>Rich says:
>
>> The bottom-line is that there should not be a situation where the bias
>> can be in the non-linear/Rx mode when the RF relays are in the Tx mode.
>
>I must admit that I've never thought it through very carefully before, but
>unless the bias is switched before the RF gets there, you must be
>effectively pulse modulating.
? indeed.
>This will give the usual sinx/x spectrum, with
>sidebands off to infinity (or at least a long way!) with nulls at offsets
>that are the inverse of the rise time.
>The overall ill effect should be
>minimised by having a long hang time, so that it doesn't switch very often -
>but it will still splatter while it does.
>
>As an alternative, how about switching the bias by means of a signal derived
>from the audio, so the PA is switched before the RF gets there?
? A DC T/R signal is available at the accessory jack in every modern
transceiver. The signal typically appears 5 to 10 mS before RF output
appears. The signal is typically +6v to +12v @ c.10mA max. . Inside
the transceiver, the same signal is typically used to power the RF reed
relay that switches the antenna between the receiver and the transmitter
sections. In Kenwood radios, this signal is designated as "RL"
(+12v@10mA). In Figure 7 on my Web site, Q3 uses the T/R signal to
actuate the two RF relays. The net result is that within 2mS of the
appearance of the T/R signal, both RF relays in the amplifier have closed
and their contacts will have stopped bouncing.
> Sounds a bit like having relays and interlocks, doesn't it?
>
? make the relays switch fast enough and interlocking is not needed.
>And as Rich says, getting switching on 'soft' syllables is difficult. Ever
>get a VOX that's really satisafctory, especially on words like 'electron'?
>
? Surely. I set the delay to drop at commas, periods and yawns.
>
- cheers, Peter
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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