OP power and Drive controls question

[bob edwards]

Had my first QSO with fellow Hermes-Lite user and long-time constructor Mike Grierson G3TSO yesterday, I was using the barefoot Hermes Lite. He said it sounded well.

My questions relate to the 'OP Power' control in the Receive window, section Radio versus the 'Drive' control in the Transmit window, TX section. Am I right in saying:-

1. The 'Drive' control sets the digital input level into the transmitter D/A.
2. The 'OP Power' control sets the analogue attenuator controlling analogue RF output and this control has 16 levels.
3. On the Hermes Lite front panel, I notice the ADC 100 LED starts to flash on at about the 3W level with 'Drive' set at 88. However, to get the 'Tx Overload' to flash in SDR Console, the 'Drive' setting has to be much higher at 97. Which indicator shows the maximum linear transmitter output? Why do the two indicators come on where they do?

Cheers, Bob G4BBY

[sdr_fan_h]

Hi Bob,

I'm not very familiar with that HPSDR derivate Hermes-Lite. The total attenuation of the TX path is 19.5 dB and determined by the modem chip AD9866. Gain can be only controlled digitally. For details see datasheet AD9866, Rev. C, page 30. If firmware and SPI control is done correctly this chip should work absolutely linear and comply with the TX path specifications of the datasheet. Possible overload of the following external amp stages is not monitored for my knowledge.

73, Helmut, DC6NY

[bob edwards]

Helmut, that's useful data about the AD9866 - I've just had another look at the data sheet. However, it doesn't help answer how the SDR Console controls and indicators relate to the hardware and how to achieve maximum undistorted output. What is the hardware indicator 'ADC 100' and the SDR Console indicator 'Tx Overload' telling me? I'll need advice from someone who's transmitted with this radio. Can I achieve the rated 4-5W output?

[Simon G4ELI]

Bob,

With SDR there's a maximum numerical value that can be sent to the radio, all software tries to get close to this value. With SDR Console you'll get very close to max. output.

Screenshots always help as I don't have all the info in my mind.

[Max]

Hi Simon

While on the subject of output power from Hermes Lite 2, is there any chance you could take 5 mins to look at the post thread linked below. I think it originally came during your mid-builder period?

Be interested if you can confirm or otherwise potential findings from F4INE re: sending of power output to HL2 of 240 not full 255?

viewtopic.php?f=15&t=818

Thanks for looking.

Max

[bob edwards]

I just did a quick tune up into a dummy load with an oscilloscope across it. With 'Drive' and 'OP Power' controls at maximum, the power output on my Hermes Lite 2 with gateware version 72.8 appeared to be:-

160m 6.8W
80m 6.7W
60m 6.8W
40m 6.1W
30m 7.4W
20m 4.9W
17m 3.7W
15m 3.3W
12m 7.2W
10m 8W

These output levels were roughly the same when driven by OpenHPSDR mRX PS v3.5.0 (HL2). So clearly, there's some variance in output by band. My Hermes Lite was bought in January and is as supplied by Makerfabs, no mods. The output power variation doesn't have anything to do with SDR Console I believe.

Why are the LEDs ADC 75 and 100 flashing during transmit when using SDR Console but don't flash with other software? I also asked this question on the Hermes Lite forum and got this answer from Steve KF7O:-

"I can shed some light at least on the hardware side. Regarding the ADC75 and ADC100 on during TX for some software and not for other, this depends on if the software reduces the RX LNA gain during TX. The HL2 always operates in full duplex whether or not software is making use of that. So RX does pickup your TX signal and the ADC75 and ADC100 LEDs indicate RX clipping. If the LNA gain is at the default +14 to +20 dB range, then you will usually see clipping during TX. Some software (like PowerSDR, piHPSDR and LinHPSDR) have a separate setting for RX LNA gain (or attenuation from +20dB) and the software will switch to that gain setting during TX. Other software (like Quisk) makes use of the the hardware managed TX LNA gain as described on the protocol wiki page to set two gain levels and then let the HL2 do the switching. The hardware managed TX LNA gain is a HL2-specific protocol extension. It appears that SDR Console does not change the LNA gain during TX nor uses the hardware managed TX LNA gain and hence your clipping during TX. Maybe there is a SDR Console setting I am missing though."

To test this, I set the receiver at minimum RX Gain and then transmitted: The ADC 75 and 100 LEDs no longer flash on speech peaks. So that seems to prove that the receiver gain is not minimised during transmit.

So it's safe to ignore those two LEDs.

Unless told otherwise, I'll be assuming that the 'OP Power' control sets the IAMP gain block after the TXDAC in the AD9866. I'm guessing this because this stage has 16 settings and so does the control.

I assume that the 'Drive' control sets the amplitude of the digital stream into the TXDAC - hence the need for overload warning.

So in driving an external linear amplifier, I would set 'Drive' at the highest level without overload flashing, so as to fill TXDAC with a near 12 bit signal. Then set 'OP Power' to suit the drive needed for the linear amplifier. If the lowest setting was still too high, then an attenuator should be fitted between Hermes Lite and linear amplifier, rather than reducing the 'Drive' control.

Both controls are mentioned in the user guide, but how to use them both together for best signal fidelity isn't clear. An extra sentence or two would clear that up - maybe as above if I've got it right.

If Simon or another Hermes Lite user can comment on the above as correct / baloney. that would be welcome.

Cheers Bob