Most of the time I see RF stages coupled with .1u caps, which are
basically transparent at RF. Sometimes I see very small values
used, like 3-10p.

A couple examples are in:

http://www.amqrp.org/kits/38spcl/schematic.html

C17 (near the middle top, connecting U2 to a tank) and then the output
of that tank to U4A (unmarked, but also 5p).

It's clearly important to the circuit (empirically) but I don't understand
what they're doing. I can sort of see C20 (the unmarked second one) acting
as a high-pass RC filter.

--
Ben Jackson AD7GD

http://www.ben.com/

Ben Jackson wrote:
It's clearly important to the circuit (empirically) but I don't understand
what they're doing. I can sort of see C20 (the unmarked second one) acting
as a high-pass RC filter.

--
Ben Jackson AD7GD

http://www.ben.com/

Andy writes:

Well, it could be that his transmitter oscillated, and he just tacked

a 220 pf to ground for C20 to make it stop.......

Putting a cap to ground on the output of an op amp is something
I have always avoided, EXCEPT when it made things work... :)))))

Andy W4OAH

Those small capacitors effect an impedance transformation into and out
of the tank. They also reduce the shunt R seen by the tank, which
increases its loaded Q and therefore selectivity.

Roy Lewallen, W7EL

Ben Jackson wrote:
Most of the time I see RF stages coupled with .1u caps, which are
basically transparent at RF. Sometimes I see very small values
used, like 3-10p.

A couple examples are in:

http://www.amqrp.org/kits/38spcl/schematic.html

C17 (near the middle top, connecting U2 to a tank) and then the output
of that tank to U4A (unmarked, but also 5p).

It's clearly important to the circuit (empirically) but I don't understand
what they're doing. I can sort of see C20 (the unmarked second one) acting
as a high-pass RC filter.

On Thu, 03 Aug 2006 17:23:31 -0500, Ben Jackson wrote:

Most of the time I see RF stages coupled with .1u caps, which are
basically transparent at RF. Sometimes I see very small values
used, like 3-10p.

Depends on the frequency and the impedences.

A couple examples are in:

http://www.amqrp.org/kits/38spcl/schematic.html

C17 (near the middle top, connecting U2 to a tank) and then the output
of that tank to U4A (unmarked, but also 5p).

It's clearly important to the circuit (empirically) but I don't understand
what they're doing.

It's part of the matching network, the upper cap is the 5p and the
lower is 100p. Also by making the value small the amount of RF
coupled is lower ( capacitive attenuator) along with tuning L2.
It is a simple circuit but has a more complex analysis.

For example U2 on recieve is a product detector and the desired
output from pin 4 is audio. On transmit it's used as a mixer to get
the VXO and BFO to produce a ~10mhz rf signal at pin 4. So making
the path for RF use a smaller value to keep any audio out of the path
is part of the circuit "tricks".

Since the design makes many of the parts serve dual duty
the analysis is often less than obvious.

I can sort of see C20 (the unmarked second one) acting
as a high-pass RC filter.

It's doing two jobs, DC blocking and a somewhat high pass coupling.
However the impedence at that point is fairly high so coupling values
can be smaller. The device there is not an opamp but a inverter
(digital logic) from a CMOS IC pressed into service as an amplifier
so there are some things done arount it to make behave are as needed
to make it work.

Allison
KB!GmX

com/

Andy writes:

Well, it could be that his transmitter oscillated, and he just tacked

a 220 pf to ground for C20 to make it stop.......

Putting a cap to ground on the output of an op amp is something
I have always avoided, EXCEPT when it made things work... :)))))

Andy W4OAH


Oops !!! I was looking at C21 instead of C20......


......... never mind .... :))))))

Andy

Ben,

The series C17 coupling cap is working against the net reactance of the
C19-TC2-L2 circuit to form a resonant impedance matching network. Although
it's not obvious, the tank network is tuned on the inductive side of
resonance in order to produce exactly enough net inductive reactance to
resonate C17. At this setting, C17 and the net inductive reactance of the
C19-TC2-L2 network form a series resonant circuit. Because the whole network
is resonant, the junction of C17 and the C19-TC2-L2 network is a higher
impedance point than the pin 8 output of U2. Assuming lossless components,
energy must be conserved, and so the signal voltage at this high impedance
point is higher than at pin 8 - yes, voltage gain in a passive network! The
unmarked coupling cap feeding into U4A is probably just a large value "DC
block", since the U4A stage design will have a fairly high input impedance
and so will not substantially load the tuned impedance step-up network.

Most people think that couping caps are just used to drop signal voltage
sort of like inserting a resistor, but if designed carefully as part of a
resonant interface as I described, they transfer energy with little or no
power loss, and can actually produce voltage gain. This is a very important
RF design concept that many folks don't understand.

Joe
W3JDR


"Ben Jackson" wrote in message
...
Most of the time I see RF stages coupled with .1u caps, which are
basically transparent at RF. Sometimes I see very small values
used, like 3-10p.

A couple examples are in:

http://www.amqrp.org/kits/38spcl/schematic.html

C17 (near the middle top, connecting U2 to a tank) and then the output
of that tank to U4A (unmarked, but also 5p).

It's clearly important to the circuit (empirically) but I don't understand
what they're doing. I can sort of see C20 (the unmarked second one)
acting
as a high-pass RC filter.

--
Ben Jackson AD7GD

http://www.ben.com/

On 2006-08-04, W3JDR wrote:
unmarked coupling cap feeding into U4A is probably just a large value "DC
block", since the U4A stage design will have a fairly high input impedance
and so will not substantially load the tuned impedance step-up network.

That was my assumption, however when I replicated that part of the
circuit, I found that when U4 was at 8V, the signal picked up a LOT
of phase noise. I'm assuming that the 10k feedback resistor that
biases the inverter for linear operation was back-driving the tank.
Or perhaps it's the fact that in the inverting amplifier configuration
the XC of C20 is required to set the gain at a reasonable ~3x instead
of essentially infinite. In that case I should have been able to
use a .1u + 3k R for the same effect.

Anyway, I've found this tiny bit of circuitry to be very informative!

--
Ben Jackson AD7GD

http://www.ben.com/