> 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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