output level bug ?

[G4WIM]

Hi,
I've just upgraded the FW in my Portsdown to the latest version and was using the console under system 2 opt 4 to set the RF level per band - however the RF level seems fixed at -8dBm regardless of setting the parameter to 0, 1, 2 or 3 and doing a reboot after the change.

However if go into sig gen mode and set the level to max then I do get much higher level of +9dBm at 437MHz in sig gen mode, then when I select normal 437MHz operation I get the same +9dBm.

Seems as though there's something blocking setting the RF level per band and the sig gen function over rides it.

23cms just seems stuck at low output and using the 'sig gen trick' doesn't seem to work.

The other possibility is finger trouble at my end as I assume than 0 , 1, 2 and 3 equates to 0dBm, +3dBm, +6dBm and +9dBm

Any thoughts ?

[G8GKQ]

Hi

I'm not clear whether you are describing the ADF4351 output level or the Filter-modulator board output level.

The Level set by system setup option 4 controls the ADF4351 output level fed to the filter-modulator board. This was included as a feature because changing the level improved the filter-mod board performance when no LO filter was used. After the LO filter was widely adopted, this feature fell into disuse and I thought that it still worked, but I may be mistaken.

However, there are 3 points to note:

1. The QPSK modulator on the filter-modulator board acts as an amplifier and a limiter. Changes to its input level make very little difference to the output level. You can expect the output levels as stated here https://wiki.batc.org.uk/Portsdown_Output_Levels whatever the input level is set to.

2. The ADF4351 datasheet http://www.analog.com/media/en/technica ... DF4351.pdf states that the levels are -4dBm to +5dBm in 3dB steps. However, the cheap eBay boards do not achieve this. The max output that I have seen at any frequency below 1500MHz is 1dBm. This is reflected in the default calibration file for the ADF4351 mode of the SigGen https://github.com/davecrump/rpidatv/bl ... gencal.txt which stores the lowest output level (level 0) as tenths of dBm.

3. The filter-modulator board output power reduces by a few dB when modulation is applied due to the filtering of the IQ signals (as opposed to I=-1, Q=-1 which is used in CW mode).

I hope that explains what you are observing

Dave

[G4WIM]

Hi Dave,

a bit of progress - I am attempting to set the drive level out of the Portsdown to match the various amplifiers I use on 146, 435 and 1249MHz.

I had imagined that adjusting the level out of the LO on the 4351 would also affect the signal level out of the modulator - I now see the error of my ways and realise I was mistaken and that adjusting the 4351 output level does indeed work and I can adjust it from -8dBm to +2dBm before it gets fed into the modulator PCB.

I've currently left it at +2dBm.

The out put of the modulator board is +5.8dBm at 23cms, +9dBm at 70cms and +10dBm at 2Mtrs - I guess these figures are fairly typical ??

I also see about a further 0.5dB loss thru' the output switcher - again pretty typical I expect.

Knowing what my output powers means I can adjust the gain of each amp by means of attenuators as necessary.

Thanks again for all your help !
73 Tim

[G8GKQ]

Hi Tim

Yes those figures are about right.

There is a variable attenuator coming. Currently only programmed for the SigGen, but high on the to-do list for the transmit software. https://wiki.batc.org.uk/Output_Attenuator

It’s tied up with this as well: viewtopic.php?f=103&t=5328

Dave

[G4WIM]

Hi Dave,

thanks for the info and links - looks interesting and may build one myself in due course.

73 Tim