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Old 12-Sep-2013, 2:11 AM   #7
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 171
Quote:
Originally Posted by Pete Higgins View Post
...Do you think his environment could be contributing a noise floor or NF threshold?
Don't know.

Years ago, I measured noise figure of VHF & microwave prototypes for military use. The prototypes would be similar to an unshielded TV preamp.

Here in Dallas, the ambient noise floor was high enough that often the results were contaminated. The problem was cured after a solid copper lined test room was built especially for this (called a screen room). The room had special RF tight ventilation.

Undoubtedly, the VHF spectrum is busier now, than then.

I can see it now, on the Sencore 1456 spectrum analyzer. I have to make a metal case for anything that I hope to take S/N measurements, here.

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However, there appears to be enough lossy components in front of both the VHF transistor and the ATF-54143 to cause the noise figures of the devices to substantially degraded.

Keep in mind that a preamp like the original CM-7777 used transistors that could have a raw device noise figure slightly above 1 dB, but the preamp specification is 2.8 dB (VHF) and 2 dB (UHF). By some measures the CM-7777 has, or had a tough time of meeting specifications. And, Channel Master used low loss coils.

Quote:
The device can handle +20 dBm RF Input Power provided IGS is limited to 2 mA”. I’m not quite sure how to interpolate that number in terms of Winegards advertised “Maximum Total Input (microvolts)” of “2,024,789” ..
That's interesting. without doing any looking at the data sheet.

That maximum total input appears to be the instantaneous Volts the device can sustain without failing. It seems much more than the device can handle in a linear fashion (without IMD and harmonic distortion).

If it linearly amplified 2 Volts, the output could be 20 Volts. Not likely with a 3+ Volt supply.

Quote:
...which in a 75 ohm system, I calculated to be a signal level of about +17.38 dBm unless they are being a little conservative with this rating (~˝ the power ?).
Winegard's Transzorbs and diodes are probably set up to limit input Voltage for some margin of safety.

Quote:
And, given both of those numbers, I don’t understand why I was seeing what appeared to be overload.
The naked ATF-54143 is superior in terms of linearity (signal handling capability), than the NPN transistors that the RCA uses.

It takes much more stringent ESD protection to keep a 54143 alive than the NPN devices.

It could be that the transient protection around the 54143 is introducing some distortion from the strong signals.

When signal strength is high, diodes and Tranzorbs are good IMD & harmonic generators.

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Last edited by tripelo; 12-Sep-2013 at 1:53 PM. Reason: typo
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