David Wooley wrote:
Geoffrey Mackenzie-Kennedy wrote:
I agree that this is one source, and one point of entry for this
signal.is K16 / K17.
Why do you say that. On my K2, the amplitude depends, in the expected
way, on the settings of those two relays, indicating an earlier injection
point. Note that the microcontroller to which they are adjacent is shut
down during receive, so one should not be picking up anything from it's
oscillator. (The relays are the attenuator and pre-amp bypass ones.)
In my K2 the amplitude of the 7MHz birdy behaves in a similar way. The
source appears to be the MCU on the Control Board, because the birdy can
still be detected below both K16 and K17 using a 'sniffer' whose exposed
'link' is approximately 1/8 inch in length, double shielded coax, when the
VCO, Reference oscillator and BFO are shut down to eliminate a 7 MHz product
from these sources. Changing the MCU's 4 MHz oscillator's frequency changes
the birdy's frequency at the rate of 7/4 as seen on a spectrum analyzer.
From 'sniffing' the pcb trace connecting K16 pin 3 to D6 (via C53) I found
maximum birdy amplitude to be at the K16 end, and removing D6 has little
effect. Between K17 pin 8 and the mixer there is not much difference in the
birdy's amplitude, the higher level being at the K17 end.
The birdy can also be found in my K2 on the pcb traces which connect to the
relay coils (pins 10) and in the nearby bundle of pcb traces, some of which
eventually lead to the Control Board. Without destroying my K2's RF board,
and with the test equipment that I have, I cannot be certain whether the
birdy is being conducted all the way or whether spatial coupling is also
present. However the existence of the birdy in the coil circuits K16 and K17
raises the possibility of the classic problem i.e. unwanted coupling between
a relay's coil and its contacts. I had intended to take a look at the RF
bypassing of the relay coils, or adding L-C filters, time permitting.
Without further investigation I suspect that this 7MHz birdy is being
introduced either via the relay coil circuits or by direct pick up by the
pcb trace between D6 and K16, or both.
If you would like to continue this discussion, I suggest that we do so
off-list. I am leaving the house soon, but expect to be here tonight.
73,
Geoff
GM4ESD
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