M04
(http://www.cel.com/pdf/datasheets/ne3508m04.pdf) can give you a device
noise figure under 0.4dB, operated at room temperature, at any
frequency across the entire "UHF" frequency range (300MHz to
3GHz)--better than that at the lower end of the frequency range.

Cheers,
Tom

Thanks for the website, my problem was i had a factory cellular preamp
from surplus equipment, it had a ceramic filter on the input set
824-850 mhz. When i removed the filter the noise figure was terrible
but the bandwith very wide, im trying to get it to operate in the 33cm
band with decent noise figure. The only thing i can figure is that by
removing the banpass filter i raised the NF?

How are you measuring noise figure? Describe your setup?

wrote:

Thanks for the website, my problem was i had a factory cellular preamp
from surplus equipment, it had a ceramic filter on the input set
824-850 mhz. When i removed the filter the noise figure was terrible
but the bandwith very wide, im trying to get it to operate in the 33cm
band with decent noise figure. The only thing i can figure is that by
removing the banpass filter i raised the NF?




--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

In article .com,
"K7ITM" wrote:

As a point of reference, a typical NE3508M04
(http://www.cel.com/pdf/datasheets/ne3508m04.pdf) can give you a device
noise figure under 0.4dB, operated at room temperature, at any
frequency across the entire "UHF" frequency range (300MHz to
3GHz)--better than that at the lower end of the frequency range.

Cheers,
Tom


Nothing like a Ruby Maser dumped in Liquid Nitrogen and pumped at
-12 DBm to bring in those extremely small signals........

all it takes is cold......

Im guessing at the noise figure, im running a low level 1mv fr0m a
cushman service monitor to the preamp input. Im using a uniden 898t
scanner as a receiver because it has a digital S meter. When i turn on
the preamp jumps up 3-4 bars
but the modulation tone stays the same volume level. By this im
guessing the noise level has increased, the preamp had a ceramic
filter with the 50ohm input going directly into it and its output thur
cap to the 1st amp, the seller on ebay said just remove the filter and
move the input to the filters output trace, thats what i done but
results are poor even thou this is a commercial preamp pulled from
analog cell site. It looks to be a very well made preamp so it should
work if i figure out the
input match?

The device in the preamp may not have the greatest noise figure to
start with. Most preamps used in those type installations are usually
more concerned with signal handling ability, so as not to produce
intermod, rather than the last ounce of signal to noise performance.
It is difficult to have a device that will give you maximum signal to
noise performance along with excellent intermod performance. Often a
preamp is used ahead of power dividers to feed several receivers and
it is there to make up for the losses in the power dividers and not
necessarily there for the sole purpose of improving signal to noise
performance.

When comparing how it is performing you need to actually measure
sinadd or quieting of the receiver to tell if it is giving you any
improvement. Just looking at stronger noise levels on the receiver and
guessing how much the signal is above the noise can be very
misleading.

73
Gary K4FMX


On 13 Apr 2006 14:49:28 -0700, "
wrote:

Im guessing at the noise figure, im running a low level 1mv fr0m a
cushman service monitor to the preamp input. Im using a uniden 898t
scanner as a receiver because it has a digital S meter. When i turn on
the preamp jumps up 3-4 bars
but the modulation tone stays the same volume level. By this im
guessing the noise level has increased, the preamp had a ceramic
filter with the 50ohm input going directly into it and its output thur
cap to the 1st amp, the seller on ebay said just remove the filter and
move the input to the filters output trace, thats what i done but
results are poor even thou this is a commercial preamp pulled from
analog cell site. It looks to be a very well made preamp so it should
work if i figure out the
input match?

If your modulation is FM, the receiver audio is going to remain the
same. And yes, the S meter will jump due to excess noise output of the
preamplifier. The Excess gain will reduce the dynamic range of your
receiver system and cause intermodulation (IM).

1. Connect a SINAD meter to your receiver .
2. Connect the Cushman directly to the receiver, adjust for +/- 3.0 KHz
deviation with 1000 Hz tone.
3. Adjust Cushman generator for 12 dB SINAD
4. Note the generator output in dBm
5. Install preamplifier between Cushman and receiver
6. Adjust generator again for 12 dB SINAD
7. Note the generator output in dBm (should be more negative value than
step 4)
8. The difference in dBm from steps 4 and 7 relate to the improvment (if
any) in noise figure between the receiver alone and the receiver with
the preamplifier.

9. Install a variable attenuator between output of preamplifier and
input of receiver. Set to 0 dB attenuation.
10. Adjust generator for 12 dB SINAD
11. Adjust variable attenuator from step 9 until the 12 dB SINAD
degrades slighly (numerically 12 dB).
12. Readjust attenuator 1 dB per step until 12 dB SINAD is restored.
13. You have optimized your receiver and preamplifier for best
intermodulation (IM) performance.
14. Install preamplifier close to antenna with a feedline, replace
variable attenuator with fixed value equal to the value from step 12
minu the measured loss of the feedline between preamplifier and receiver.

15. If Cushman doesn't display dBM, use microvolts, fewer microvolts for
12 dB SINAD is the goal!

16. The above assumed an unmodified, properly operating preamplifier.
Your preamplifier must have some sort of preselector filter between the
antenna and it's input. Removing the stock filter exposes the
preamplifier to overload from out of band signals, may affect the
impedance (stability) of the preamplifier. Try operating it in its stock
configuration and frequency prior to modifieing it. Then replace the
stock filter with a properly tuned filter for the frequency you desire.
Yes, you might have to retune the preamp if the frequency change is too
far. Use the SINAD method and a low level signal to tweak the
preamplifier sensitivity. Make sure your generator and cables are 50
ohms. A 10 dB 50 ohm pad between generator output and the system, may
improve your results if the generator does not provide a 50 ohm source.




wrote:

Im guessing at the noise figure, im running a low level 1mv fr0m a
cushman service monitor to the preamp input. Im using a uniden 898t
scanner as a receiver because it has a digital S meter. When i turn on
the preamp jumps up 3-4 bars
but the modulation tone stays the same volume level. By this im
guessing the noise level has increased, the preamp had a ceramic
filter with the 50ohm input going directly into it and its output thur
cap to the 1st amp, the seller on ebay said just remove the filter and
move the input to the filters output trace, thats what i done but
results are poor even thou this is a commercial preamp pulled from
analog cell site. It looks to be a very well made preamp so it should
work if i figure out the
input match?




--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P