Re: Lowering the noise figure in a preamp

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.




tucker@xxxxxxxxxxx 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

.

Relevant Pages

  • Re: Lowering the noise figure in a preamp
    ... Connect a SINAD meter to your receiver. ... Adjust Cushman generator for 12 dB SINAD ... Install preamplifier between Cushman and receiver ...
    (rec.radio.amateur.homebrew)
  • Ailtech 13680 preamplifier
    ... Does anyone know anything about these Ailtech models? ... AIL-13610 Receiver ... I tried googling for these models - but found nothing on them. ... I'm guessing the 13680 is a voltage preamplifier with a 50Ohm input ...
    (sci.electronics.basics)
  • Ailtech 13680 preamplifier
    ... Does anyone know anything about these Ailtech models? ... AIL-13610 Receiver ... I tried googling for these models - but found nothing on them. ... I'm guessing the 13680 is a voltage preamplifier with a 50Ohm input ...
    (sci.electronics.equipment)
  • Re: What is SINAD?
    ... by setting up a test where the receiver produces output from a SSG (typically for a 1KHz audio output) and notching out the 1KHz output to measure the noise and distortion wrt the filtered 1KHz output. ... RF output reduced to find the input level for 12dB SINAD. ...
    (rec.radio.amateur.antenna)
  • Re: What is so special about 12 dB SINAD?
    ... > SINAD port on the test equipment. ... > the receiver at 12 dB SINAD. ... The INPUT level, ... condition, IS the sensitivity. ...
    (rec.radio.amateur.antenna)