Topband: ...Recovery time???

tom.mcdermott4 at tom.mcdermott4 at
Mon Feb 19 21:09:18 EST 2007

Tom, W8JI wrote:
> I looked at my 1000MP MK V's (actually two of them) just by 
> on-the-air testing and didn't notice that problem, both in 
> VOX and QSK. I can hear any signal above noise floor below 
> 25-30WPM dots between a string of dots on QSK. As a matter 
> of fact if I send a string of dots I can easily copy a 5 WPM 
> S-6 station while I'm rattling off the high speed dots.

My MP is packed away for awhile, so it's not possible to
recheck my recollection of previous measurements, but:

	* If I sent one very short 'dit' the FT1000MP receiver
	recovered almost normally.

	* If I sent a longer single 'dit' the AGC jumped to 60-over-9,
	and then had to recover from that level (slow in either AGC
	mode, since the AGC has a long way to go).

	* The AGC overshoot was temperature dependent (or perhaps
	operating time since power up).

In looking at the receiver response, there are some 7 millisecond timers
(at the minimum possible delay settings) during break-in:

	A. The first one mutes the receiver on key closure, then waits
	7 msec to start emitting a transmit carrier.

	B. The transmit carrier stops on key-up, the receiver then waits
	7 msec (or was it 14 msec ?) to unmute.

I suspect that if the 'dit' is shorter than 7 msec (14 msec key-down),
it works OK, and if longer than that, the receiver AGC can get hosed.

This appears to be independent of the 'thump' which on the scope looks 
like a capacitor being opened at the unmute time. If the receive audio
line into the mute circuit happened to be different than zero volts at
the instant of unmute, then a coupling capacitor could get a big jolt
across it, causing a thump related to the value of the capacitor, the
resistance of the muting transistor, and the voltage in front of the mute
circuit at the instant it was unmuted.

Previous recollection of an article in QEX about a different receiver
was that the L.O. injection into one of the mixers was supressed during
Transmit. The author found that it was reduced a lot of dB, but not
sufficiently killed, and this caused the mixer to actually mix and thus
produce an I.F. signal resulting AGC overshoot.

So there's likely 2, maybe even 3 separate issues related to break-in.

	-- Tom, N5EG

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