Apache Labs Community Forum

Using an 8000DLE, currently using 3.4.9 (I don't believe related to this issue).

Descriptives used:
S Meter : S Meter set to SIG AVG (just had it there, could be SIG)
NFlr: measured noise floor on the panadapter - hovering over the "average" signal in the noise with the yellow horizontal part of the '+' lines.
Filter: same on both, Preamp gain, 16db, same on both (unless indicated otherwise)
All other settings, I BELIEVE, are the same

RX1 TEST:
No Antenna connected: S Meter: -138dbm NFlr: -162dbm
Antenna connected: S Meter -127 NFlr: 152.7

If I change the preamp gain from 16 db to 0 the S Meter value goes up 16 db to -111 and the noise floor, NFlr, goes to -136.7

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RX2 TEST:
No Antenna connected: S Meter: -138dbm NFlr: -147.9 dbm
Antenna connected: S Meter -127 NFlr: 137.6

If I change the preamp gain from 16 db to 0 the S Meter value goes up 16 db to -111 BUT the noise floor, NFlr doesn't change, still at -136.7!

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Preamp on RX2 bad? That would suggest that the difference "properly" seen on the S meter on RX2 w/ and w/o the preamp is because it is just math - i.e. the 16 db is being added into the measured value? Vs. the noise floor itself which doesn't change and is high, same level seen on RX1 with the preamp gain set to 0.

Gary
K9RX

Gary,

You can't "change the preamp gain". You can change the calibration of the preamp gain, but not the actual preamp gain.

The 6M preamp gain is fixed. It can be on or off, that's it. The preamp is a Mini Circuits PGA-103+. The data sheet shows a huge amount of gain at 50MHz, 26.5dB with 5VDC power on the preamp. On the filter board the PGA-103+ is followed by a fixed attenuator. I do not know what the value of that attenuator is. However, the default settings in Thetis lead one to the assumption that the intended gain of the preamp/attenuator path is 13dB.

I've never worked 6M with my ANAN-8000, but I was just fooling with things and with the preamp on and off I saw no appreciable change in noise floor on the panadapter or S-meter. This is as expected since the noise figure of the PGA-103+ is a very low 0.5dB at 50MHz. Remember that filter bypass cannot be on for this to work.

Unfortunately, there were no signals on the 6M band that I could receive (I don't have a 6M antenna), so I could not judge the effects of the pre-amp on an external signal. However I had thought the software is supposed to account for the additional gain, i.e. if a signal is -100dBm at the rear panel antenna connector then it will be shown as -100dBm on the panadapter regardless of the state of the preamp.

Perhaps some regular 6M operators could chime in here.

73,

Scott

Scott - ok, that makes even more sense. If I'm not changing the actual gain but its relative value used in setting the S Meter response, I believe that is correctly stated, this still doesn't explain why the same actions on RX1 do not reflect the same response when done on RX2. THAT was my issue/question.

Also the actual NF of the radio, just for reference, is closer to 5 db and not 0.5db due to the connections/cables and relays involved. This is what the ARRL got in their testing of the unit. I questioned their result and a response from them says it is an accurate number.

g.

K9RX wrote:Also the actual NF of the radio, just for reference, is closer to 5 db and not 0.5db due to the connections/cables and relays involved. This is what the ARRL got in their testing of the unit. I questioned their result and a response from them says it is an accurate number.

I haven't measured it, but just eyeballing it it seems worse than 5dB by quite a bit. However I was referring only to the NF of the 6M preamp by itself.