Re: [mi0bot/OpenHPSDR-Thetis] Drive control range insufficient for external linear (Issue #3)

["Christoph v. Wüllen"]

There is a symmetric attenuator (R94, R97, R98, R100) between the
"low power output" and the PA, here you could add additional
TX attenuation.

According to my measurements, the minimum HL2 output is even as high as 1.25 Watt
since the nominal 5W output are achieved with a setting of the on-chip
attenuator of -1.5 dB, and when you go then to -7.5 dB you have 6 dB more,
so the result is more than 1 Watt.

The connection with PURESIGNAL is as follows:

in principle, you could get lower output by down-scaling the TX IQ samples as they
go from the computer to the HL2. However, PURESIGNAL only works if these samples
have full amplitude. So it is very easy to do the scaling (it is built into
standard SDR programs since the old "Penelope" transmitter needed this) but
in my view this is a no-go for HL2 since you want PURESIGNAL.

P.S.: there are LDMOS PAs which you could even better drive directly from the
RF1 output ("Low Power TX"), then you could switch off the HL2 PA.



> Am 04.06.2023 um 08:17 schrieb Aleziss <notifi...@github.com>:
>
>
> Hello to all, so if I understand this correctly, there is no way to get lower signal output from the HL2 than 500mW at -7.5dB ? This is the lowest setting I can get but still my pre-drive amp delivers 25W which is too high to drive the final amp and I need to tune the final amp at 400W ! I would only require like 10-15W would be plenty.
> Thetis version cannot go lower than 500mW because of the pure signal algorithm ?
> —
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[Steve Haynal]

Your options to lower the HL2 output are:

• Add an inline attenuator to the RF output
• Lower audio input signal level (unfortunately throws away DAC bits)
• Reduce 7.5 dB gain of AD9866 to lowest possible value
• Do not use the main RF output but instead use the low power output RF1 with max output of ~20dBm
• Lower the gain of the HL2 preamp by adjusting R55. See the schematics at www.hermeslite.com and search this group for R55 https://groups.google.com/g/hermes-lite/search?q=R55
• Lower the gain of the HL2 amp by adjusting R92 and R99. Search this group for R92 https://groups.google.com/g/hermes-lite/search?q=R92
• As Christoph suggests, populate the preamp/PA attenuator R94, R100, R96 and R97 with values for your application. See the schematic https://github.com/softerhardware/Hermes-Lite2/raw/master/hardware/hl/hermeslite.pdf

The HL2 is primarily a QRP transceiver. It is not designed to connect to a wide variety of external amplifiers. Many people have had success with the HardRock 50.

73,

Steve

kf7o