Joe,
Excellent information.
The T/R system I designed with the UM2110 devices replaced the QSK system in
a Ten Tec Omni Six where output power is held to 100W and generally, VSWR is
much better controlled in between the Tx and amp, or between the Tx and
transmatch. Part of the reason for the upgrade was to also improve port
isolation between the main and Rx ports where it was improved from ~ 40 dB
to well over 70 dB.
I had not really considered the effects that you and Lon are discussing at
high power and the required HV needed for reverse biasing the Rx path -- all
of which makes perfect sense.
Paul, W9AC
----- Original Message -----
From: "Joe Subich, W4TV" <lists@subich.com>
To: "'Paul Christensen'" <w9ac@arrl.net>; "'Amp Reflector'"
<amps@contesting.com>
Sent: Saturday, December 19, 2009 12:43 AM
Subject: RE: [Amps] Response the "QSK switching time"
>
>
>> The tick is caused by the moderately-high switching voltage
>> being applied to the T/R diodes. I haven't taken a good look
>> at that part of the schematic, but I'm curious as to why such
>> high voltage (400V) is needed for switching?
>
> The 400 Volts is used to baas off (reverse bias) the receive
> path (antenna to transceiver) during transmit. In a 50 Ohm
> system at 1500W the RF voltage is approximately 275 Volts
> (RMS) or about 385V peak. A back bias of less than 400V will
> allow the diodes to conduct during part of the RF cycle and
> will result in feedback which can turn the amplifier into an
> oscillator, destroy finals in the transceiver or many other
> nasty things.
>
> As pointed out by K5JV just a couple days ago, 400V is really
> too low to survive system voltages at 1500 W with at SWR levels
> above 1.5:1 in SSB or any SWR in constant carrier modes like
> RTTY. This is one reason that the QSK-5 is rated as it is.
> This lack of understanding is one reason the QSK-5 and old
> DEO QSK-1500 have a bad reputation, particularly when operated
> with 8877 based amplifiers with "all knobs to the right."
>
> Lon is correct, to be reliable the diode QSK units really need
> to use 1000V reverse bias for the "transceiver to antenna" leg
> of the switch in order to prevent breakdown under the combination
> of high power and high SWR. By the way, the 400V reverse bias
> is reasonable for a 1000W amplifier like the SPE 1KFA or a
> single 3-500Z and the diode T/R switch is likely to survive
> anything the amplifier can deliver into a 2:1 SWR.
>
> 73,
>
> ... Joe, W4TV
>
>
>
>
>> -----Original Message-----
>> From: amps-bounces@contesting.com
>> [mailto:amps-bounces@contesting.com] On Behalf Of Paul Christensen
>> Sent: Friday, December 18, 2009 7:16 PM
>> To: Amp Reflector
>> Subject: Re: [Amps] Response the "QSK switching time"
>>
>>
>> > I have to comment on Carl's thread. I have not been able to get
>> > through to them lately, either. My bone with them is that their
>> > Quality Control is awful, almost non-existent. Since I
>> have started
>> > messing with QSK, I have bought two QSK-5, stand alone units, from
>> > them.
>>
>> I recently purchased a used QSK-5 to use with an SPE
>> solid-state amp. The
>> electrical concept of the QSK-5 is quite good although
>> mechanically, it can
>> use some improvements.
>>
>> The low-level SO-239 panel connectors use the aluminum
>> cabinet exclusively
>> for the RF return. I detest construction practices where
>> only the case and
>> hardware are used to complete return paths. Same issue with
>> their RCA
>> keying connectors. The panel is used for the DC returns. If
>> the KEY 2 jack
>> becomes slightly loose -- and looses contact against the
>> cabinet, the QSK-5
>> will hot-switch - guaranteed.
>>
>> To deal with these issues, I installed toothed solder lugs on
>> the SO-239 and
>> RCA jacks. On the RF connectors, I then brought individual
>> leads from each
>> jack directly back to circuit ground. On the RCA connectors,
>> I grouped the
>> returns together with #20 buss bar back to circuit ground.
>> At least now,
>> the QSK-5connections are solid.
>>
>> Finally, I replaced the line cord with a detachable IEC type with
>> combination RFI/EMI filter. I really hate having to find the
>> end of an AC
>> cable when it's time to perform some maintenance on a piece
>> of equipment.
>> The IEC cord makes it easy to pull the unit from the desktop.
>>
>> I did find it odd that the QSK-5 makes a "ticking" sound when
>> keying - with
>> or without RF applied. Although a frame signal relay is used
>> as part of the
>> unit's bypass system, the relays are not the root cause. The
>> tick is caused
>> by the moderately-high switching voltage being applied to the
>> T/R diodes. I
>> haven't taken a good look at that part of the schematic, but
>> I'm curious as
>> to why such high voltage (400V) is needed for switching?
>> Fifteen years ago,
>> I designed a PIN diode T/R switch using Microsemi UM2110
>> types and the
>> diodes are fully biased with low voltage and moderate levels
>> of forward
>> current. The UM2110 is optimized for HF switching and easily
>> handles 1.5KW
>> with a reverse voltage rating of 1KV.
>>
>> Paul, W9AC
>>
>>
>>
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>
>
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