HL2 - Output/Drive control functions with PA disabled

Max
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HL2 - Output/Drive control functions with PA disabled

#1

Unread post by Max »

Hi Simon

I'm using HL2 for the first time with low power output from HL2 RF1 to drive a transverter. I'm seeing some pretty weird behaviour.

So as a memory jogger, RF2 on the HL2 is actually labelled "Ant" and is the onboard 5W PA output (via changeover relay). RF1 is the "unswitched" low power output for transverters and the like that becomes active when the PA is not enabled.

So with PA disabled, and transverter connected to RF1, when I hit "Tune", even with TX DSP panel Drive control set to zero, I see power out of RF1 from the HL2 (well, I actually see output power from the transverter output, hence I assume some signal out of HL2 RF1). So I figure, perhaps the TX DSP panel Drive control is non-functional in the low power mode, and only the "OP Power" slider in the TX "Radio" panel is active.

But no. Transverter output (10W PA) sits around maybe 5-7 Watts with TX DSP Panel slider set to zero (and "OP Power" set to maximum), but increasing the Drive slider results in increasing power from the transverter up to around the full 10W (hence presumably, by deduction, more power out from RF1).

So the question is, what exactly is the function of the TX DSP panel "Drive" control when the HL2 is in Low Power mode (i.e. PA disabled)?

Thanks for looking

Max

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Simon G4ELI
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Re: HL2 - Output/Drive control functions with PA disabled

#2

Unread post by Simon G4ELI »

Max,

Drive is the IQ level as sent to the HL2. If zero then there should be no output. Also, from memory I'm not sure how you can disable the PA - I'm sure this must be documented somewhere.

Anyway - please add a screenshot!
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Max
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Re: HL2 - Output/Drive control functions with PA disabled

#3

Unread post by Max »

Hi Simon. Thanks for reply. Screenshots attached. One shows the PA disabled in the HL2 Radio settings. In the main screenshot you can see I am in "Tune" mode, with the ALC meter confirming audio tone present. You can see in the TX DSP panel the "Drive" control is set to zero.

"OP Power" slider in the TX "Radio" panel on left is set to "Max".

So the low power RF from HL2 is outputting signal to the transverter input via HL2 RF1 output (as expected). See ref to RF1 on this page:
https://github.com/softerhardware/Herme ... onnections

In this state I see about 7W at the output of the transverter. But what's odd is when I increase "Drive" control to 100 in the TX DSP panel, the power at output of transverter goes up again to full 10W output. In the low power mode I didn't think the Drive control would do anything because I assumed IQ out level is controlled by the Radio "OP Power" slider? I thought Drive control would only affect PA drive with PA enabled. Yet they seem to interact in some way at the low power RF1 output with the PA disabled?

Max
Attachments
Main.jpg
Disable-PA.jpg

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Simon G4ELI
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Re: HL2 - Output/Drive control functions with PA disabled

#4

Unread post by Simon G4ELI »

OK,

Bear in mind that I don't use the HL2 often so have forgotten a lot. The screenshots really help.

1) In the Radio panel OP Power will be a power level control in the HL2.
2) The Drive slider is simply the IQ level sent to the HL2 over the ethernet cable.

So, both will affect the output level. Not all hardware has a OP Power option, also Drive is used in the Tune / Tone options.
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Re: HL2 - Output/Drive control functions with PA disabled

#5

Unread post by Max »

OK Simon. What's perhaps counter intuitive is that when used in this configuration with both controls set to minimum, in low power "PA disabled" mode and in "Tune" there is still power being sent out of HL2 RF1 socket even with both sliders on minimum, yet in normal "PA enabled" mode, with both sliders at minimum there is no detectable output from the PA on the HL2, and with "OP Power" set to Max, and Drive (TX DSP slider) set to zero there is also no output out of the Ant/RF2 socket on HL2. Strange that in "PA enabled" mode, with the TX DSP slider, zero really is zero, yet with the PA disabled, TX DSP slider zero is not zero out of RF1 (low power output).

Thanks anyway and no worries, I know it's hard to keep track of the foibles of every radio.

Max

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Re: HL2 - Output/Drive control functions with PA disabled

#6

Unread post by Simon G4ELI »

OK,

If you get any power anywhere with Drive at zero then it's a residual carrier I think. Please monitor this signal with another SDR, let's see what it actually is.
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Re: HL2 - Output/Drive control functions with PA disabled

#7

Unread post by m3ghe »

Max wrote: Sun Jul 25, 2021 2:12 pm OK Simon. What's perhaps counter intuitive is that when used in this configuration with both controls set to minimum, in low power "PA disabled" mode and in "Tune" there is still power being sent out of HL2 RF1 socket even with both sliders on minimum, yet in normal "PA enabled" mode, with both sliders at minimum there is no detectable output from the PA on the HL2, and with "OP Power" set to Max, and Drive (TX DSP slider) set to zero there is also no output out of the Ant/RF2 socket on HL2. Strange that in "PA enabled" mode, with the TX DSP slider, zero really is zero, yet with the PA disabled, TX DSP slider zero is not zero out of RF1 (low power output).

