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Author Topic: Overshoot and Solid State Amps  (Read 5571 times)
WE1X
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Posts: 350




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« on: July 23, 2011, 06:07:49 AM »

There has been much discussion on the TS-590S and IC-7410 Yahoo groups regarding leading edge overshoot from these rigs and the potential problems presented to hollow state amps. Can solid state amps (e.g., I'm using an Icom 7410 with a THP HL-1.2Kfx) be damaged from overshoot?

Harry WE1X
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M0HCN
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Posts: 473




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« Reply #1 on: July 23, 2011, 12:00:33 PM »

Assuming we are talking AGC loop induced overshoot then yes, you can damage a FET amp that way, but really you should be setting the drive power so the AGC loop at most barely activates.

If you Have say a 1KW amp with a known gain of say 17db (so it needs 20W of drive for full output), then provided you set the rig to output 20W maximum the AGC loop will never activate and thus never overshoot......

If the rig is overshooting even without the AGC loop involved, then the answer is probably to run the rig at full power and just stick a suitable pad between the rig and the amp (This will also improve the broadband match seen by the rig as well as the input match seen by the PA).

Regards, Dan.
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W8JX
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« Reply #2 on: July 23, 2011, 01:02:13 PM »

There has been much discussion on the TS-590S and IC-7410 Yahoo groups regarding leading edge overshoot from these rigs and the potential problems presented to hollow state amps. Can solid state amps (e.g., I'm using an Icom 7410 with a THP HL-1.2Kfx) be damaged from overshoot?

Harry WE1X

When 590 first shipped it has a issue with output power very briefly spiking to full power regardless of power setting or ALC. A firmware flash/update fixed problem.
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M0HCN
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« Reply #3 on: July 23, 2011, 01:31:08 PM »

OUCH!
That is a nasty bug.

Regards, Dan.
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ZENKI
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Posts: 980




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« Reply #4 on: July 23, 2011, 07:13:53 PM »

This is also the reason why so many Tetrode amplifiers cause excessive amount of splatter. Since tetrode amplifiers  are very sensitive to  grid current, any ALC power  overshoot causes splatter. This is especially problematic when you trying to  stay legal with a very high gain tetrode amplifier. Unless you very sure about your transceivers ALC characteristics  you should be aware that you probably producing splatter.  The popularity of Russian tetrodes has also brought with their popularity a huge increase of splatter on  the bands.

The blame can be leveled firstly at the transceiver designers and then at the amplifier designers who can easily make tetrode amplifiers less sensitive by increasing the drive level required. This is easily done  by changing the grid termination or simply putting in a simple attenuator. Tetrodes in the hands of new hams  are disaster unfolding on  the ham bands with the excessive splatter that they produce mainly because of the transceiver ALC issue.
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W8JI
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« Reply #5 on: July 24, 2011, 06:08:11 AM »

Assuming we are talking AGC loop induced overshoot then yes, you can damage a FET amp that way, but really you should be setting the drive power so the AGC loop at most barely activates.

If you Have say a 1KW amp with a known gain of say 17db (so it needs 20W of drive for full output), then provided you set the rig to output 20W maximum the AGC loop will never activate and thus never overshoot......

If the rig is overshooting even without the AGC loop involved, then the answer is probably to run the rig at full power and just stick a suitable pad between the rig and the amp (This will also improve the broadband match seen by the rig as well as the input match seen by the PA).

Regards, Dan.
   

When there is ALC overshoot, it generally gets worse as a percentage of desired power when power level is reduced. The usual options are either to reduce transmitter IF system gain or use an attenuator pad.

This is because virtually all power limit systems simply rely on more internal ALC. There are a few rigs, like the old FT1000 and 1000D, that have a front panel transmitter IF gain control.  Most just share power level with ALC.

73 Tom
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K7ZRZ
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« Reply #6 on: July 24, 2011, 07:18:39 AM »

Tom,

Please define transmitter "IF system gain."

Thanks,
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Brian K7ZRZ
W8JI
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« Reply #7 on: July 24, 2011, 01:10:23 PM »

Tom,

Please define transmitter "IF system gain."

Thanks,

Transmitters have IF systems. That's where ALC and power control voltage is applied. They often have some means of adjusting transmitter IF system gain, either a control or a menu item.  For example in the FT1000MP MKV:

http://www.w8ji.com/ft1000mk_v.htm

Quote
The FT-1000 MK V  has hidden transmit gain menus. They are accessed by pushing and holding FAST and LOCK while turning the POWER switch on. Both of my MK V's  and every MK V serviced here has had the TX IF gain set too high. This causes first character clicks on CW and spits and splatter on SSB.  Here is how to correct the IF gain to prevent ALC clipping on leading edges of CW and voice:

Press and hold FAST and LOCK before and during initial POWER on.

Press FAST and ENT at the same time. You are now in the MENU's and the display should say "0-1 GrPI-cH".

Turn the VRF/MEM CH counter-clockwise to 9-2. The display should say "t iF - GA in" This is the transmit IF gain menu.

Turn the SUB VFO knob clockwise one position to  " t iF - 018". This is the 1.8MHz transmit IF gain.
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K7ZRZ
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« Reply #8 on: July 24, 2011, 08:36:33 PM »

Thanks, Tom...  however I was more interested in your reference to the FT-1000D.  I presume that you are referring to the front panel control "RF Power" - but could you be referring to the "Drive" control?  There's much about the FT-1000D that I don't well understand.... not in those basic functions, just how that functionality is accomplished under the hood.

I'm not trying to get us off track with the original poster.  Thanks.

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Brian K7ZRZ
W8JI
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« Reply #9 on: July 25, 2011, 08:29:57 AM »

Thanks, Tom...  however I was more interested in your reference to the FT-1000D.  I presume that you are referring to the front panel control "RF Power" - but could you be referring to the "Drive" control?  There's much about the FT-1000D that I don't well understand.... not in those basic functions, just how that functionality is accomplished under the hood.

I'm not trying to get us off track with the original poster.  Thanks.



I'd have to go outside and look at mine, but the 1000/1000D has a control on the front panel that sets the transmitter IF gain. There are TWO power controls. One sets the absolute power limit and the other sets the TX IF gain without ALC.
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ZENKI
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Posts: 980




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« Reply #10 on: July 27, 2011, 02:57:21 AM »

SM5BSZ has made extensive measurements on the FT1000Ds ALC performance. He clearly demonstrates how reducing the drive by making the ALC inactive the FT1000D becomes a superbly clean radio.

http://www.sm5bsz.com/dynrange/alc.htm

Leif has been documenting the poor ALC designs in all brands of amateur radio transceivers now for years.
The TS-590S is the latest new release  radio with this issue despite Leifs widely published work in many ham magazines.
The ham companies have their heads buried in the sand regarding the ALC design issue and the poor TX IMD issue. They all really need to clean up their act. Even the 10,000 dollar IC7800 has this design problem.

All radios can can cause excessive splatter if the ALCs become over active. I heard  ZL2JBR the other day telling other IC7700 users  to driver the ALC right to the end of the scale. This kind of idiocy and poor understanding of how ALC should be used is the root cause of the crap signals we hear so much of on the bands.

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