Geoff, Don,
As an ex sufferer of this disease, my 2p worth.
My investigations, admittedly only with the limited kit available to me
including 20MHz scope and 2 or 3 different wattmeters, I think it was,
showed that the output across all the bands up to 21MHz was remarkably
constant for a given Q1,2 base voltage (and attempt at current
calculation). However, there was a very marked voltage (estimated
current) loss across the base drive circuits on the affected bands
(C80,81/R19,20). Readings prior to this in the RF line merely
confirmed the high drive requirement as did the Hi Cur on 20/30m. With
much help and advice from guys at my club, we too considered the
possibility of other spurious resonant circuits but could find no
evidence for these in my kit at a level that affected performance.
73,
Ted G7BQM K2#4732 and first year RF engineering student!
On 24 Jun 2005, at 02:31, Geoffrey Mackenzie-Kennedy wrote:
Hello Don,
Yes I understand the purpose of the RC compensation circuit in the
drive
path to each transistor to offset their decreasing gain as the working
frequency increases, and which cannot be fully dealt with by the RCL
network
in the primary circuit of T1. As you say say they are not intentional
traps,
which is why I use the term "spurious resonant circuits". The
possibility
of unwanted resonances in the feedback networks eg R39, C59, R35 is
very
real. If for example R39 and C59 were to be a series resonant circuit
at X
MHz, the in-circuit gain of Q2 would decrease at X MHz.
RF design is an unforgiving mistress!!
73,
Geoff
GM4ESD
Geoff,
These are not intentional traps, but are a parallel RC circuit to
adjust
the
base drive with frequency (at low frequencies, the resistors are the
main
path, at higher frequencies, the capacitors pass more RF). This is
done
to
compensate for the gain vs. frequency characteristics of the
transistors.
The inductance of the resistors is just an unfortunate by-product (but
must
be considered just the same in the design).
73,
Don W3FPR
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