Thanks anyway and no worries, I know it's hard to keep track of the foibles of every radio.

Max
In a lot of radios it isn't unusual for the driver stages to produce power some way before the final stages start to amplify. So a drive level that appears to be zero when the PA is not outputting RF might still be enough to 'excite' the transverter.
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Re: HL2 - Output/Drive control functions with PA disabled

#8

Unread post by Max »

Simon G4ELI wrote: Sun Jul 25, 2021 5:24 pm OK,

If you get any power anywhere with Drive at zero then it's a residual carrier I think. Please monitor this signal with another SDR, let's see what it actually is.
No, it's not that Simon. By way of proving it, in "PA disabled" mode (monitoring transverter output on another SDR) if I select USB, TX DSP Drive at zero, HL2 OP Power slider at minimum.
With no audio input there is no power out at all (as expected). Then select tone input and increase audio input gain, the signal appears on the separate receiving SDR, together with about 2/3 power out at the transverter output. All this with Drive control at zero.

So now taking the transverter totally out of the equation and listening purely for the HL2 low power output on the separate SDR on the fundamental frequency (with no transverter shifts enabled in SDRC), still in "PA disabled" mode, it all behaves exactly as above (a lot weaker of course), but it's very clear. OP Power minimum, Drive zero, I still here tone output audible on the separate SDR only when audio gain is increased, and at the correct audio frequency too.

So, there is definitely power coming out of HL2 low power output with TX DSP Drive set to zero. Maybe as Martin says, the same is happening with the PA enabled, but it's masked by the PA not being driven into action until gain is increased somewhat, although Martin, this would surely imply that the PA output is not linear? I don't want to conflate two unrelated issues here!

Max

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Re: HL2 - Output/Drive control functions with PA disabled

#9

Unread post by m3ghe »

Max wrote: Sun Jul 25, 2021 8:14 pm
Simon G4ELI wrote: Sun Jul 25, 2021 5:24 pm OK,

If you get any power anywhere with Drive at zero then it's a residual carrier I think. Please monitor this signal with another SDR, let's see what it actually is.
this would surely imply that the PA output is not linear? I don't want to conflate two unrelated issues here!

Max
Not necessarily even if the PA is correctly biased the device being used will still have a minimum drive requirement that must be reached before it starts to amplify.
One important point is that I assume SDR console doesn't know the the PA is disabled?
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Re: HL2 - Output/Drive control functions with PA disabled

#10

Unread post by Max »

Max
Not necessarily even if the PA is correctly biased the device being used will still have a minimum drive requirement that must be reached before it starts to amplify.
One important point is that I assume SDR console doesn't know the the PA is disabled?
Hi Martin

Not so. The PA is disabled from within SDRC. See my screenshots in the third post in this thread. But if what Simon says is correct and the Drive control controls the IQ output, then when Drive set at zero then the output should indeed be zero (I would have thought?) and this should be exactly the same whether the PA is enabled or not surely? If drive control on any transmitter is set to zero I would not expect any output regardless of what type of TX it is. This is what I see with PA enabled. Setting of zero on the TX DSP Drive slider gives 0 Watts output from the PA. All good. It's with PA disabled when it all goes odd.

So here is a post from Steve Haynal (HL2 designer) on the Hermes Lite group:
"The HL2 has a low power output mode that can provide 14 to 20 dBm output."
(Although the spec says low power output is 17dbm)

This seems to be confirmed by the approx 6dB output range available from Min to Max on the OP Power slider, both with PA enabled and also disabled. So it's just what's happening with the TX DSP Drive slider that seems to be the issue.

I hear your point about the PA, but if this was the issue, then what I would expect is, with the PA enabled, no output for the first small part of travel on the Drive control. That's actually not even the case with here, because with PA enabled, I can see tiny power output meter deflection (external meter) even with Drive set to position "1" out of 100, and at a setting of 100 I see the full 5W out of the HL2 (as expected), so a full range of adjustment is available from zero to 5 Watts out.

But with the PA disabled, I see about 60% power output with the TX DSP Drive Slider set at zero (I guess this approximates to around 15dbm at the low power output socket), and 100% power output (17dbm) with it set to 100. Strange.

Max

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